Standard or Optional calculation

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blueheels2

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Raleigh, NC
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Electrical contractor
I have a 400 amp service fed 120/240 from the poco. This is a 400 amp meter main. There are 2 200 amp breakers fed off the bus of the meter, 1 feeding the 200 amp panel in the main and then the other 200 amp breaker feeds a 200 amp main breaker panel in the garage. Inspector is requesting a calculation on the exterior panel because it has a lot in it. In this scenario can I use the optional calculation. The panel is fed by a single set of wire coming into the meter main.

But I bet you will tell me no.

Thanks
 
You need to do three calculations, one for each of the 200's and one for the 400A service. The 200A calcs can't use the optional method because you're not doing the whole house, but the 400A can be done with the optional method. Per 220.82(A)

(A) Feeder and Service Load. This section applies to a
dwelling unit having the total connected load served by a single
120/240-volt or 208Y/120-volt set of 3-wire service or feeder
conductors with an ampacity of 100 or greater. It shall be permissible
to calculate the feeder and service loads in accordance
with this section instead of the method specified in Part III of this
article.
Click to expand...

If you were only to add the results of the two 200A calcs to get the 400A results, you'll be more conservative than with the optional method.
 
JoeStillman said:
You need to do three calculations, one for each of the 200's and one for the 400A service. The 200A calcs can't use the optional method because you're not doing the whole house, but the 400A can be done with the optional method. Per 220.82(A)



If you were only to add the results of the two 200A calcs to get the 400A results, you'll be more conservative than with the optional method.
Click to expand...
This part I don’t understand. The optional or existing calculation on the individual panel is significantly less than the standard calculation. The general lighting load is reduced but the demand factors for equipment only allows a 25% deduction for fixed equipment right? Where in the optional and existing everything but Havc loads is allowed to be at 40%. Seems like it would be possible to calculate a 400 amp service with the optional calculation that will be exceeded by the standard calculation in the panels.

What I’m saying is I could run an optional calculation on a service and it will say 400 amps is sufficient. But when you perform the standard calculation it’s 450 amps. Break that into 2 separate panels and you are over both with the standard calculation. But I haven’t really run the numbers on that, but I’m my head it seems possible.
 

I believe this article will show what I am talking about. They run the optional and the standard calculation on the same dwelling and the standard calc is like 143 amps and the optional is 108. Seems like it’s pretty easy to calculate a 400 amp service with the optional method but the individual panels will be overloaded using the standard calculation. That seems like a code flaw to me.
 
Here in PA, it's a rare house indeed that needs more than 200A. An all-electric house with one kitchen and heat pump/air conditioning can be 4,500 ft² on a 200A service.

I checked with NFPA once about whether the loads in the Art 220 calcs are continuous or non-continuous. They told me non-continuous. So there's no 125% factor on either of those calcs.
 
Yeah house I’m working on was started by another electrician. He upsized the service to 400 amps.

There are 6 HVAC units, 2 dryers. Cooktop, oven, speed oven/micro, 2 disposals, dishwasher, steam shower, 85 amps of infrared heat on the porch, and a pool with about 40 amps of load. It definitely needs 400 amps. Might need 600 but if so that’s not my problem. I’ll build that but I’m not eating it!
 
JoeStillman said:
Come build me a house like that!
Click to expand...
1 million dollar addition and pool. The addition was 1500 square feet and and additional 500 square foot gut and rewire (living room, kitchen, Mudroom, and foyer. Electrical, my part only was 71k. Addition was 850k pool was 150 k.
 
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