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#169524 10/07/07 11:14 AM
Joined: Oct 2007
Posts: 2
E
New Member
Hi All,
I have 2 questions and hope someone can clarify them please.

1. In a 3 phase panel rated at 225 amps, does this mean that EACH PHASE can supply 225 amps?

2. A 3 phase breaker rated at 45 amps, does this mean that the breaker can supply 45 amps at each phase or 15 amps at each phase giving it a total of 45 amps?

Thanks in advanced and great site by the way.

Joined: Mar 2005
Posts: 399
A
Member
1. Yes
2. Yes. See 1.


Alan--
If it was easy, anyone could do it.
Joined: Mar 2005
Posts: 1,213
S
Member
If you're asking about overcurrent protection, I also concur with Alan- yes/yes. If you're asking how much load you can put on it- it depends whether you're talking delta or wye. Wye is straightfoward- the load on the phase conductor perfectly matches the current on the phase conductor. But for delta, if you put 100A on AB, 100A on BC, and 100A on CA, each phase conductor will be carrying 173A because a portion of the components are additive. (3-phase always has a reduction by the square root of three, appx 1.73. For wye, the L-N voltage is the L-L voltage reduced by root 3; for delta, the L-L current is reduced by root 3.) Any 3-phase equipment you hook up will have this taken into account- the only time you really need to worry about it is when you're hooking up individual pieces of 1-phase equipment L-L.

Be aware the neutral is also rated at 225A in a 3-phase panel, as 225A is the max current if the phases are imbalanced.

And, to confuse the issue even more, the neutral is rated at 150-200% for applications with a lot of DC power supplies because the neutral current from switch power supplies is not true RMS, but essentially square wave, creating overload heating akin to (IIRC) 318A. In this case, you don't have to worry about the specifics; it's safer just to pull a 150% oversized neutral. That way you don't end up with neutrals glowing cherry red at the transformer (true story!).

Last edited by SteveFehr; 10/07/07 04:57 PM.
Joined: Jul 2004
Posts: 101
J
Member
And just to pile on, unless you have a 100% rated breaker (non-standard), it is only capable of being loaded to 80% of it's rating. So your 45A breaker can really only be loaded to 36A.


JRaef

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