# Thread: Does anyone exceed 80% on primary/redundant circuits?

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## Does anyone exceed 80% on primary/redundant circuits?

So the rule of thumb is if you have a 30A circuit to not exceed 80% or 24A.

If I buy a 30A circuit, I am assuming it is protected by a circuit breaker that is 30A, correct?

In *theory* we could run 29-30A all day and be fine.

So, I understand, the rule of 80% to be to the effect that "during startup, equipment uses more power ... or during runtime if it heats up, the fans go faster using more power" so it is safe to be at 24A in case you get spikes to 27-29A.

Is my reasoning correct?

Further, if you run primary/redundant circuits, is there really harm in running 13A on one circuit and 14A on the other.. and in the unlikely event of a failure, you are running 27A on one circuit (at 90% instead of 80%).

Any thoughts on this? Getting an extra 3A out of a 208V circuits is like putting in 3-4 more servers. If someone is doing this please let me know. I think it would work fine but i would like to know from the experts that do the colo stuff for a living what problems i could encounter, if any.

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It is POSSIBLE however there is a VERY HIGH potential of the circuit breaking if over 80% is consumed.

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Originally Posted by shunter1
So the rule of thumb is if you have a 30A circuit to not exceed 80% or 24A.

If I buy a 30A circuit, I am assuming it is protected by a circuit breaker that is 30A, correct?

In *theory* we could run 29-30A all day and be fine.

So, I understand, the rule of 80% to be to the effect that "during startup, equipment uses more power ... or during runtime if it heats up, the fans go faster using more power" so it is safe to be at 24A in case you get spikes to 27-29A.

Is my reasoning correct?

Further, if you run primary/redundant circuits, is there really harm in running 13A on one circuit and 14A on the other.. and in the unlikely event of a failure, you are running 27A on one circuit (at 90% instead of 80%).

Any thoughts on this? Getting an extra 3A out of a 208V circuits is like putting in 3-4 more servers. If someone is doing this please let me know. I think it would work fine but i would like to know from the experts that do the colo stuff for a living what problems i could encounter, if any.

The National Electrical Code says you can not exceed 80% of a breakers rating for continuous loads. Computers are continuous load per the NEC. If you exceed 80% you are violating the NEC, local fire code, and likely your contract with your colo company. And on top of that, your circuit is at risk for tripping.

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I've seen people run a 20A circuit at 19A all day, every day, for years and have had no problems.

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Originally Posted by Jacob Wall
I've seen people run a 20A circuit at 19A all day, every day, for years and have had no problems.
Some people get away with it. Others do not. For every 20A circuit I see at 19A, I see another one running at 18A that trips on occasion.

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Originally Posted by ST-3
Some people get away with it. Others do not. For every 20A circuit I see at 19A, I see another one running at 18A that trips on occasion.
Precisely, it's risky and can be an expensive risk to take.

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In a data center it's not going to matter whether you can or not.

You won't be permitted to run steady state at more than 80 percent of the ciruit rating. If you don't reduce the load when asked, they'll just shut you down until you do.

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Originally Posted by shunter1
So, I understand, the rule of 80% to be to the effect that "during startup, equipment uses more power ... or during runtime if it heats up, the fans go faster using more power" so it is safe to be at 24A in case you get spikes to 27-29A.

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Coresite slaps you with a \$500/fee if you go over 80% on your circuits.. each.. time... its called their "branch circuit monitoring"

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Anyway, it's still illegal...

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Originally Posted by shunter1
So the rule of thumb is if you have a 30A circuit to not exceed 80% or 24A.
No, you can't go by rules of thumb for data centres; you have to ask the electrical engineer what the limit is for your circuits.

Personally, I would expect a 30-amp supply to have a 30-amp breaker, and be capable of delivering up to 30 amps continuously. The US wiring regulations allow this, provided the underfloor cables are adequate. But what is adequate depends on the temperature rating of the underfloor cables, and how many cables there are per cableway. That's why you have to ask the electrical engineer what the limit is.

However, if the power strip is shared with other customers, the data centre management will likely impose an 80% limit for management purposes - to reduce the risk of one customer overloading the strip and taking out other customer's equipment (imagine a rack running fine, till some customer upgrdes all their servers with extra memory, disks, CPUs ...). And even if you are the only person using the cabinet, they may still impose a limit to keep the air conditioning within its capacity.

By the way, electrical engineers do not necessarily know how server power supplies work; you may have to explain that for a dual-powered server, the load is normally split equally between the two power supplies. If one can no longer deliver power, its load will instantly be taken by the other; the overall load will likely *fall* slightly, because the server's power supply will be operating at a higher load hence higher efficiency.
Last edited by tim2718281; 10-18-2009 at 09:57 PM.

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The problem is that you are assuming that the electrical equipment is in-spec all the time.

The 80% is partially to avoid cases where the sum of all the components in the power feed trip. That is, you might buy two different circuit breakers - the manufacturing tolerance is so wide that one might go at 90% continuous load all the time, the other might trip at 81% continuous load.

Actually they are both OK from the standpoint of the NEC.

Remember too that there is some resistance at each point and the purpose is to avoid there being too much warmth which could cause a fire.

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Originally Posted by MrZillNet
Remember too that there is some resistance at each point and the purpose is to avoid there being too much warmth which could cause a fire.
Wiring regulations or standards will specify a maximum permitted voltage drop, thus limiting the heat generated.

Typically the max voltage permitted drop will be 2%; so, in the case of 120V 30 amps, the load is 4 ohms, and a 2% voltage drop means the cabling resistance is 0.08 ohms, and so the power dissipation within the cable will be 72 watts, distributed alonmg the length of the cable ... maybe 2 watts per foot.

But put a number of cables together in a cable tray and you can see how the heat can build up; which is why the wiring regulations stipulate increasing thickness of cables (hence less voltage drop and resistance) where there are higher numbers of cables in a tray.

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Originally Posted by tim2718281
I would expect a 30-amp supply to have a 30-amp breaker, and be capable of delivering up to 30 amps continuously. The US wiring regulations allow this, provided the underfloor cables are adequate.
A helpful overview of when 100%-rated breakers is permissible is

http://static.schneider-electric.us/...0600DB0101.pdf

I have only seen 100%-rated breakers in distribution circuits of 200A or more, usually when the branch circuit is a longer length. At the branch circuit level, it's cheaper to move to a larger amperage, higher voltage, or more phases.

But I have a feeling this discussion isn't about what is permissible per code, but rather why you can't use that extra 25% capacity. Thankfully, the world is moving to usage-based power billing, so hopefully we will lose the unhelpful incentive to pack as much as possible onto a branch circuit.

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In Tier-2 designs, it may be permissible to have primary and redundant circuits loaded to 50%, depending on who you ask (heck, if it's your own datacenter, load them both to 80% if you like, so they pop during your first failover... it's a free country!). But to meet Tier-3 criteria, no power system component may exceed 90%, even when operating in Maintenance Mode (e.g. primary circuit taken off-line for maintenance). So that would limit load to 45% on each of primary and redundant, in Tier-3 datacenters.

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Originally Posted by Spudstr
Coresite slaps you with a \$500/fee if you go over 80% on your circuits.. each.. time... its called their "branch circuit monitoring"
Damn Zak - that's crazy.

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Originally Posted by PhPhear
Damn Zak - that's crazy.
Thats not such a bad policy, if a customer was to have sustained "violations" as opposed to a 10 minute spike into 80%+ or some short period that is caused by increased activity or something of that nature.

Basically it's a good heavy handed method to force compliance. It was always amusing to me to walk around in the facility we colo'd in before we got our own and see what other colocation customers were doing, there was always one in particular that was riding 90% all day long and you would always seem them tripping circuits.

I guess to some people that extra 10% usage really helps their bottom line? (until the outage comes) lol...

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