Residential Feeder 83% rule .

[Seekthetruth]

310.15 B7 states that you can put 2/0(rated for 175 amps) on 200 amp breaker using the 83% rule. If your load calculation is 190 amps based on NEC 220 can 2/0 still be used or does NEC 215.2 A apply?

 

[infinity]

The load calculation gives you the OCPD size which is 200 amps. You take the 200 amps then apply the 83% rule and that is your minimum conductor size.

 

[Seekthetruth]

The load calculation gives you the OCPD size which is 200 amps. You take the 200 amps then apply the 83% rule and that is your minimum conductor size.

Thanks . So you are saying 2/0 is good for that calculated load?

 

[infinity]

Thanks . So you are saying 2/0 is good for that calculated load?

For a dwelling service yes. The assumption is that the dwelling load calculation is so generous that reducing the conductor size by 17% will still be a safe installation.

 

[winnie]

Remember that this only applies directly to services and feeders which supply the _entire_ load of a dwelling.

 

[Tulsa Electrician]

Good point, I see this rule mis used quite often.

 

[Seekthetruth]

For a dwelling service yes. The assumption is that the dwelling load calculation is so generous that reducing the conductor size by 17% will still be a safe installation.

Thanks for the input. It seems that NEC 215.2 A doesn't give that option

 

[infinity]

Thanks for the input. It seems that NEC 215.2 A doesn't give that option

My response should have included the word feeder. A service or feeder supplying the entire load of a dwelling can use the 83% rule.

310.15(B)(7)(2)
For a feeder rated 100 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.

 

[wwhitney]

Remember that this only applies directly to services and feeders which supply the _entire_ load of a dwelling.

Well, it also applies to any other feeder of the same size on such a service, as there's no reason to use a larger wire for a subset of the load.

Cheers, Wayne

 

[winnie]

Well, it also applies to any other feeder of the same size on such a service, as there's no reason to use a larger wire for a subset of the load.

Cheers, Wayne

Agreed. That is why I said 'directly'.

For example, consider a residence with a 200A service, with an outside meter main.

In the outside enclosure you have a 40A breaker for a heat pump, and a 200A feeder going into the house.

The 83% rule applies directly to the service conductors, but not to the 200A feeder going into the house.

But the 200A feeder doesn't need to be 'larger' than the service conductors. So _if_ you used the 83% rule to size the service conductors, then the 83% rule would come into play for the feeder.

However if you used full size service conductors then you couldn't later use the 83% rule for the feeder.

I forgot if as currently worded 'larger' means actual conductor size, or ampacity.

Jon

 

[wwhitney]

However if you used full size service conductors then you couldn't later use the 83% rule for the feeder.

As I read (2017) 310.15(B)(7)(3), you could.

Cheers, Wayne

 

[infinity]

For example, consider a residence with a 200A service, with an outside meter main.

In the outside enclosure you have a 40A breaker for a heat pump, and a 200A feeder going into the house.

The 83% rule applies directly to the service conductors, but not to the 200A feeder going into the house.

But the 200A feeder doesn't need to be 'larger' than the service conductors. So _if_ you used the 83% rule to size the service conductors, then the 83% rule would come into play for the feeder.

However if you used full size service conductors then you couldn't later use the 83% rule for the feeder.

As I read (2017) 310.15(B)(7)(3), you could.

Cheers, Wayne

I agree with Jon. If the SEC's are #3/0 Copper for 200 amps then you could not use #2/0 copper to size a 200 amp feeder.

 

[frofro19]

The 83% rule applies directly to the service conductors, but not to the 200A feeder going into the house.

Is that because it's not serving the ENTIRE load of the house? I never understood that. If the heat pump Was included you could use the 83% rule but if you take some load away you can't. Makes no sense. I no you have the Ole diversity thing but if the conductors are adequate enough for everything they surly should be adequate for a lesser load.

 

[wwhitney]

I agree with Jon. If the SEC's are #3/0 Copper for 200 amps then you could not use #2/0 copper to size a 200 amp feeder.

I'm having trouble understanding how (2017) 310.15(B)(7)(3) could be read that way.

"(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2)."

So if (1) specifies that a 200A service need only have an ampacity of 166A, then any downstream feeder need only have an ampacity of 166A. You might choose to upsize some conductors above the minimum size and end up with a higher ampacity, e.g. for voltage drop, but that doesn't change that the maximum required ampacity is 166A.

Cheers, Wayne

 

[Dennis Alwon]

As I read (2017) 310.15(B)(7)(3), you could.

Cheers, Wayne

I think the way it is worded Wayne is correct but it surely defeats the purpose of the 83% only if it carries the entire load of the dwelling.

 

[winnie]

I concur with Wayne's reading of the 2017 code. I believe an earlier version of the code went by service conductor size, however.

This allowance does not defeat the intent of the 83% rule, because in only applies in situations where the feeder rating is as large as the service rating.

In other words you can apply the 83% rule to a 200A feeder supplied by a 200A service, even if that feeder only carries part of the residential load. But you couldn't use the 83% rule to size a 100A feeder on a 200A service.

Think about it: if 166A service conductors are sufficient for a 200A rated service, why would they not be sufficient for 200A less the heat pump load?

Jon

 

[wwhitney]

I believe an earlier version of the code went by service conductor size, however.

Yes, the last version of the NEC without the 83% formulation was 2011, and the relevant sentence from 310.15(B)(7) there is "The feeder conductors
to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors." Which goes off the actual ampacity installed, rather than the minimum size allowed per Table 310.15(B)(7). Which was obviously a problematic formulation, leading to a variety of undesirable corner cases.

Cheers, Wayne

 

[Seekthetruth]

I notice that they changed the wording of 215.2.A.1 in the 2020 code

 

[infinity]

I'm having trouble understanding how (2017) 310.15(B)(7)(3) could be read that way.

"(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2)."

So if (1) specifies that a 200A service need only have an ampacity of 166A, then any downstream feeder need only have an ampacity of 166A. You might choose to upsize some conductors above the minimum size and end up with a higher ampacity, e.g. for voltage drop, but that doesn't change that the maximum required ampacity is 166A.

Cheers, Wayne

You're correct. I was thinking of the pre-83% version like this from the 2008 which stated that "the feeder conductors
to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors."

 

[Rex Hungerford]

I thought I watched a video here that said a feeder need not be larger than the service conductor. This being outside the 310.12(A) (B) where it is feeding the whole dwelling load.
I have a 400A service that terminates in a main panel. loads spit off in the main 400A panel. Then I have a set of 400A feed through lugs that will go to another 400Amp panel. Table 310.12 says I can use Copper 400 from the meter to the main service panel. Is there not an exception somewhere that says I don't have to make the wires from the main service panel to the subpanel larger than the service wires?