Service Rating and Branch Circuit Rating

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jsinclair

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This is definately a silly question to be asking, but I need some input on this to back my position.

I have a situation where an outside firm is completing some AC power audits on various panels. They have comeback saying that every panel surveyed will need to be upgraded.

The basis for this statement is their method of calculating the loads. The person making the calculations says that NEC requires the circuit to be loaded no more than 80% of its OCP value. (ie - 20A breaker can not exceed a 16A load) This I agree with when sizing OCP devices and wire. However, when it comes to a load study, I am more interested in the actual values rather than what the circuit can handle. Thus, if a circuit is protected by 30A breaker, and the connected load is only 17A. The 17A should be used in the calculation, not 24A.

Any opinion on this?

Conversely, the same problem with the service. I have always understood that the service should not be loaded to more than 80% of its rating. Can anyone point out a NEC reference for this? I am at a loss. The loading is actual load, not calculated load correct?

Thanks,

Me
 
Re: Service Rating and Branch Circuit Rating

There is no requirement to go back after the fact in an existing installation and change your wiring to meet the current conditions. this would be silly and horrendously expensive.

OTOH, if this is a new installation they may well be correct.
 
Re: Service Rating and Branch Circuit Rating

What about 210.20 A exception . If the Breakers were changed to 100 % rated??If The assembly was Rated 100 % >?
 
Re: Service Rating and Branch Circuit Rating

I was under the impression that the 80% rule is not absolute. Doesn't it generally only apply to continuous loads?

It seems to me that unless the loads in question last longer than three hours, OCPDs need not be sized at 80% of circuit ampacity.

Or am I missing something here?
 
Re: Service Rating and Branch Circuit Rating

Originally posted by jsinclair: Thus, if a circuit is protected by 30A breaker, and the connected load is only 17A. The 17A should be used in the calculation, not 24A. Any opinion on this?
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I agree. I also agree with Jeff?s statement that the 80% rule applies to continuous loads.

But I am not certain that I understand the method that the ?outside firm? is using to calculate the loads. You certainly can (i.e., it is permissible, under NEC rules) add up the rated load of everything connected to every breaker. But that is not a requirement. Article 220.35 has a provision for evaluating the existing loads on the existing panels within an existing facility. It?s intent is to allow you to determine whether there is room to add more load. But it applies, even if you are adding ?zero? new load. You can add up a year?s worth of utility bills, or you can take load readings over a 30 day period. Since you are talking about loads on individual panels, you would need a meter at each panel of interest. If the calculated or measured load, plus 25%, does not exceed the rating of any panel, then the existing configuration is acceptable.
 
Re: Service Rating and Branch Circuit Rating

80% applies to continuous loads only. for example, if you have a non-continuous load of receptacles, your maximum load could be up to 2400VA, or 20 amps, on a 20amp circuit breaker.

For the main service, there is no such thing as 80% of the main OCPD. the OCPD of the main service is to protect the service conductors.

First you size the srvice conductors, then protect it with the next size up OCPD.

for example, if your load is 370 amps, then you choose #500MCM conductors(good for 380amps, copper @75 deg), then you use a 400 amp OCPD to protect feeder, assuming that the breaker is 100% fully rated.
 
Re: Service Rating and Branch Circuit Rating

Sounds like the consultant is adding up the branch breaker handles and taking 80% of that and comparing to the rating of the panel.
That would surely lead to a lot of business.

A lot of homeowners try that too. :) My panel would need to be replaced, especially since I have a lot of underloaded receptacle and lighting circuits. :)
 
Re: Service Rating and Branch Circuit Rating

potential load is a world apart from demand load.How does one equate one with another?
 
Re: Service Rating and Branch Circuit Rating

This raises a very interesting question.. Does the code define how "loads" should be looked at? Is actual load a proper way to look at an existing installation or is potential load? Either way can cause problems in the end.

To answer some of the above questions, this is an existing installation. The group doing the "audits" is taking all breakers in the panel at 80% and adding that up to get the "total load" on the panel. As far as the service load of 80%, I gather that is just a general rule of thumb??
 
Re: Service Rating and Branch Circuit Rating

Originally posted by jsinclair:
To answer some of the above questions, this is an existing installation. The group doing the "audits" is taking all breakers in the panel at 80% and adding that up to get the "total load" on the panel.
Click to expand...
This seems to me to be a really silly way of auditing anything. Receptecle loads tend to be underused, while often other loads are segregated off a single breker simply for convenience or to provide a place to shut off power rather than because the load requires the full current rating of the breaker.

Might be you need new auditors. I'd find out just what their qualifications are and why they think this is a valid way of doing things as opposed to calculating or measuring the true loads being used.
 
Re: Service Rating and Branch Circuit Rating

You ned to find an auditor that understands the basics of the NEC. This bunch is so off-base as to be laughable. In order to determine the adequacy of the service they are going to have to do a complete article 220 calcualation. There are no shortcuts or "rule-of-thumbs". The calculation will have to be done, the proper de-ratings made, and the adequacy may be determined. Some of those breakers which they are assuming are loaded to 80% may in fact have zero load. They may find that some don't even have conductors connected to them! The service has to be adequate for the load as determined by article 220 period. It looks to me that this bunch is looking for a big paycheck for doing practically nothing.
 
Re: Service Rating and Branch Circuit Rating

Its sad to say, but I have to agree with you. It is very hard to find people who understand both the electrical theory and the NEC well enough to perform such a simple task. Your assumption is correct that most of the circuits have no load on them and used for such things as emergency light circuits, convience recpts, halon system monitors, and even a dedicated recpt for a portable phone. This puts me between a rock and a hard place. If the AE firm gets their way, it will take me about 6-12 months to get the services upgraded. An upgrade that is not needed at this point in time. Its very difficult to teach the old dogs the new tricks. When I questioned about method and procedure used, I was told, "That is how I have done it for 30 years and that is how I will continue to do it." If that is truly the case, there are a lot of homes and businesses out there with 600 Amp plus services that feed a total connected load of less than 60A. Who would have thunk it?
 
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