Another unique electrical issue I’m trying to work though here.
1970’s house. The distribution panel for the majority of the home is located in a bedroom closet. It has no main breaker. The main service disconnect (150 amps) for the panel is located near the meter (no label of any kind of course) this main disconnect also supplies a small separate sub panel with one circuit (not labeled) located directly near it (as well as the distribution panel inside the home).
As I understand it, this means that the panel inside the home must now be considered a “remote distribution panel” (aka sub panel) that does not serve the entire dwelling unit (anymore). Therefore, the feeder to the sub panel from the service disconnect outside must be larger than the service entrance cable going from the meter to the service disconnect outside because of the requirement 310.12 (B) to serve the entire load for the dwelling unit in order to use table 310.12(a).
This is because the cable going from the service disconnect is now a feeder for a sub panel. The service main can’t be considered a “meter main” and has no label stating “Meter Disconnect not service equipment” or any other label, and supplies a separate 60 amp circuit that is not associated with the other circuits inside the house.
The service main as stated is 150 amps which can be size 1 AWG (copper) for service entrance to the service main, but should be 1/0 copper to the remote distribution panel (sub panel) inside the home.
I have included pictures, I know the other issues you’re going to see but don’t have any questions about them.
My main question is do you agree that the main service disconnect is oversized to supply 150 amps to the sub panel with only 1awg size cable (as labeled on the cable although it actually looks larger). Said another way, the feeder to the subpanel is undersized.
I’m not sure how this happened except that at some point the main service disconnect was tapped for a separate circuit
Thank you. Keep in mind its a question for me because the service main breaker also serves another panel with a single circuit other than the one in the bedroom closet, so the sub panel doesn’t actually serve the entire dwelling unit.
I would think that the conductors feeding both subs would be considered “feeders.” So the SEC, and both sets of feeders fall under table 310.12 that I posted above. But yeah, it can get complicated and there may be a nuance here that one of the resident electricians might comment on. For me, I have been using the table for all SEC’s and feeders, so I would like to confirm that is correct also. I think a while back there was a NACHI graphic or other well-circulated graphic that had this info wrong. It had the feeders sized like branch circuit conductors instead of SEC’s.
My understanding is 310.12 (B) requires the sub panel to serve the entire dwelling in order to qualify to use table 310.12(A) .
Otherwise because the sub panel doesn’t serve the entire dwelling unit, it is more likely that it will use all available current to serve an outbuilding, etc and needed to be sized larger.
In this case it is a catch 22 because originally when installed it was designed to serve the entire dwelling unit. some time later they tapped into the main service disconnect outside, which means it no longer serves the entire dwelling unit
That is a question I surely don’t know the answer to. 310.12(B) is posted below, but I don’t fully understand how it differentiates between “entire load” and “partial load” I guess.
I’m just going to refer it to an electrician, stating exactly what I said that basically it doesn’t serve the entire load although it likely did when the home was built, and is a unique circumstance since they tapped off the main service disconnect that should have further evaluation. There are other more important issues so the electrician should be out there anyway
In this case the feeder is not required to be larger than the service conductors. The service conductors end at the 150 amp circuit breaker. The feeder conductors to the panel are not required to be larger than the service conductors even if they do not carry the full load of the service. The set of smaller conductors in the photo are improperly connected to the load side of the CB and would constitute tap conductors and would be required to be installed with one of the tap rules in 240.21.
310.12(A) Services.
For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.
310.12(B) Feeders.
For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.
310.12(C) Feeder Ampacities. In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).
In the case of what the OP is saying, the service is 150A. Assuming the service conductors are at least 1 AWG copper or 2/0 aluminum (or larger) the feeders to the distribution panel inside the home would need to have an ampacity rating of at least 125A and could potentially be 2 AWG even though 1 AWG would be more suitable?
1 AWG copper is rated for 150A (depending on the temperature). 83% of 150A is 124.5A. 2 AWG copper is rated at 125A
The question is since the feeder to the panel is protected at 150 amps can those feeder conductors be sized at less than 150 amps even though they do not carry the entire load of the service? If the conductors to the 150 amp breaker are sized at 83% as permitted by Table 310.12 (#2/0 Al) then the feeder conductors are not required to be larger than the service conductors. {310.12(C)}
So for example say you have #2/0 Al (135 amps) service conductors to the breaker which are sized at a minimum 83% of 150 amps. (#1/0 is 120 amps so that it too small to meet 83% 150*83%=124.5 amps) On the load side of that breaker the feeder to the panel does not carry the entire load of the dwelling so those feeder conductors cannot use the 83% rule but those conductors are also not required to be larger than the service conductors so #2/0 would still be code compliant.
In another scenario say the service conductors to the 150 amp breaker are #3/0 Aluminum (155 amps) then the feeder conductors to the panel could not be #2/0 because the 83% rule is not applicable to a feeder that does not carry the entire dwelling load. That feeder would now require a minimum conductor size of 150 amps or #3/0 Al.
So to summarize the feeder which is not carrying the entire load of the dwelling must be sized at a minimum of 150 amps but by the code exception it is not required to be sized larger than the service conductors feeding the 150 breaker.
