Main panel ampacity for subpanels


Old 08-20-07, 08:04 PM
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Join Date: Aug 2007
Posts: 7
Main panel ampacity for subpanels

We have a rather complicated new construction project going on and I have a simple question that may end up with a long answer. I'm going to describe as many details as possible, although I'm not sure whats relevant. For the record, I did go through the NEC handbook to find the answer to this question with no avail.

We have a 400A main service which feeds from the underground pedestal to a Trans-S CT cabinet via 3 500kcm cables in a 4" conduit. The feed run is about 20ft long from the transformer to the cabinet. From there, the feed goes into a 400A main disconnect with is also an ATS for a 62KW generator. The main supply comes in one side, the generator comes in the other and the ATS does the magic in between to provide a single switched output. The main disconnect and ATS are one unit and are approved for service entrance usage. Somewhere in all of that is 7.6KW of solar power that gets fed into the meter in a grid connected design, most likely off of a 100a breaker in the main panel.

From there, we are planning on feeding a 400a panel. This main panel will feed 3 sub panels, one 100a and two 200a, basically one for each floor. It will also feed some of the basement level mechanicals, perhaps another 100a total. Each of the sub panels will have a breaker of the appropriate size (100a or 200a).

Each of the subpanels on each floor will then feed the lights, outlets, small appliance runs, etc.

My question is, where, if anywhere, does the code specify the amount of downstream ampacity that can exist from a main panel to a sub panel? It doesnt seem to be addressed anywhere. The only thing I can find is that the sub panel must be protected by a breaker of suffcient ampacity for the box that it is connecting to. So the 400a main panel must be connected to a 400a main disconnect (it is) and the 200a sub panels must be connected to 200a breakers in the main panel (they will be). But nothing stops me from connecting ten 200a subpanels to one 400a main panel, provided that they are all connected to 200a breakers. Is that correct?
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Old 08-20-07, 08:51 PM
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Join Date: Sep 2005
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There's no code that specifies how many breakers, subpanels or circuits that can be put on a service. In theory, you could have 100 breakers (across multiple subpanels) on a 100A service. The trick is the demand load calculation. It helps determine how much load can be safely placed on a subpanel and main panel. You don't want to run into the problem of tripping main breakers or anything.

Google Demand Load Calc for more details.
Old 08-20-07, 10:35 PM
Join Date: Aug 2004
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Posts: 1,065
"But nothing stops me from connecting ten 200a subpanels to one 400a main panel, provided that they are all connected to 200a breakers. Is that correct?"

No, you must have a calculated load for the service based on the loads for the dwelling. Article 220 prevails. So the addition of the sub-panel must not cause that calculated load to increase over the 320 amps continuous for a 400 amp service class 320 meter socket. >> .80 x 400 = 320 amps.

62KW generator .......Wow!! that's a big one

Old 08-21-07, 07:37 PM
Thread Starter
Join Date: Aug 2007
Posts: 7
We had to do a demand calc for the utility so that they could size the CTs. This was done using the worksheet below. If every single thing in the house was turned on at the same time it would be 108000w, which is 454amps, which is in excess of our main panel capacity. However, from 220.82 I saw that I can "derate" various items, like the first 10kw is at 100%, then after that its 40% for the living area/small appliance circuits, etc., plus the "largest" of the HVAC stuff, and so on. So think we are ok from that standpoint. My main concern was that the main panel can handle the downstream panels, which from further reading, I see that panels are sized for the number of conductors, not the total ampacity, so a 200a panel has so many cubic inches for wires and a 400a panel has so many more.

Thanks for the quick and thoughtful responses!


Square footage living area @ 3w/sqft 9600 28,800

20A Small appliance dedicated circuits @ 1500w each 9 13,500

Washing machine @ 1500w 1 1,500
Dryer @ 1500w 1 1,500

EB-388 Convection oven @ 7100w each 2 14,200
Warming drawer @ 1800w 1 1,800
Viking VGIC365-6B @ 1000w 1 1,000
Viking VGGT240 @ 1000w 1 1,000
Exhaust hood @ 1000w 1 1,000
Jade RJRS 4880 refridgerator @ 1500w 1 1,500

Pressure booster @ 1000w 2 2,000

Electric baseboard heater @ 1000w each 11 11,000
Trane XV90 / TUY Furnace @ 1300w 3 3,900
Trane XL16i 3 Ton Heat pump @ 5500w 1 5,500
Trane XL16i 4 Ton Heat pump @ 6700w 1 6,700
Trane XL16i 5 Ton Heat pump @ 8000w 1 8,000

Master bath hot tub @ 6000w 1 6,000

Total wattage 108,900

Calculated amps (watts/240) 454
Old 08-22-07, 02:18 PM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
70% Feeder Factor

I advise that you calculate the number of Feeders based on a maximum Feeder current that is 70 % of the ampacity of the Feeder conductors.

If the ampacity of the Feeder Conductors = 200 , then connect loads with a total current of 140 amps to each sub-panel. 3 X 140 = 420 amps = Three 200-amp Feeders.

If you use 100 amp Feeders, then 70 X 6 = 420 = 6 100-amp Feeders.

Of course, you could design with both 200 and 100 amp Feeders, but the amp-ratings of the Service Conductors and the Main- Breaker do not limit the number of Feeders connected.

Good Luck, & Learn & Enjoy from the Experience!!!!!!

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