service and back to back panel

[kwired]

If you understand the point I was attempting to make, you would see it draws the same. It just draws less after the A/C circuit. The feeder starts at the load side of the 200A main breaker in the meter/main panel. It may be busbar at that point, but it is still a feeder.

Listen to what I mean, not what I say

I know the bus in exterior panel is essentially a part of the feeder, I meant the field installed portion of the feeder, the panel bus isn't exactly covered by NEC in as many ways as field installed conductors that connect to it are.

 

[texie]

In the case described by the OP this feeder does NOT require a feeder ampacity of more than the service conductors per 310.15(B)(7)(3). Period, plain and simple. It is recognized that if the feeder does not carry the entire load then you lose the diversity factor and could overload the conductors. For example, if you were to change the design of the OPs service and have the service split up with 2, 100 amp disconnects with each supplying a separate feeder/panel board in the dwelling, you would be required to use the standard ampacity table for these feeders because you have lost the overall diversity factor. The service conductors up to the disconnects would still qualify to use 310.15(B)(7). The point to 310.15(B)(7)(3) is to recognize that if the feeder has a least the same ampacity as the service conductors, the fact that you add load at the origination point of the feeder may change the diversity on that feeder but it still could never see any more load than the service conductors do.
Here is another scenario. Assume the same situation as the OP but he has decided to add a 100 amp breaker in the meter/main to supply an additional panel in the house. This feeder would need to use the standard ampacity tables as it does not carry the entire load and does not have ampacity as large as the service conductors. The other 200 amp feeder could still use 310.15(B)(7) because, while it does not carry the entire load, it is at least as large as the service conductors and will never see more current than the service conductors.
Over the years the intent of these rules haven't changed much, just the location and wording. The big changes in the 2014 NEC for this were intended to help clarify things and also to recognize the fact that the old table did not take into account that ampacity adjustments may need to be made due to conditions of use.
If you look at the 50 or so year history of this allowance for reduced sizing for service conductors residential applications, it is based on the known fact there is a lot of load diversity in these services and history has shown that there has not been an issue with over heated conductors.

 

[winnie]

But we are not feeding an individual dwelling anymore, one circuit got left off at the supply end of the feeder.
Quite frankly I doubt it would hurt to run same size feeder as the service conductor, but that is not what is written IMO.

I believe that _is_ what is written.

The first part of the paragraph says that in order to be allowed to have reduced conductor sizing, the conductors must either be service conductors or feeder conductors that supply the entire load of the dwelling.

So if a feeder only supply part of the load of the dwelling, then you don't get to directly apply the reduced conductor sizing.

But then there the sentence about 'not be required to have an allowable ampacity rating greater than their service-entrance conductors'. I read that to mean that once you have permission to use a given reduced conductor size for a given breaker rating, you do not have to increase conductor ampacity for feeders of that same breaker rating, even for feeders that do not carry the full load of the dwelling. For the specific case of feeders with the same breaker rating as the main, you get to indirectly apply the reduced conductor sizing.

So, if you have a 400A service (using reduced conductor sizing) and then have a 200A feeder for part of the load, you would need a full size 200A feeder.

But if you have a 200A service (using reduced conductor sizing) and then have a 200A feeder for part of the load, you get to continue using conductors of an ampacity equal to the service conductors.

Just to be clear am agreeing with texie's interpretation.

-Jon

 

[GoldDigger]

I believe that _is_ what is written.

The first part of the paragraph says that in order to be allowed to have reduced conductor sizing, the conductors must either be service conductors or feeder conductors that supply the entire load of the dwelling.

So if a feeder only supply part of the load of the dwelling, then you don't get to directly apply the reduced conductor sizing.

But then there the sentence about 'not be required to have an allowable ampacity rating greater than their service-entrance conductors'. I read that to mean that once you have permission to use a given reduced conductor size for a given breaker rating, you do not have to increase conductor ampacity for feeders of that same breaker rating, even for feeders that do not carry the full load of the dwelling. For the specific case of feeders with the same breaker rating as the main, you get to indirectly apply the reduced conductor sizing.

So, if you have a 400A service (using reduced conductor sizing) and then have a 200A feeder for part of the load, you would need a full size 200A feeder.

But if you have a 200A service (using reduced conductor sizing) and then have a 200A feeder for part of the load, you get to continue using conductors of an ampacity equal to the service conductors.

Just to be clear am agreeing with texie's interpretation.

-Jon

But you could just as well argue that the reduced size service conductors DO have the full rated ampacity, they are just allowed to claim it with smaller wire. In that case the feeder could be the same ampacity as the service but need larger wires to get there.