# Thread: 80% rule in datacenters?

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Originally Posted by funkywizard
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.

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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Originally Posted by ramnet
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.

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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Originally Posted by funkywizard
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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Originally Posted by funkywizard

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