Subpanel Load Calculation Frustration


Old 08-22-06, 06:00 AM
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Join Date: Apr 2006
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Unhappy Subpanel Load Calculation Frustration

I an an engineer, but not a trade electrician, that has been doing my own electrical work for many years, passing many permit inspections. But recently I was failed on an inspection by the AHJ on what should have been a straight-forward project and need some advice.

I had installed an automatic transfer switch (ATS) with integrated subpanel to feed selected circuits from a generator. The ATS and genset came from the same manufacturer as a kit with DIY instructions. The total wattage of the selected circuits was calculated using a worksheet provided by the genset manufacturer which used estimates for appliances and basic values outlets in dwelling areas.

Following the manufacturers directions I simply connected the selected circuits breakers to backup face value for face value (i.e., connect a 15A breaker included by manufacturer in the ATS subpanel to an existing 15A circuit in main panel).

The reason given by the AHJ for not passing this work was that since this was an automatic rather than manual transfer switch then the total of breakers in the ATS subpanel cannot exceed the max amperage output of the generator at continuous load. I was advised that I need to do "NEC" load calculations at worst case load conditions (i.e., all circuits in demand simultaneously at time of power transfer). For me I thought a UL approved device in kit form would already comply with NEC code since the manufacter had preloaded this ATS sub with the particular breakers provided. I realize the AHJ can override a manufacturers instructions but I would like to know where in the NEC I can look to remedy this and pass this on round 2.

Load calcs are a weak spot for me. And obviously the wattage calculation, which is typically used to size a genset, is not acceptable to the AHJ. But knowing my main panel is 200A service yet there's 490A of breakers at face value in there I know this cannot be because of the face value summation in the ATS subpanel.

So I had planned to follow article 220.14(A) & (C) of the 2005 NEC taking actual manufacturer face plate data on appliances and motors amperage and calc the VAs and then use table 220.12 for dwelling space sq ft to calc the bedroom circuits potential VAs (e.g., 10x12 @ 3VAs sf = 360VA), etc for each circuit connected to the ATS subpanel. Add it all up, divide by the voltage and show that the amps for the subpanel meet the required demand. I had planned to show two tables, one at normal load and another at surge load (compressor and pump motor starting) to show that both met manufacturers specs at worst case conditions of simultaneous demand.

I have two questions. One, is my approach for addressing the AHJ concern sound or is there a different article 220 load calculation approach needed on genset subpanels?

And two, I am unsure how to explain that the surge load exceeds the subpanel breaker face value. Since the manufactrer rates the genset for a surge amperage that exceeds the safety breaker on the genset device itself by 30A (e.g., rated 66A cont load and 92A surge load @ 240VAC but only 65A 240V breaker at the alternator output) there must be some allowance that I am not familiar with in the NEC for this?

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Old 08-22-06, 09:07 AM
Join Date: Feb 2002
Location: port chester n y
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It is incumbent on the AHJ to cite the specific Code Article that is the basis for his issue of a violation. The relevant Art is 702, Optional Standby Systems.

Art. 702.3, Application of other Articles states---

" Except as modified by this Art., all applicable articles of this Code shall apply" This means that the total CONNECTED load, as distinguished from the MAXIMUM load, is subject to the application of Demand Factors, which is the "standard" calculation for Branch-Circuits, Feeders, and Services, Art 220.

The conductors that supply the "Emergency" panel are "Feeder" Conductors, and Art 220.82 , Dwelling Units, permits an "optional" calculation for Feeder loads. The load on a Feeder is NOT the sum of the ratings of the individual Branch-Circuit breakers supplied by the Feeder.

Ask the AHJ which specific Article prohibits the application of Art. 702.3 , and which specfic article requires that the load be calculated on the sum of the ratings of the Brach-Circuit Breakers.He is required to provideyou with specfic, exact information.

Good Luck, & Learn & Enjoy from the Experience!!!!
Old 08-22-06, 09:07 AM
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Does the generator feed have a "main" breaker which is sized to the generator's capability? Usually 20A or 30A.
Old 08-23-06, 06:31 PM
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Is this a medical facility with life support equipment? a surgery room? elevators? emergency lighting?

If so, the inspector is correct: these loads must be 100% supplied at worst-case demand (still should be less than the sum of the breakers).
If not, he is confused and your set up is correct.
Old 08-23-06, 07:32 PM
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Join Date: Apr 2006
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This is just a residential optional standby system - Art. 702 of the NEC. Not even backing up air conditioning equipment. Just rooms (wall outlets & ceiling lighting), fridge and a well water pump.

Major appliances run on propane already.

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