310.12 and 83% service rating

[Electromatic]

I was recently trying to explain dwelling service sizing to some coworkers and of course came out confusing myself. I was thinking through meter-mains with branch breakers, 400A services with 2 panels, etc. Now I'm wondering what the actual "service rating" is when it says, "...conductors...ampacity not less than 83 percent of the service rating." The handbook says the 83% is based on the "...disconnect rating. The calculation is not based on the overcurrent device protecting the main feeder."
Say I had a calculated load of 210A. 210 * .83 = 174.3A If I use 4/0Al with 180A ampacity, I'm good for the load and the re-inserted Table 310.12 and can next-size-up to a 200A breaker/disconnect. What if I choose 2/0Cu? It's good for 175A. The table says it's good for a 200A service, but 175A is a standard OCPD size.
I think I read another forum member saying to choose an OCPD size then take 83% of that to size your conductors. That seems contrary to how you approach most other calculations.
I might just be thinking about it too much!

 

[Electromatic]

Thinking about the handbook commentary (I've never been a big handbook user): When it says 83% based on the disconnect rating, does it mean the meter base? I've never really considered the meter base as the basis for the "service rating", but I suppose, for residential it kind of is.

 

[jaggedben]

The handbook says the 83% is based on the "...disconnect rating. The calculation is not based on the overcurrent device protecting the main feeder."

That strikes me as complete nonsense. Note that the associated table uses standard overcurrent device ratings, notably 110. The handbook is really not helpful sometimes, and appears to be just wrong in this case. And in any case, if the disconnect is a circuit breaker then they are the same thing.

Like you, I find the wording of this section to be pretty bad, but nonetheless, the only logical conclusion is that the 'rating of the service/feeder' is the overcurrent device rating under this rule. There's nothing else to go on really, and the table corroborates this interpretation.

I think I read another forum member saying to choose an OCPD size then take 83% of that to size your conductors. That seems contrary to how you approach most other calculations.

Yes that's how you do it and it IS contrary. The contrary method is specifically what this section allows only where the conditions apply (single phase, one or two family dwelling, etc.)

Note that the 240.4(B) allowance for the 'next size up' overcurrent device does not apply here. The normal conductor ampacity times must actually be 83% or greater than the OCPD rating.

 

[jaggedben]

Thinking about the handbook commentary (I've never been a big handbook user): When it says 83% based on the disconnect rating, does it mean the meter base? I've never really considered the meter base as the basis for the "service rating", but I suppose, for residential it kind of is.

If there are multiple service disconnects then the meter base might be the basis for determining the meter rating. (You have to understand the ratings precisely, like for example that a 320A rated meter is typically rated for 320A continuous and therefore can be protected from overload by two 200A overcurrent devices.) So for example a 400A service with a 320 meter that can use the table could use 400kcmil copper for service conductors. Whether the two feeders each supplied by a 200A breaker could use 2/0 per 310.12 would depend on whether they served two separate dwellings (they can use 2/0) or both served one dwelling (they can't, they'd need 3/0). (If you're wondering whether that last part makes any science or physics sense ... It doesn't. It's just what the code says.)

 

[Dennis Alwon]

If you have 2 panels you cannot take the 83% you must use 100% since neither panel carries the load of the entire service. So 4/0 alum which is really on 360 amps but 200 amp breakers are fine based on 240.4(B) or 3/0 copper

 

[infinity]

If you have 2 panels you cannot take the 83% you must use 100% since neither panel carries the load of the entire service. So 4/0 alum which is really on 360 amps but 200 amp breakers are fine based on 240.4(B) or 3/0 copper

He said meter mains so would that make these feeder conductors to the panels?

 

[Electromatic]

Yes that's how you do it and it IS contrary. The contrary method is specifically what this section allows only where the conditions apply (single phase, one or two family dwelling, etc.)

Note that the 240.4(B) allowance for the 'next size up' overcurrent device does not apply here. The normal conductor ampacity times must actually be 83% or greater than the OCPD rating.

Really? So, if I have a calculated load of 210A, I don't apply the 83% to that but use the calculation to know that I need at least a 225A service, then apply 83% to 225?
I've never done much ground-up, new residential, so I've never thought a lot about these things.

The whole adding a branch breaker to a meter main then meaning that the feeder to the erstwhile "main" house panel is then not supplying the entire load causes a headache, too. But I go by the last paragraph of that section stating that the feeder need not be bigger than the service conductors. So if I have meter -- 4/0 -- 200A meter main (w/branch circuits) -- 4/0 -- 200A inside panel, I don't sweat it.

 

[Dennis Alwon]

He said meter mains so would that make these feeder conductors to the panels?

Yes it would so he still couldn't use 83%

But he said below...... Not sure what he is saying. The one I have seen have been 2- 200 amp breakers without 400 amp main but either way it doesn't supply the entire load of the dwelling

" I was thinking through meter-mains with branch breakers, 400A services with 2 panels, etc. "

 

[Electromatic]

Just thinking about all the scenarios has gotten me confused as to what actually gets 83% applied to it -- the calculated load? the rating of the meter base? the rating of the primary OCPD? The section reads 83% of the "service rating." What is the "service rating"?
Also, it's late in the day on Friday, and I think I might just need a beer to clear my head.

 

[Dennis Alwon]

Just thinking about all the scenarios has gotten me confused as to what actually gets 83% applied to it -- the calculated load? the rating of the meter base? the rating of the primary OCPD? The section reads 83% of the "service rating." What is the "service rating"?
Also, it's late in the day on Friday, and I think I might just need a beer to clear my head.

If you meter main is rated 400 amps then you service is 400 amps

 

[jaggedben]

If you have 2 panels you cannot take the 83% you must use 100% since neither panel carries the load of the entire service. So 4/0 alum which is really on 360 amps but 200 amp breakers are fine based on 240.4(B) or 3/0 copper

It's not that simple. It's like I said at the end of post #4. If each feeder is for one dwelling, you can use 83%. If they both feed the same dwelling, you can't. (Which, if you think about it, is probably the opposite of how it should be, but nobody said that anything about this rule really makes sense.)

 

[wwhitney]

Note that the 240.4(B) allowance for the 'next size up' overcurrent device does not apply here. The normal conductor ampacity times must actually be 83% or greater than the OCPD rating.

I think I don't agree.

230.90(A) Exception 2 clearly allows the use of 240.4(B) for a regular (non 310.12) service. So you could use 4/0 Al (75C ampacity = 180A) service conductors protected by a 200A OCPD, and that would be fine for a calculated load up to 180A. Would the service rating then be 180A or 200A? I'd say 180A.

If so, then the rating goes by the allowable load, not the OCPD size. Then if the service is for a residential dwelling unit, you could use 3/0 Al (75C ampacity = 155A) to get a service rating of 155/.83 = 187A, good for a calculated load up to 187A, and still protect it at 200A. I think.

That is, what says you can't combine 230.90(A) Exception 2 with 310.12 / 230.90(A) Exception 5?

Cheers, Wayne

P.S. I've seen it expressed here that 310.12 has the effect of increasing the ampacity of service conductors, in which case you certainly could apply 240.4(B), but I don't agree that 310.12 changes the ampacity. Conversely, the idea of 310.12 would be simpler if it were replaced by an additional 83% reduction factor in the load calculation , and in which case you certainly couldn't apply 240.4(B) on top of that 83%. Right now it's a wishy-washy middle ground.

 

[jaggedben]

Just thinking about all the scenarios has gotten me confused as to what actually gets 83% applied to it -- the calculated load? the rating of the meter base? the rating of the primary OCPD? The section reads 83% of the "service rating." What is the "service rating"?
Also, it's late in the day on Friday, and I think I might just need a beer to clear my head.

The 'service or feeder rating' is not defined or made clear. It could be either the rating of the primary OCPD (if there is only one) or the load calc (because if there's multiple OCPDs then the load calc determines the size of service conductors, see 230.90 exception #3). You could say it's whatever you start with. Most of the times in residential no one does a load calc and if they say e.g. '200A service) they mean it will have a 200A main breaker. So the breaker is what is defining the size of the service. If you have a load calc, then maybe you can argue what Wayne says in the post above, but good luck talking your AHJ through that.

 

[jaggedben]

I think I don't agree.

230.90(A) Exception 2 clearly allows the use of 240.4(B) for a regular (non 310.12) service. So you could use 4/0 Al (75C ampacity = 180A) service conductors protected by a 200A OCPD, and that would be fine for a calculated load up to 180A. Would the service rating then be 180A or 200A? I'd say 180A.

If so, then the rating goes by the allowable load, not the OCPD size. Then if the service is for a residential dwelling unit, you could use 3/0 Al (75C ampacity = 155A) to get a service rating of 155/.83 = 187A, good for a calculated load up to 187A, and still protect it at 200A. I think.

That is, what says you can't combine 230.90(A) Exception 2 with 310.12 / 230.90(A) Exception 5?

Cheers, Wayne

P.S. I've seen it expressed here that 310.12 has the effect of increasing the ampacity of service conductors, in which case you certainly could apply 240.4(B), but I don't agree that 310.12 changes the ampacity. Conversely, the idea of 310.12 would be simpler if it were replaced by an additional 83% reduction factor in the load calculation , and in which case you certainly couldn't apply 240.4(B) on top of that 83%. Right now it's a wishy-washy middle ground.

Yeah, I can see your point, but usually there isn't a load calc in resi anyway. I feel like I'm the only person I know who ever does one. And the chance of the AHJ not having ever thought of this and listening to you might not be worth the cost of the next size up wire.