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I recently got a service upgrade to 200A.

My old sub panel (from which all branch circuits connect) was 100A and recently got upgraded as well but the feeders are unchanged. To support this, the main breaker from the sub panel (200A which came with the panel) was removed. The feeder is currently protected by 100A in the Main Service Panel (MSP).

If I wanted to put the 200A breaker back in, which wire would I need? Do I really need AWG 0/3?

Is it correct that I would need at least a 1-1/2 EMT conduit?

Of course, after replacing the feeders I would like to replace the breaker in the MSP from 100A to 200A as well.

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  • Is this conduit being run inside a wall, on the surface of a wall, or...? Nov 18, 2023 at 13:18
  • Is there a breaker in the main panel for the sub panel? If so what size is it? Sub panel main breakers become mainly local disconnect in use, it does not protect the feeder wires. That is done by the breaker in the main panel.
    – crip659
    Nov 18, 2023 at 13:20
  • You need to know the size of the breaker for the sub panel, and the size/gauge of the feeder wires. Then check on a feed wire size chart, which will be different than the regular gauge sizes for normal circuits.
    – crip659
    Nov 18, 2023 at 19:41
  • @ThreePhaseEel first ~3ft exterior wall, a few inches through exterior wall and remaining part (15ft?) through crawl space.
    – divB
    Nov 18, 2023 at 19:49
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    @crip659 The NEC 310.12 table in the feeder size chart is for "conductors supplying the entire load associated with a one-family dwelling." That means the service from the utility to the main disconnect, or the feeder from the main disconnect to the main breaker panel. A feeder going to a subpanel, though, doesn't get the 83% discount -- a 200 A feeder to a subpanel, ie a panel not serving the entire load associated with a dwelling, would require 250 kcmil aluminum rather than 4/0.
    – Greg Hill
    Nov 18, 2023 at 20:48

1 Answer 1

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When sizing a subpanel feeder for a one- or two-family dwelling under NEC rules, after all the load calculations are done, there's another question that must be answered. It is this: does that subpanel serve the entire load associated with the dwelling? In other words, are there (or could there be) circuits added in the main panel so that some of the load is served at the main panel and other of the load is served at this subpanel?

When the entire load associated with the dwelling is carried through a given feeder (or a service), NEC allows a little break in the wire sizing. The wire may be sized for 83% of the overcurrent protection limit. For example, if the overcurrent protection is at 200 A the wire may be sized for 166 A.

Feeders and services often use THWN or XHHW insulated wire and are sized for 75° C. When using aluminum conductors the 4/0 gauge with 180 A ampacity fits the bill for 166 A sizing. When the 83% de-rate does not apply, though, aluminum 250 kcmil would have to be used to meet the full 200 A ampacity.

A short feeder, or one where conduit size is expensive, might opt for copper conductors instead of aluminum. The same de-rate rules apply.

This does seem counter-intuitive. I don't know but I could speculate that the NEC load calculation procedure causes a whole-home load to be a little over-stated, so they give a little back in the service/feeder sizing. Such excess may not be incorporated in the general case, though, so no de-rate is offered generally.

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