Calculating Panel Backwards

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bcm

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Hi gang,

I have been tasked with calculating the load on some small 120/240V, 200A panelboards (each one serves equipment at a different cell tower site). A number of the breakers are unlabeled, so I was taking the CB rating x voltage as "worst case".

E.g. 60A/2P breaker is 60Ax240V = 14.4 KVa (7.2 KVa per phase).

My boss (who's a structural) is telling me that the Code says I should be taking 80% of that 14.4 KVa, but I'm not finding that in the Code so far. I have a couple of panels where NOTHING is labeled, so I have no idea of the actual load on each circuit breaker beyond the rating of the breaker.

Can somebody clue me in as to the correct approach and where I can find that in the NEC?

Thanks!
 
Article 220.

Simply stated, calculating load by breaker rating is invalid.

See examples in Annex D.
 
Calculating Panel Backwards

Bosses don't want to hear that, so I'm still stuck trying to cobble up a way to calculate these. :/
 
What I think your boss is trying to say is that breakers can only handle 80% of their rating of continuous load.
 
ASG said:
What I think your boss is trying to say is that breakers can only handle 80% of their rating of continuous load.
Click to expand...
Yes, and up to 100% for non-continuous load.

And on many motor circuits, the load per calculation may be substantially less than the breaker rating.

The thing about going up to 80% with continuous load is, the feeder and service load calculation results in 100% of the rating if loaded to that degree. That is where the discrepancy comes in... seldom are all breakers loaded to the max degree.

It is not unimagineable that a panel could have a breaker ratings sum to 200% of the panel rating. But on the lesser side of that, as an example, I have seen 42 space 200A lighting panels chock full of 20A CBs. The summed ratings would errantly translate to 42 * 20 * 120 / 208 / 1.732 = 280A... and we all know that can't be right :p
 
Sounds like you need to mark the panels before you can proceed. There are ways to find out what breakers control what circuits without shutting off the power if that is the problem. Perhaps a circuit tracer or/and an amprobe would help. Educated guessing is really not an option if you want an accurate load calc. Once you have accurate circuit loads use Art 220.
 
fisherelectric said:
Sounds like you need to mark the panels before you can proceed. There are ways to find out what breakers control what circuits without shutting off the power if that is the problem. Perhaps a circuit tracer or/and an amprobe would help. Educated guessing is really not an option if you want an accurate load calc.
Click to expand...

I thought tracers start at the load and help you find what CB it is circuited to. He needs to find out what load is on each unlabeled circuit breaker. And you can't really use an amp probe unless you know whatever is connecting to the circuit breaker is operating at peak when you are taking the measurement. If it is heating something, it's not winter yet. If it is cooling something, it's not summer anymore. The real answer is that you have no way of knowing the actual load.
 
Why not just do same thing you do when designing a new installation? Get a listing of equipment, find out what the ratings are and do a load calculation based on that information? Apply demand factors where applicable, use the larger of heating or cooling, or any other equipment that will not run simultaneously. It is very likely much less than just adding all the values of installed overcurrent devices. Is not really that hard to do with a small facility.
 
Even though it isn't a valid approach, I wouldn't do the 80% thing. Doing it the right way can illustrate:

A 12A continuous load goes into the load calc as 12*1.25 = 15A.
A 15A non-continuous load goes into the load calc as 15A.

Either of these can be on a 15A circuit. So if you're adding up breakers, you need to use the breaker rating and not 80% of it as a worst case load.
 
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