DeRating main panel breaker...Load calculations required?

[RAM1of4]

Scenario (generalized): A new Solar PV system to be installed. Main panel has 200A bus and breaker. PV backfed breaker to be 70A for load-side connection to bus. Design shows main breaker to be derated to 150A in order to stay below 120% of bus rating...(150+70=220A<240A).

Me: I feel the contractor needs to perform load calculations to verify the remaining loads will be properly served with the derated main.

Electrician: Providing calcs is excessive and unnecessary. The system will simply be safer with reduced main breaker capacity. Worst-case scenario is that the customer may experience nuisance tripping.


I think the reasoning is absurd and absolutely clear that a little professional responsibility should be afforded to the customer. Regardless, I prefer to have black and white references to back up my reasoning as opposed to pushing my weight around just because I can.

QUESTION: Does the NEC require that load calculations be performed to ensure proper sizing of the main breaker for purposes of derating?

Thank you,
RAM

 

[Dennis Alwon]

IMO the inspector has every right to require a load calculation. That being said if this is a residence I rarely need to do a calculation because I can pretty much tell if the service will be large enough by knowing what the loads are. For instance most 200 amp services around here will rarely pull over a 100 amps and probably never reach 150 amps.

I also have never been asked to do a calc on a dwelling. Commercial units definitely

 

[jaggedben]

RAM, I agree with you. One doesn't want to go to all the trouble and expense of installing a solar system only to find that a downsized breaker trips all the time. What do you tell your customer then? That they need a service upgrade?

Also agree with Dennis. With residential you're taking less of a chance if you don't do calcs. With commercial you'd better do it.

Browsed the NEC a little, didn't find anything that clearly requires it. I see references to conductor ampacity as calculated in Art 220, but not overcurrent protection.

 

[Smart $]

...

Browsed the NEC a little, didn't find anything that clearly requires it. I see references to conductor ampacity as calculated in Art 220, but not overcurrent protection.

Perhaps this...

230.79 Rating of Service Disconnecting Means. The service
disconnecting means shall have a rating not less than
the calculated load to be carried, determined in accordance
with Part III, IV, or V of Article 220, as applicable.

Service equipment overcurrent protection is not required to be any size other than not more than the ampacity of the service conductors. However, when that OCP is afforded by a circuit breaker, that breaker also serves as the service disconnecting means.

 

[Zee]

175 Amp main brkr....

 

[gar]

121215-0821 EST

How can the installation of a PV system into a main panel cause more current to flow thru the main breaker (disconnect) than the main breaker rating, if the maximum current of the PV system is less than the main breaker rating, and there are no other sources of energy feeding the main panel.

In this first paragraph I am not describing whether the bus could be overloaded.

Bus overload is a different issue. But not likely to occur.

.

 

[don_resqcapt19]

... Bus overload is a different issue. But not likely to occur.

Preventing bus overload is the purpose of the 120% rule.

 

[jaggedben]

Perhaps this...

230.79 Rating of Service Disconnecting Means. The service
disconnecting means shall have a rating not less than
the calculated load to be carried, determined in accordance
with Part III, IV, or V of Article 220, as applicable.




Service equipment overcurrent protection is not required to be any size other than not more than the ampacity of the service conductors. However, when that OCP is afforded by a circuit breaker, that breaker also serves as the service disconnecting means.

Okay, there we go. Although it does seem to lead to an odd situation where load calculations would be required for circuit breakers but not for fuses? And for downsizing service breakers but not those for feeders?

 

[jaggedben]

121215-0821 EST

How can the installation of a PV system into a main panel cause more current to flow thru the main breaker (disconnect) than the main breaker rating, if the maximum current of the PV system is less than the main breaker rating, and there are no other sources of energy feeding the main panel.

.

You've missed the point. The example has an existing 200A main which is proposed to be downsized to 150A. If the building has routinely been drawing, say, 165 amps at certain times, that will not trip the existing 200A. But it will trip the 150A after the solar is installed, if it happens at night or when there are clouds.

 

[Smart $]

Okay, there we go. Although it does seem to lead to an odd situation where load calculations would be required for circuit breakers but not for fuses? And for downsizing service breakers but not those for feeders?

I agree, an odd situation regarding service fuses... but fuses are more easily replaced than breakers. The downside is upsizing of fuses may be after inspection to compliance.

As for feeders...

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of
Part I of Article 240. Where a feeder supplies continuous
loads or any combination of continuous and noncontinuous
loads, the rating of the overcurrent device shall not be less
than the noncontinuous load plus 125 percent of the continuous
load.

Exception No. 1: Where the assembly, including the overcurrent
devices protecting the feeder(s), is listed for operation
at 100 percent of its rating, the ampere rating of the
overcurrent device shall be permitted to be not less than the
sum of the continuous load plus the noncontinuous load.

Exception No. 2: Overcurrent protection for feeders over
600 volts, nominal, shall comply with Part IX of Article 240.

Doesn't call for a load calculation, but it does call for the need to list, factor, and total... and if one goes to that degree, may as well do a load calculation for the feeder. Not doing a comprehensive load calculation or determination may indicate an undersized feeder and its OCP.

 

[RAM1of4]

Thanks

Thanks

Thank you all for your input and help...RAM

 

[tallgirl]

If you'd like to do a load calculation cheap and dirty, see if the customer has a smart meter. Then ask for the 15 minute interval data from the PoCo for the highest consumption month. Find the peak, divide by 240, that's the maximum load, more or less.

If they have a really smart meter, it may display the peak register during the display cycle -- my peak, through a 125A service, is around 5.4kW. Peak demand time for me is August or September, late enough in the day that I'm not making enough power for that to be anything other than what I drew from the PoCo.