Optional calculation, 220.82, dwelling unit with multiple feeders

[UBG]

Existing single family home, one 120/240 V service (400A/320 continuous) supplies 3 subpanels: pool, upstairs, and downstairs.

Seems like the optional method can be used for service calculations, but not for the feeder calcs, based on 220.82(A):

"...total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors..."

Reason for asking this question about an existing system is that at least one of the subpanel feeders was grossly undersized for the load that is connected to it, and I'm making sure that I'm accurately calculating what it should have been, when I give report to the owners. (I'll probably make a separate post with those details).

Thanks in advance

 

[UBG]

I guess I didn't actually phrase that as a question, so I'll try again

Am I interpreting 220.82(A) correctly?

The language there (quoted in my OP above) seems to me to imply pretty clearly that

• IF a home has more than one feeder, THEN
• NONE of those feeders loads can be calculated using the optional method

But maybe I'm missing something?

I'm reminded of 310.12(B) and (C), which also applies to the same home described above:

310.12(B) says "the feeder conductors supplying the entire load associated with a one-family dwelling... shall be permitted to have an ampacity not less than 83 percent of the feeder rating". That seems to imply that if a home has multiple feeders, then the 83% rule can NOT be used...

But then 310.12(C) 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)", and that seems to me to suggest that in a dwelling with multiple feeders, all feeders may utilize the 83% rule.

I'd love to get your thoughts on all of the above, thanks

 

[tortuga]

You are correct that is one of the many pitfalls of the optional calc.
That calc is limited to the entire load of one single dewlling unit and thats it.
So if your feeding the dwelling unit with more than one feeder no go.
And if you are feeding say a dwelling unit and a guest suite or additional dwelling unit no go.

 

[UBG]

Thanks tortuga...any thoughts on 310.12?

 

[wwhitney]

But then 310.12(C) 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)", and that seems to me to suggest that in a dwelling with multiple feeders, all feeders may utilize the 83% rule.

No, just that any feeder need not be larger in size than the feeder/service supplying the whole house.

E.g. 200A residential service with 2/0 Cu service conductors per 310.12. Service panel has a few loads and feed-thru lugs to another 200A panel. The feeder supplied by the feed-thru lugs doesn't meet the requirements of 310.12(B), and needs to be 200A, so you might think it has to be 3/0 Cu. But no, 310.12(C) recognizes that makes no sense, you get to use 2/0 Cu.

Whereas in this example if the service panel has a 125A breaker for a feeder, you don't get to use the 83% rule on that feeder.

Cheers, Wayne

 

[UBG]

No, just that any feeder need not be larger in size than the feeder/service supplying the whole house.

E.g. 200A residential service with 2/0 Cu service conductors per 310.12. Service panel has a few loads and feed-thru lugs to another 200A panel. The feeder supplied by the feed-thru lugs doesn't meet the requirements of 310.12(B), and needs to be 200A, so you might think it has to be 3/0 Cu. But no, 310.12(C) recognizes that makes no sense, you get to use 2/0 Cu.

Whereas in this example if the service panel has a 125A breaker for a feeder, you don't get to use the 83% rule on that feeder.

Cheers, Wayne

Thanks Wayne, that makes sense now.

And, so that I'm perfectly clear, in the home in my example with 3 feeders, the 83% rule is NOT allowed for the feeders either, although it IS allowed for the SEC?

 

[Dennis Alwon]

The only time 83% is used is for the service conductors or any feeder that carries the entire load of the home. So 2/0 copper to the service disconnect which has no other loads and then 2/0 to the interior panel.
If you had a 200 amp service you could use 2/0 copper however if you ran a cable to 2-100 amp panels those feeders could not use the 83%.