NEC 310.12(B) Reduction in Feeder Size

[jar546]

I understand that NEC 310.12(B) allows for the reduction in size of feeder conductors when they supply the entire load of a one-family dwelling, by permitting an ampacity of not less than 83% of the feeder rating. However, I'm struggling to see practical scenarios where this reduction makes sense. For example, if I have a feeder that comes from a combo panel located outside a home, with the combo panel serving as the service disconnect and meter combo, and this feeder comes into the garage to the main panel for the house, which serves 100% of the dwelling unit's load, it is code compliant. But if I install a circuit for an AC condenser unit originating from the combo panel, the feeder size cannot be reduced because it no longer serves 100% of the dwelling. This does not make sense to me.

 

[wwhitney]

But if I install a circuit for an AC condenser unit originating from the combo panel, the feeder size cannot be reduced because it no longer serves 100% of the dwelling. This does not make sense to me.

Typically 310.12(C) means that you could still use the reduction in size.

Cheers, Wayne

 

[ICE]

(A) Services. For a service rated 100 amperes through400 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.
(B) Feeders. For a feeder rated 100 amperes through400 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.

(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).

The scenario described by Jeff has the service conductors reduced to 83% of the service rating. There is a feeder to a panel and a branch circuit or possible feeder to an AC condenser. That does not fit neatly, within (A) or (B).

Having had this discussion previously I anticipate a response to be, "Well then the code writers would not need (C)." To which I say Jeff's example is the problem, not the code.

There is also the premise that a feeder would never be required to exceed the service. That part about the entire load gets in the way.

I should mention that the code I provided is not 2023.

 

[wwhitney]

(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).

This part means that no feeder on such a service needs to have conductors of higher ampacity than the service conductors. So it covers the scenario in the OP.

Cheers, Wayne

 

[ICE]

The feeder in Jeff's example has less calculated load that does the service entrance conductors. Would you then allow the 83% rule to apply to that lessor load? If I understand Jeff's question, that is what he wants to do.

 

[wwhitney]

The feeder in Jeff's example has less calculated load that does the service entrance conductors. Would you then allow the 83% rule to apply to that lessor load? If I understand Jeff's question, that is what he wants to do.

Given that a feeder that doesn't supply the full load of a dwelling unit can't use 220.82 to calculate the load, but has to use the standard method, I'm not convinced that the load for "dwelling unit minus one HVAC unit" will have a lower load than "all of the dwelling unit"

But suppose for the moment that the dwelling unit requires a 175A service and the feeder for the dwelling unit less HVAC requires only a 150A feeder. Then the 175A service would normally require 2/0 Cu, but because of 310.12(A) (the 83% rule) would only require 1/0 Cu. The 150A feeder would normally require 1/0 Cu, and you can't use 310.12(B) to reduce that. But if do you run 1/0 Cu, you get a 175A feeder for free, because of 310.12(C).

Cheers, Wayne

 

[jar546]

This part means that no feeder on such a service needs to have conductors of higher ampacity than the service conductors. So it covers the scenario in the OP.

Cheers, Wayne

This helps to clarify but does not necessarily make sense in some applications. If the POCO provides a typical UG lateral to an exterior combo meter and the bars of the combo meter provide voltage to the service disconnect, at no point are there any customer-supplied service conductors. We can never control what size conductors the POCO provides, so in many cases, the feeder that goes from the bus bar lugs of the load side to the main panel is the closest thing to the actual service conductors. The contractors always take advantage of the 83% rule in SFR, but per the letter of the NEC, if the feeder does not control 100% of the dwelling unit load, they are not supposed to be allowed that reduction.

What I am essentially saying is why have that rule at all if they can reduce the feeder size regardless, even when there technically are no customer-supplied service conductors?

 

[wwhitney]

This helps to clarify but does not necessarily make sense in some applications. If the POCO provides a typical UG lateral to an exterior combo meter and the bars of the combo meter provide voltage to the service disconnect, at no point are there any customer-supplied service conductors.

That doesn't necessarily affect the applicability of 310.12(C), as it does not refer to the ampacity of the service conductors actually installed. It refers to the ampacity "specified in 310.12(A) or 310.12(B)". So as long as those service conductors are supplying a single OCPD service disconnect for the dwelling unit, you know the rating of the service, and you can do the 310.12(A) calculation, and any downstream feeder need not be larger than that.

You can throw a wrench in the above if you have an underground service to the type of combo meter/main/no distribution that has 2 separate OCPD service disconnects, with each OCPD up to 200A. Now it's less clear what the service rating is, but generally it would be over 200A when using such equipment, so 310.12(C) wouldn't help you with either of the two feeders supplied. But you could have a case where the load calc comes out to under 200A, and for whatever reason you use such a setup with a 50A OCPD for an HVAC unit, and a 200A OCPD for the rest of the load.

Still, here I would say that the load calc determines the service rating, as if there were any customer supplied service conductors, it would determine the necessary conductor size. So I would argue that 310.12(C) would apply to the 200A feeder.

Cheers, Wayne

 

[jar546]

That doesn't necessarily affect the applicability of 310.12(C), as it does not refer to the ampacity of the service conductors actually installed. It refers to the ampacity "specified in 310.12(A) or 310.12(B)". So as long as those service conductors are supplying a single OCPD service disconnect for the dwelling unit, you know the rating of the service, and you can do the 310.12(A) calculation, and any downstream feeder need not be larger than that.

You can throw a wrench in the above if you have an underground service to the type of combo meter/main/no distribution that has 2 separate OCPD service disconnects, with each OCPD up to 200A. Now it's less clear what the service rating is, but generally it would be over 200A when using such equipment, so 310.12(C) wouldn't help you with either of the two feeders supplied. But you could have a case where the load calc comes out to under 200A, and for whatever reason you use such a setup with a 50A OCPD for an HVAC unit, and a 200A OCPD for the rest of the load.

Still, here I would say that the load calc determines the service rating, as if there were any customer supplied service conductors, it would determine the necessary conductor size. So I would argue that 310.12(C) would apply to the 200A feeder.

Sticking with one service disconnect, I see no point to the verbiage where they are basing the ability to reduce on the feeder supplying 100% of the load. it is almost a moot point.

 

[wwhitney]

Sticking with one service disconnect, I see no point to the verbiage where they are basing the ability to reduce on the feeder supplying 100% of the load. it is almost a moot point.

The point is that on a 200A residential service, a 100A feeder doesn't get to use the 83% factor.

Cheers, Wayne

 

[jar546]

The point is that on a 200A residential service, a 100A feeder doesn't get to use the 83% factor.

Cheers, Wayne

Agree, and neither does a feeder that does not provide 100% of the dwelling load.

Example:
POCO provides a 4/0 copper lateral to the combo meter box for a dwelling with a 186A load calc and 200A service disconnect. The only wires leaving the combo box are the 2/0 copper feeders going to the main panel in the garage that handles 100% of the dwelling load. Legal.

After a few months the owners decide to install a pool and the electrical runs that feeder from the exterior combo panel. At that point, the feeders to the garage panel are no longer code compliant with 310.12(B).

 

[wwhitney]

Example:
POCO provides a 4/0 copper lateral

From what I hear (no direct experience), that's pretty unlikely. But the size of the POCO conductors is immaterial.

to the combo meter box for a dwelling with a 186A load calc and 200A service disconnect. The only wires leaving the combo box are the 2/0 copper feeders going to the main panel in the garage that handles 100% of the dwelling load. Legal.

No, 1/0 Cu has a 75C ampacity of 150A, and would need to be protected at 175A under 310.12. So they would not suffice for a 186A load.

Change them to 2/0 Cu, and all is good.

After a few months the owners decide to install a pool and the electrical runs that feeder from the exterior combo panel. At that point, the feeders to the garage panel are no longer code compliant with 310.12(B).

This pool only increases the total load calc by 14A or less? If so, and if the feeder conductors were 2/0 Cu as required above, adding the pool feeder doesn't make those 2/0 Cu conductors non-compliant. 310.12(C) still applies.

Cheers, Wayne

 

[jar546]

This pool only increases the total load calc by 14A or less? If so, and if the feeder conductors were 2/0 Cu as required above, adding the pool feeder doesn't make those 2/0 Cu conductors non-compliant. 310.12(C) still applies.

The existing feeder would have to be 3/0 and not 2/0 even if they decide to install a single circuit breaker for a receptacle in the combo box.

 

[wwhitney]

The existing feeder would have to be 3/0 and not 2/0 even if they decide to install a single circuit breaker for a receptacle in the combo box.

No, when there is no derating required, 310.12(C) says that on a 200A residential service, no feeder has to be bigger than 2/0 Cu.

Edit: 310.12(A) and (B) both have the condition "supplying the entire load associated with a one-family dwelling, . . ." 310.12(C) does not have that condition. When 310.12(C) says "a feeder for an individual dwelling unit" it means any feeder installed to serve any portion of the loads of that dwelling unit.

Cheers, Wayne

 

[jar546]

No, when there is no derating required, 310.12(C) says that on a 200A residential service, no feeder has to be bigger than 2/0 Cu.

Edit: 310.12(A) and (B) both have the condition "supplying the entire load associated with a one-family dwelling, . . ." 310.12(C) does not have that condition. When 310.12(C) says "a feeder for an individual dwelling unit" it means any feeder installed to serve any portion of the loads of that dwelling unit.

Cheers, Wayne

I don't agree. 310.12(B) is specific to the feeder. 310.12(A) is not relevant because it is specific to services, and feeders are not services. 310.122(B) is specific when it does not feed the entire load of a dwelling unit.

 

[wwhitney]

I don't agree.

Then you are missing the point of 310.12(C). Which is why you are getting the nonsensical result in the OP.

What does 310.12(C) say? It says "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)."

So to see what this means for your example, we have to calculate the ampacity specified in 310.12(A).

What does 310.12(A) say? It says that for a 200A service to a dwelling unit, the service conductors need not have an ampacity greater than 166A. And 310.12(C) is just referencing the _ampacity_ from 310.12(A), nothing else about 310.12(A) is incorporated into 310.12(C). So 310.12(C) says that for a dwelling unit supplied by a 200A service, in no case shall a feeder be required to have an ampacity greater than 166A.

It's that simple.

If you still don't believe me, you probably have access to the NEC Handbook or some NFPA-specific commentary about 310.12(C) (I don't have such access). Why don't you post that information? It will say the same thing I say above, as there is no other reason for 310.12(C) to exist.

Cheers, Wayne

 

[jar546]

Then you are missing the point of 310.12(C). Which is why you are getting the nonsensical result in the OP.

What does 310.12(C) say? It says "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)."

So to see what this means for your example, we have to calculate the ampacity specified in 310.12(A).

What does 310.12(A) say? It says that for a 200A service to a dwelling unit, the service conductors need not have an ampacity greater than 166A. And 310.12(C) is just referencing the _ampacity_ from 310.12(A), nothing else about 310.12(A) is incorporated into 310.12(C). So 310.12(C) says that for a dwelling unit supplied by a 200A service, in no case shall a feeder be required to have an ampacity greater than 166A.

It's that simple.

If you still don't believe me, you probably have access to the NEC Handbook or some NFPA-specific commentary about 310.12(C) (I don't have such access). Why don't you post that information? It will say the same thing I say above, as there is no other reason for 310.12(C) to exist.

Cheers, Wayne

Here is what it says:
The main service or feeder to a dwelling unit is permitted to be sized at 83 percent of the disconnect rating. The calculation is not based on the rating of the overcurrent device protecting the main feeder. The minimum disconnect rating for a dwelling unit is 100 amperes according to 225.39 and 230.79. This calculation applies only to conductors carrying 100 percent of the dwelling unit’s diversified load.

If a 120/240-volt single-phase service supplies a one-family dwelling or an individual unit of a two-family or multifamily dwelling, the reduced conductor size is applicable to the service-entrance conductors or feeder conductors that supply the dwelling unit. The feeder conductors to a dwelling unit are not required to be larger than its service-entrance conductors.

See my example a few posts up with the 4/0 lateral from the POCO. You say it is rare, but it is not. There are a lot of pad mounted transformer that are far away from the homes they serve and the POCO chooses larger wire due to voltage drop.

 

[wwhitney]

Here is what it says:

Surprisingly unhelpful, it doesn't seem to address 310.12(C) specifically. So let me ask you, if 310.12(C) doesn't mean what I said, what does it mean?

See my example a few posts up with the 4/0 lateral from the POCO.

Again, the size of POCO conductors, or the size of the actual conductors run as service conductors, doesn't matter for applying 310.12(C). What 310.12(C) references is the minimum ampacity specified in 310.12(A).

You could have a residential 200A service has the service point far from the dwelling unit, with the underground service conductors subject to the NEC upsized well above 2/0 Cu to account for voltage drop, maybe to 4/0 Cu or even larger. They hit an exterior meter/main/distribution that supplies an outdoor HVAC unit as in your example, as well as feed-thru conductors to an interior 200A panel for all the other loads.

Since 310.12(A) says the service conductors only need to have a 166A ampacity, 310.12(C) says that the feeder to the interior panel may be 2/0 Cu, which has an ampacity in excess of 166A (assuming no derating). And if that feeder is short, say to a back-to-back panel, making it 2/0 Cu doesn't contribute significantly to voltage drop, so that wouldn't necessarily be counterproductive.

Cheers, Wayne

 

[wwhitney]

BTW, we already went over all of this back in October:

NEC 310.12(A) One-Family Dwelling Service Conductors & Feeders

In 2013 I built a home. I remember talking to the electrician about his choice of conductor sizes for the following scenario: The service conductors were fed from the POCO pad-mounted transformer between houses in the development. The service conductors came directly to a 200A combination...

www.thebuildingcodeforum.com

Cheers, Wayne

 

[steveray]

I believe it is all about the disco on the outside now and all "services" being feeders in to an interior panel and allowing them to size that one particular feeder the same way you did the service....And not having to downsize the main which is what we used to do on 100A exterior disco conversions...But I am not that smart on electrical...

 

[wwhitney]

I don't agree.

Have you had a chance to review this thread, the earlier thread, and 2023 NEC 310.12, and reconsidered the meaning of 310.12(C)?

Cheers, Wayne