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  1. #41
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    Quote Originally Posted by funkywizard View Post
    Had I been exceeding this? The datacenter provided graphs showing that I had at times exceeded this. Prior to seeing the graphs, this was not at all clear to me, as I had assumed I was using substantially less than this.
    Exceeding 80% "at times" is perfectly fine.

    If you have a dedicated breaker all to yourself, and it hasn't tripped, it really isn't any of your datacentres business how close you might be to tripping it and exceeding the NEC limits. That is something for you to worry about, not them.

    That was my point.

    If I got a "nastygram" from a datacentre moaning about this, I would ignore it. If they threatened to cut me off, I'd move facilities, and if I was under contract and sufficiently P-O'd by it, I'd sue them for violating it.

    When you buy a dedicated breaker, the datacenter shouldn't then turn around and moan because you might be close to tripping it. That isn't any of their concern.
    Last edited by ramnet; 05-05-2012 at 04:40 AM.
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  2. #42
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    Quote Originally Posted by ramnet View Post
    Metered where you share a breaker with others is handled differently, as opposed to 1 customer having their own dedicated breaker, which is what is meant by purchasing breakered power.
    Ahhh, ok, now that's not what I meant by metered power. I meant that if a customer wants, say, 4kw in a rack, then the DC is responsible for laying out enough power to be able to provide that power (on a non-shared breaker, or more likely pair of breakers). The breakers supplying that power are higher rated than anything the customer is likely to need and the power is charged based on what they use.

    For example, we lay out our power based on 32A and 63A (230V) breakers. If a customer wants a 4kw rack, their rack will be supplied by 2 32A breakers (for redundancy). We measure their power and as long as it stays below 4kw (real, measured) then all are happy. If they use 6kw, then they get a bill. If they start getting close to the limits for the 2 x 32A, then with their agreement, we will provide the power through 2 x 63A breakers.

    No breakers trip - better for the DC and the client. No incentive to leave empty rack space - again, better for the DC and the client.

    I know this isn't the way most US DC's operate but doesn't a scheme like that make a lot more sense than being incentivised by the costings to fill the rack to the point just before the breaker trips then continuously crossing your fingers???
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  3. #43
    A couple of things about the 80% rule. If you are running 80% you are only using primary power, not A/B, right? If you were A/B you should be under 40% on each circuit. So you have only one power circuit and you are running it at over 80%. That should be terrifying to you.

    Most DCs have Branch Circuit Monitoring in the PDUs so they are alarmed whenever you go over a threshold. They also see when you get back under it so it is really easy to see how much time you are spending above 80%. You rebooted something in your rack and spiked, shouldn't be a problem. But every day for hours you should be concerned as should the DC.

    For those people that think it is just about code you might want to google an infrared picture of a breaker at 90% load. You will see that it is much warmer than a circuit at 70%. We've already established that you are only running primary power and not redundant, so now you want to run that single feed up where the breaker is at risk of failure? That is not a good idea. If you have a breaker that trips how are you going to get back up since the servers will be at peak during boot. What is the cost to your business for that downtime?

    You should be thanking the DC for telling you that you are over on your power. You only have a single feed so you should be doing everything you can to make sure it stays up. I can assure you that more people will be upset about tripping breakers without being notified than will be upset because they had to order more power.
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  4. #44
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    Quote Originally Posted by Colo4-Paul View Post
    You should be thanking the DC for telling you that you are over on your power.
    Agreed, however,

    Quote Originally Posted by Colo4-Paul View Post
    What is the cost to your business for that downtime?
    as a supplier, it really isn't any of your concern how your client's run their businesses. This of course assumes all your clients are business customers in the first place, and not buying a rack of gear for their own personal use.

    Quote Originally Posted by Colo4-Paul View Post
    If you have a breaker that trips how are you going to get back up since the servers will be at peak during boot.
    Again, this really isn't your concern either. One can simply stagger start servers. APC calls this cold-start delay. Other PDU manufacturers have similar features.

    Quote Originally Posted by Colo4-Paul View Post
    I can assure you that more people will be upset about tripping breakers without being notified than will be upset because they had to order more power.
    So, just to clarify, would you force your clients with dedicated breakers in their racks to upgrade their power service if they were regularly spiking above 80% (and threaten to cut them off if they didn't), or would you leave them to decide if they want to take the risk of blowing their breaker?

    If one of my servers has pegged it's CPU for several hours doing something and is consuming twice the amount of power it normally would, pushing the average on my dedicated circuit to say, 82%, I shouldn't be forced by my datacenter to buy more power.

    This is really what the key issue is here.

    I firmly believe it should be left up to the client to decide how much they want to push their dedicated power allotment, the same way it should be left up to the client on any other dedicated resources a client might purchase (such as how close they push a dedicated network link to the max for example).
    Last edited by ramnet; 05-05-2012 at 08:36 AM.
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  5. #45
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    Basically what ramnet is saying is, don't sell him service because he will be using over 80% without a doubt.
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  6. #46
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    as a supplier, it really isn't any of your concern how your client's run their businesses. This of course assumes all your clients are business customers in the first place, and not buying a rack of gear for their own personal use.
    Hope dont mind me chiming in. So yes its a DC's business to make sure a customer stays online and does not affect his own or adjacent rack or servers just because a customers thinks he can use above 80% power.


    Again, this really isn't your concern either. One can simply stagger start servers. APC calls this cold-start delay. Other PDU manufacturers have similar features.
    Ofcourse its gonna be DC's business again because a customer tripped their breaker and I know a few datacenters that charge customers about $300 to turn the power back on and if they are using above 80% they should buy more power.

    So, just to clarify, would you force your clients with dedicated breakers in their racks to upgrade their power service if they were regularly spiking above 80% (and threaten to cut them off if they didn't), or would you leave them to decide if they want to take the risk of blowing their breaker?
    Most DC's do because a breaker is tripped from panel and not your rack alone, and no one is going to let a customer keep blowing off their breaker again and again.

    And lastly 80% is set by firecode and is in place for fire safety and not to upsell or rip customer. Specially here we have annual fire inspection and a fire Marshall comes in inspects and certifies that the facility is safe to operate. So if he sees a couple of customers regularly spiking above 80%, entire business is at risk of being non compliant and NOT SAFE TO OPERATE
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  7. #47
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    Quote Originally Posted by Kusai View Post
    yes its a DC's business to make sure a customer stays online and does not affect his own or adjacent rack or servers just because a customers thinks he can use above 80% power.
    1) Overloaded your own dedicated circuit breaker affects no other customers.

    2) The NEC does allow you to use over 80% power. It has done since 1996, and so does the latest version from 2011:

    Overcurrent Devices Rated 800A or Less [240.4(B)]

    Conductor to comply with 210.19(A), 215.2, and 230.42(A). Sections 210.19(A), 215.2 and 230.42(A) require the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load.

    Quote Originally Posted by Kusai View Post
    Ofcourse its gonna be DC's business again because a customer tripped their breaker
    Let's assume the customer doesn't trip their breaker, as is the case with the OP in this thread.

    Quote Originally Posted by Kusai View Post
    I know a few datacenters that charge customers about $300 to turn the power back on
    Remote hands fees are irrelevant.

    Quote Originally Posted by Kusai View Post
    Most DC's do because a breaker is tripped from panel and not your rack alone, and no one is going to let a customer keep blowing off their breaker again and again.
    I'm not disputing this, however, as I said previously, you can run a breaker up to 100% if the load is non-continuous. If I regularly spike a power circuit to, say, 90%, more likely than not, it will be fine.

    Quote Originally Posted by Kusai View Post
    And lastly 80% is set by firecode
    I've already quoted the relevant sections from the National Electric Code. I encourage you to read them and stop spreading misinformation.

    Quote Originally Posted by Steven
    Basically what ramnet is saying is, don't sell him service because he will be using over 80% without a doubt.
    I'll ignore the fact that you have defamed my name in public for the moment.

    I run my power responsibly, however, I am aware of MANY people that spike their racks at much higher than 80% on a regular basis. There are plenty of datacenters that allow this, and there are plenty of datacenters that don't even have power metering on individual breakers, and have no idea how many amps a client is pulling anyway.

    If a breaker trips, then it's too much. If a customer doesn't trip a breaker, and is abiding by the law and the NEC, then the datacenter's should leave the customer alone.
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  8. #48
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    ramnet,
    Until you build and run your own datacenter, you have no room to make "demands" on what THEIR power infrastructure rules and guidelines are. You follow them as they are told to you, or you don't get service. Being asked to keep your circuit below 80% constant load (except for perhaps the -odd- SMALL spike) is not an unrealistic or unreasonable request.
    However don't expect any sympathy or people rushing around for you when you trip your circuit.
    Also, don't forget to factor in the remote hands fees that will most undoubtedly be incurred from having to perhaps manually stagger the turn up of the machines in your rack, as well as the troubleshooting / recovery from unclean shutdown to your machines (and potential complete data loss).

    I don't see how its worth all the effort and risk. Just makes no sense at all.
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  9. #49
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    Quote Originally Posted by ramnet View Post
    If a breaker trips, then it's too much. If a customer doesn't trip a breaker, and is abiding by the law and the NEC, then the datacenter's should leave the customer alone.
    This reply is not a display that you run your breakers responsibly.

    This is why we always buy breakered power instead of metered power.
    Neither is this reply. It just screams that you overload it on a frequent basis.

    I firmly believe it should be left up to the client to decide how much they want to push their dedicated power allotment, the same way it should be left up to the client on any other dedicated resources a client might purchase (such as how close they push a dedicated network link to the max for example).
    The same with this one, obviously have no respect for power usage rules.
    I don't see how you can relate power usage limits to the amount of resources used on a dedicated network link. Its apples to oranges. If you max out a dedicated network link, its not a safety issue.

    ---

    As a colo customer you should be designing your colo enviroment to abide by the usage rule, not skate by the rule in the hopes the DC won't catch on to save some money.
    Last edited by Steven; 05-05-2012 at 03:30 PM.
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  10. #50
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    Quote Originally Posted by CGotzmann View Post
    ramnet,
    Until you build and run your own datacenter, you have no room to make "demands" on what THEIR power infrastructure rules and guidelines are. You follow them as they are told to you, or you don't get service.
    I don't make demands of anyone. You're free to run your datacenter as you see fit.

    You should not assume what my experience is.

    Quote Originally Posted by CGotzmann View Post
    Being asked to keep your circuit below 80% constant load (except for perhaps the -odd- SMALL spike) is not an unrealistic or unreasonable request.
    So, you apparently are reasonable then.

    My point was people have this delusion of "OMG you can never go over 80% never never never" nonsense in their minds, and it's completely wrong.

    Quote Originally Posted by CGotzmann View Post
    Also, don't forget to factor in the remote hands fees that will most undoubtedly be incurred from having to perhaps manually stagger the turn up of the machines in your rack
    As previously stated, any decent managed PDU can stagger the power ports from cold start automatically.

    And this of course, also assumes that you will use more power during initial startup then you might spike to during the course of computing activities.

    I can point to several cases where peak power usage does not occur when computers are first turned on.
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  11. #51
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    With my colo stuff I am doing, I designed what I am doing to leave me a few amps spare for future use. It only costs a small amount of money for me to do, and doesn't hurt my margins at all.
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  12. #52
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    Quote Originally Posted by ramnet View Post
    I can point to several cases where peak power usage does not occur when computers are first turned on.
    I agree with this, many server arrangements don't really peak anymore.
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  13. #53
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    Quote Originally Posted by funkywizard View Post
    Hi Everyone,

    We've been getting a nastygram from our datacenter regarding "bursting" to 16-17a. Whenever I look at the circuit / PDU, it's at 14-15a, although I can certainly believe that it might go over 16a occasionally for spikes (intel speedstep, servers rebooting, etc), I'm not certain, and they didn't say for how long it might be doing this.
    Using power at 16 amps is legal, but continuous loads greater than that are not.

    Quote Originally Posted by funkywizard View Post
    My understanding is that the national electrical code 80% rule is meant to apply to continuous loads. I.E. if I reboot a server and it goes over 80% for a minute or two, that's not against electrical codes, but if I continuously used 85%, then it would be.
    Non-continuous load periods are 'three hours or less.' Any load for a period longer than that must be classified as continuous.

    Quote Originally Posted by funkywizard View Post
    I was wondering, first of all, what is permissible for this "bursting", i.e., what is the definition of continuous or intermittent, how long and how much can something go over before it is considered unacceptable? And, where in the electrical code does it specify this so that I can make a valid argument as to how much power the circuit should / should not be using?
    Following the rules above, "bursting" would be load periods of 'three hours or less.' But a lot of datacenters I have worked with treat power usage like data usage and have some other strange definition of "bursting." So, YMMV.

    Quote Originally Posted by funkywizard View Post
    According to my PDU, the circuit seems to be routinely only using about 14a, sometimes 15a. It would really suck to have to drop 1-2a from here just to not get a nasty gram from the datacenter. I know that codes would dictate that I stay under 16a "continuously", but if I'm really only going over 16a for a few seconds here a few seconds there, it seems silly to take 1 or 2 servers off this PDU, putting my typical power use on a "20a" circuit at more like 12-13a, just to keep the datacenter happy. Complicating matters, looking through the list of devices connected to the PDU, the only thing I can remove without anyone noticing is a wifi access point, which really uses almost zero power anyway.
    What datacenter is this? Also, a lot of PDUs are not entirely accurate on their power consumption data. Yours may be under-reporting, or the DC PDUs may be slightly over-reporting.

    If the datacenter has someone who is not afraid of electricity, the most accurate way to determine who is in the wrong here would be to stick two probes onto the circuit and measure the EM resistance between the probes, using ohm's law.

    How many servers are you running on this circuit?
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  14. #54
    Quote Originally Posted by nenolod View Post
    Using power at 16 amps is legal, but continuous loads greater than that are not.



    Non-continuous load periods are 'three hours or less.' Any load for a period longer than that must be classified as continuous.



    Following the rules above, "bursting" would be load periods of 'three hours or less.' But a lot of datacenters I have worked with treat power usage like data usage and have some other strange definition of "bursting." So, YMMV.



    What datacenter is this? Also, a lot of PDUs are not entirely accurate on their power consumption data. Yours may be under-reporting, or the DC PDUs may be slightly over-reporting.

    If the datacenter has someone who is not afraid of electricity, the most accurate way to determine who is in the wrong here would be to stick two probes onto the circuit and measure the EM resistance between the probes, using ohm's law.

    How many servers are you running on this circuit?
    Our PDU only reads out to the nearest amp, and I could certainly be convinced it's not even 100% accurate there. I got a graph showing I was reasonably well over 16a (around 16.5) for a significant portion of the last month, so I'll go ahead and reduce my usage shortly.

    Looking at racktables, it looks like we've got 18 servers and 3 ethernet switches on this circuit. From previous experience, one of the 3 ethernet switches uses about 1.5a, the second one uses a little less than 1a, and the third uses about 0.3a. So really doing quite well in terms of amps used for each server, with 18 servers using a little under 14a it would seem. Go go gadget E3 Xeons.
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  15. #55
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    Quote Originally Posted by ramnet View Post
    1) Overloaded your own dedicated circuit breaker affects no other customers.

    2) The NEC does allow you to use over 80% power. It has done since 1996, and so does the latest version from 2011:

    Overcurrent Devices Rated 800A or Less [240.4(B)]

    Conductor to comply with 210.19(A), 215.2, and 230.42(A). Sections 210.19(A), 215.2 and 230.42(A) require the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load.
    240.4(B) means that if you have a circuit rated for 16 amps, then it must be able to carry 20 amps for a non-continuous load period, not that you can go over 16 amps continuously.

    [Edit: To clarify: the circuits you get from the DC are rated at 16 amps, not at 20 amps. Many people sell circuit capacity based on maximum non-continuous load rate. In reality, you're being sold a 16A circuit and you're allowed to use 100% of that 16A. This is kind of like dedicated RAM verses burst RAM on OpenVZ -- the burst RAM may be available to you, but you shouldn't be using it constantly.]

    Quote Originally Posted by ramnet View Post
    I'm not disputing this, however, as I said previously, you can run a breaker up to 100% if the load is non-continuous. If I regularly spike a power circuit to, say, 90%, more likely than not, it will be fine.
    NEC and IEEE standards define going over the rated continuous load (even for a non-continuous load period) may cause undefined behaviour in the power equipment. This may translate to deterioration of the circuit breakers, making them more likely to trip in the future.

    Quote Originally Posted by ramnet View Post
    If a breaker trips, then it's too much. If a customer doesn't trip a breaker, and is abiding by the law and the NEC, then the datacenter's should leave the customer alone.
    Given that overcurrent state on a circuit even for a non-continuous load period may cause undefined damage, it is not unreasonable for both the building and the datacenter itself to impose rules designed to protect their distribution equipment.
    Last edited by kaniini; 05-05-2012 at 04:04 PM. Reason: clarification
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  16. #56
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    Quote Originally Posted by funkywizard View Post
    Looking at racktables, it looks like we've got 18 servers and 3 ethernet switches on this circuit. From previous experience, one of the 3 ethernet switches uses about 1.5a, the second one uses a little less than 1a, and the third uses about 0.3a. So really doing quite well in terms of amps used for each server, with 18 servers using a little under 14a it would seem. Go go gadget E3 Xeons.
    A good rule is to allocate 1 amp for each device, yielding a maximum of 16 devices per circuit. Even if you are undershooting as a result, it will ensure that your circuit won't trip.

    Limiting the number of devices per circuit is also a good idea because crappy PSUs may cause line noise which could damage the equipment.
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  17. #57
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    Quote Originally Posted by Steven View Post
    This reply is not a display that you run your breakers responsibly.
    So conforming to the requirements set out in the National Electric code is not responsible?

    You should have quit while you were ahead.

    Quote Originally Posted by Steven View Post
    Neither is this reply. It just screams that you overload it on a frequent basis.
    No, I simply prefer not to have to be concerned about power overages, or a datacenter claiming I used more power than I did.

    I prefer flat rate network ports for the same reason. I like predictability.

    You should not assume just because I have a certain opinion, that automatically means I must be abusing resources, or assuming I must somehow be violating a datacenter's AUP/TOS.

    Quote Originally Posted by nenolod View Post
    Given that overcurrent state on a circuit even for a non-continuous load period may cause undefined damage, it is not unreasonable for both the building and the datacenter itself to impose rules designed to protect their distribution equipment.
    Yes, this is true. Likewise, however, nobody should assume all datacenters define rules regarding this that go beyond what the law says.

    Quote Originally Posted by Steven View Post
    As a colo customer you should be designing your colo enviroment to abide by the usage rule
    Same thing. You should not assume that all datacenters set usage rules that go beyond what the law requires.

    This is a portion from an Equinix contract I have been privy to:

    A Customer may order power to the extent offered and approved by Equinix. Power Circuit Policy: A power circuit at all times meets following conditions: 1) The aggregate draw of such Power Circuit does not exceed the rated capacity pursuant to the National Electrical Code.
    The colo contracts Level 3 use have similar language as well. 80 percent, or maximum usage power, is not mentioned, anywhere. They refer to the NEC rules, and do not expand beyond that. The NEC rules allow spiking over 80%, so one can assume based on a contract like this, that datacenters like Equinix and Level 3 do as well. These are not the kinds of datacenters like the ones that troll this forum for clients, they are big enterprise players and know what they are doing.
    Last edited by ramnet; 05-05-2012 at 04:12 PM.
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  18. #58
    Quote Originally Posted by nenolod View Post
    A good rule is to allocate 1 amp for each device, yielding a maximum of 16 devices per circuit. Even if you are undershooting as a result, it will ensure that your circuit won't trip.

    Limiting the number of devices per circuit is also a good idea because crappy PSUs may cause line noise which could damage the equipment.
    We find that we routinely use dramatically less than 14a on a circuit if we only have 16 outlets, we switched to 24 outlet PDUs and will never look back. It's extremely difficult to balance a load properly with only 16 outlets. You'll have one PDU hit 16 with only 12 outlets, and the other with 10a of use and no outlets free. Then you either have to put up with paying for 5a of power you're not using (1/3 of a circuit essentially), or you have to play the game where you hope you can move things around without causing any problems. Really not a game worth playing.

    On this PDU for example, we're using 22 outlets, 18 for servers, 3 for switches, and 1 for a wifi access point. If we need to hook up a crash cart it would be nice to be able to do that too, and luckily we have a spare outlet for that. Impossible on a 16 outlet pdu. Even with our current overusage, we could move simply one device (the 1.5a power hungry switch) off of the circuit and then be far enough under our allowed power to safely hook up another of these 0.75a servers we love so much, so we'd still be at 22 outlets either way.
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  19. #59
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    Quote Originally Posted by nenolod View Post
    NEC and IEEE standards define going over the rated continuous load (even for a non-continuous load period) may cause undefined behaviour in the power equipment. This may translate to deterioration of the circuit breakers, making them more likely to trip in the future.
    While I won't dispute that, I will add that
    Underwriters Laboratories (UL) tests all thermal circuit breaker designs to ensure they can handle 100% of the stated load on the breaker in open air at 40C ambient temperature.

    A 20 amp breaker is actually capable of handling 20 amps. Just not constantly (as NEC rules restrict this further).
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  20. #60
    Quote Originally Posted by ramnet View Post
    So conforming to the requirements set out in the National Electric code is not responsible?

    You should have quit while you were ahead.



    No, I simply prefer not to have to be concerned about power overages, or a datacenter claiming I used more power than I did.

    I prefer flat rate network ports for the same reason. I like predictability.

    You should not assume just because I have a certain opinion, that automatically means I must be abusing resources, or assuming I must somehow be violating a datacenter's AUP/TOS.



    Yes, this is true. Likewise, however, nobody should assume all datacenters define rules regarding this that go beyond what the law says.



    Same thing. You should not assume that all datacenters set usage rules that go beyond what the law requires.

    This is a portion from an Equinix contract I have been privy to:



    The colo contracts Level 3 use have similar language as well. 80 percent, or maximum usage power, is not mentioned, anywhere. They refer to the NEC rules, and do not expand beyond that.
    After being able to actually look at the NEC rules, it appears to be a little more strict than you're making them out to be.

    The circuit size needs to be

    1) Equal to 1.25 times the size of the continuous load

    PLUS

    2) Equal to 1 times the size of the non-continuous load

    If you never drop below 14a of usage, then your continuous load is at least 14a. 1.25 times 14a is 17.5a. If you have a 20a circuit, then your "non continuous load" is allowed to be an extra 2.5a from this point. Meaning your bursts should be no more than 16.5a according to NEC under this scenario. Not even close to "tripping the breaker" as you seem to think you should be able to do.

    I do appreciate everyone in this thread pointing me in the right direction such as to the relevant electrical codes, definition of continuous / non continuous etc. I also appreciate the datacenter getting back to me with a proper usage graph so I can square it against those NEC rules.

    However, ramnet, even though it sounds like you're "on my side", really it just feels like trolling. I should know, I'm good at it. "oh wait, we can really get this thread going if I claim it's every customer's right to trip their breakers, bet that's good for a half a dozen replies".

    Mods can please close this thread, I've gotten the information I needed, thanks.
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  21. #61
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    Quote Originally Posted by funkywizard View Post
    After being able to actually look at the NEC rules, it appears to be a little more strict than you're making them out to be.

    The circuit size needs to be

    1) Equal to 1.25 times the size of the continuous load

    PLUS

    2) Equal to 1 times the size of the non-continuous load

    If you never drop below 14a of usage, then your continuous load is at least 14a. 1.25 times 14a is 17.5a. If you have a 20a circuit, then your "non continuous load" is allowed to be an extra 2.5a from this point. Meaning your bursts should be no more than 16.5a according to NEC under this scenario. Not even close to "tripping the breaker" as you seem to think you should be able to do.
    What your continuous load may be, and what the maximum allowable continuous load is, is different.

    If you have a 20 amp breaker, max continuous load is 16 amps. This is not questioned by anyone (20 / 1.25).

    16 amps continuous load times 1.25 allowed spike load is, guess what? 20 amps - the size of the breaker.

    If your datacenter doesn't define operating requirements in their environment stricter than those defined by laws / the NEC, then you should be able to spike non-continuous load to 20 amps without an issue.
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  22. #62
    Quote Originally Posted by ramnet View Post
    16 amps continuous load times 1.25 allowed spike load is, guess what? 20 amps - the size of the breaker.
    Nope, that's wrong. If you continually use 16a, it means you REQUIRE a 1.25 times sized circuit / breaker in order to be allowed to do that, leaving ZERO room for bursting on a 20a circuit. If, however, you have ZERO continuous load, you may then use the entire circuit size for your non continuous load.

    Multiply continuous load times 1.25
    add non continuous load

    Result of the two added together must be less than rated circuit / breaker size.

    In my case, I'm never using under 14a on that circuit, so there is no room for argument that my continuous load is under 14a. 14 times 1.25 = 17.5. If I'm using a 20a breaker, this means there is 2.5a "left over" for non continuous loads, which would mean, a total of 16.5a.
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  23. #63
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    Quote Originally Posted by funkywizard View Post
    Nope, that's wrong.
    The NEC code does actually provide examples on how to size a circuit. It's fairly easy to follow.

    The 125% sizing of the Over Current Protection Device (or 80% loading) is only applicable when continuous loads are involved. Circuit Breakers and other OCPDs can be sized at 100% of their rating for noncontinuous load applications.

    What this means is, if you mix continuous and non-continuous loads on the same circuit, the circuit must be rated for the larger 125% sizing, and not simply 100% sizing. You do not add these together to come up with 225%

    It is extremely rare for continuous and non-continuous loads to be on the same circuit, which is why this isn't very clear from reading the NEC as it assumes you will use separate circuits for the different loads types. If you mix loads you use the stricter continuous load definition at 125%, otherwise you use the 100% definition. You don't use both definitions at the same time.
    Last edited by ramnet; 05-05-2012 at 04:48 PM.
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  24. #64
    Join Date
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    Quote Originally Posted by funkywizard View Post

    Mods can please close this thread, I've gotten the information I needed, thanks.
    Okie dokie!
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