What Exactly Is The "Service Rating"?

[jim dungar]

Yes. I was replying to Jim, who said that the service rating is the rating of the utility's equipment.

Are you saying the utility uses 100A meters for what they say is a 200A service?

But regardless, the service rating is what the POCO say it is, or you have to add all of the NEC sized main devices together.

 

[jim dungar]

Yeah I totally disagree that the NEC cares what the utility thinks it is.

You are correct that the NEC doesn't usually care about the utility.

But I had a situation where the contractor was trying to do his customer a favor and ended up loosing big time. The spec was for a 4 position meter stack with 600A main lugs, the were two active 200A meters. The contractor decided to upgrade the service, for future expansion and ordered an 800A meter center and an 800A service from the utility. They lost money when the utility quoted an available fault current of 38kA based on any transformer they might install to service 800A and the city EI didnt care what was actually installed. It has been 35 years and no expansion has taken place.

 

[electrofelon]

the utility quoted an available fault current of 38kA based on any transformer they might install to service 800A and the city EI didnt care what was actually installed.

FWIW, the inspector was incorrect. The NEC does not say you have to provide for a future AFC value or a value that is provided by the POCO.

 

[jim dungar]

FWIW, the inspector was incorrect. The NEC does not say you have to provide for a future AFC value or a value that is provided by the POCO.

If you don't start with a fault current value from the POCO, or other source, then you are guessing at your final calculated results.
The NEC says maximum fault current, for the service equipment. If the utility has the contractor install a pad and conduit locations for a potential 500kVA transformer, based on an emergency replacement from their spare inventory, then the EI is within this rights to see this as a source maximum fault current. If the EC had stayed with the specified 600A, the largest transformer the utility could provide would have been 225kVA.

This situation occurred during the final inspection. The EC ended up reworking the installed meter center to include a 600A main breaker with series ratings with the downstream equipment.

 

[kwired]

I think its covered in 230.79 & 230.80?
Its gets tricky if you don't have a single main, say you have one underground lateral sized and provided by utility into a utility vault or tap box then two Main switch distro gear with 6 buckets in each? 230.40 Ex1.

Yes but no. To use the 83% the conductor must be supplying the entire load of a dwelling. A dwelling with multiple service disconnecting means would not be able to use this 83% for load side conductors of any of the service disconnecting means. A common supply conductor to multiple disconnects could use the 83% though.

You also can use the 83% for multifamily on each individual dwelling feeder, or for that matter any feeder that serves a NEC defined dwelling unit, which could be a unit within say a hotel or resort that meets the NEC definition of dwelling unit.

 

[gene6]

Yes but no. To use the 83%

I was not talking about the 83% rule, a service with more than one disconnect has a 'rating' which is the ampacity of the wire if its existing and or calculated load per article 220 --see 230.71

230.80 Combined Rating of Disconnects. Where the service
disconnecting means consists of more than one switch or
circuit breaker, as permitted by 230.71, the combined ratings of
all the switches or circuit breakers used shall not be less than
the rating required by 230.79.

The last thing we need is to have them add more stuff in the code that we don't need. The code is clear enough on this issue. The rating of a breaker is it's size. The rating of a service as it relates to downsizing the conductors is the breaker's size. Multiple disconnects don't come in to the picture because it is conductors serving the entire load.

When you have multiple disconnects tapped off one set of service entrance conductors the rating of the entire service is the calculated load, not the combined rating of the disconnects.
230.80 just says that at minimum the sum of those disconnects cant be less than the 230.71 calculated load, if they add up to more than the calculated load it does not affect the rating of the service.

 

[jaggedben]

I was not talking about the 83% rule, a service with more than one disconnect has a 'rating' which is the ampacity of the wire if its existing and or calculated load per article 220 --see 230.71


When you have multiple disconnects tapped off one set of service entrance conductors the rating of the entire service is the calculated load, not the combined rating of the disconnects.
230.80 just says that at minimum the sum of those disconnects cant be less than the 230.71 calculated load, if they add up to more than the calculated load it does not affect the rating of the service.

Kwired is correct that the permitted size of the service conductors hinges on whether they serve the 'entire load associated with an individual dwelling' and not how many disconnects there are. Also this thread is essentially about the 83% rule. Because 310.12's use of the phrasings "For a service rated xxx amperes" and "83% of the service rating" raises the question of how that rating is determined. It cannot be determined by the conductors because that would be circular logic.

 

[kwired]

Kwired is correct that the permitted size of the service conductors hinges on whether they serve the 'entire load associated with an individual dwelling' and not how many disconnects there are. Also this thread is essentially about the 83% rule. Because 310.12's use of the phrasings "For a service rated xxx amperes" and "83% of the service rating" raises the question of how that rating is determined. It cannot be determined by the conductors because that would be circular logic.

Is clearer in the next section that mentions basically same thing for feeders. Feeder rating is basically the rating of the supply side of the overcurrent device. A service just happens to have the overcurrent device on the load end of the service conductor, but I think the whole intent of this is you can have 83% of normal conductor ampacity protected by a particular overcurrent device - be it service or feeder overcurrent protection. And of course the other part of this as mentioned - it must be supplying an entire dwelling unit load.

 

[Strathead]

I was not talking about the 83% rule, a service with more than one disconnect has a 'rating' which is the ampacity of the wire if its existing and or calculated load per article 220 --see 230.71


When you have multiple disconnects tapped off one set of service entrance conductors the rating of the entire service is the calculated load, not the combined rating of the disconnects.
230.80 just says that at minimum the sum of those disconnects cant be less than the 230.71 calculated load, if they add up to more than the calculated load it does not affect the rating of the service.

I disagree. The service rating is the sum of the disconnects. As such the service conductors can be 83% of the service rating. However they can’t be smaller than the calculated load, regardless of the above math. Simple as that. I again state, stop finding things that require more codes.

 

[Strathead]

Is clearer in the next section that mentions basically same thing for feeders. Feeder rating is basically the rating of the supply side of the overcurrent device. A service just happens to have the overcurrent device on the load end of the service conductor, but I think the whole intent of this is you can have 83% of normal conductor ampacity protected by a particular overcurrent device - be it service or feeder overcurrent protection. And of course the other part of this as mentioned - it must be supplying an entire dwelling unit load.

The thing I think people are forgetting is that the ampacity can never be less than the calculated load, period. That code isn’t superseded by the 83%.

 

[wwhitney]

The thing I think people are forgetting is that the ampacity can never be less than the calculated load, period. That code isn’t superseded by the 83%.

Are you proposing that if a dwelling unit has a calculated load of 185A, that it is not permissible to supply it with 2/0 Cu service conductors protected at 200A? That is contrary to the common understanding of 310.12.

Cheers, Wayne

 

[electrofelon]

I disagree. The service rating is the sum of the disconnects.

. And I disagree with you

I again state, stop finding things that require more codes.

I certainly don't want more codes, but it's annoying when the NEC uses a term and doesn't provide a definition so you don't know what exactly it means.

 

[gene6]

How about a typical 4 floor town house.
Existing 200A MLO residential panel that has six 2-pole breakers. (6 main rule)
3-100A breakers to sub panels one in kitchen and two on upper floors,
a 2-pole 60 to a basement sub panel,
2-pole 30 for a dryer and a 2-pole 50 for a range.
Sum of the main breakers is 440A
Service entrance conductors are 4/0 AL
Calculated load is 180A
What is the rating of this service?

 

[LarryFine]

What is the rating of this service?

I consider it to be 200a.

 

[wwhitney]

What is the rating of this service?

For 75C termination temp, 75C or 90C conductors, and no ampacity adjustment or correction, I'll go for 180A (ampacity) / 0.83 (factor from 310.12) = 217A.

Cheers, Wayne

 

[jaggedben]

I agree with Larry because the 200A rating of the service panel is less than Wayne's result.

 

[wwhitney]

I agree with Larry because the 200A rating of the service panel is less than Wayne's result.

If the service conductor bus has an ampacity of 200A, then 310.12 assigns it a service rating of 200A / 0.83 = 240A. So the lower rating of 217A for the 4/0 Al conductors controls.

Cheers, Wayne

 

[kwired]

The thing I think people are forgetting is that the ampacity can never be less than the calculated load, period. That code isn’t superseded by the 83%.

What NEC is doing here is saying the load diversity of a dwelling allows 83% of normal ampacity.

If you have a load calculation of 100 amps you still need a 100 amp breaker but they are saying that it would be rather uncommon for it to heat the conductor enough to be a problem if that conductor normal ampacity were only 83 amps because the nature of the loading in a dwelling.

How this was determined I have no idea, but at same time I do believe in many cases you likely could use even lesser conductor size and it still wouldn't be a problem. HVAC loads, maybe pool pumps or other rather continuous items like that could cause more problems but without those dwellings do tend to only have some the heavier loading only for short time and then sit with low draw a majority of the time.

 

[gene6]

I consider it to be 200a.

But the size of the overcurrent protection is 440A
Then when do you go with

The service rating is the size of the overcurrent protection at the service.

and when not?
I suspect when there is one main breaker is the only time its safe to say the rating of a service is the rating of the OCPD.
Here is another one I recently have seen in the wild:
Service to a single family dwelling with a 3/0 AL service entrance that hits a wireway then feeds two 125A disconnects with 125A fuses? The disconnects each feed a 125A MLO subpanel
What is the rating of that service?

 

[wwhitney]

If you apply a strict reading to these matters, all 310.12 does is act as an expanded version of 240.4(B), as explicitly referenced in 2023 NEC 240.4(H). Since 310.12 does not change the ampacity of the conductors, and as 215.2, 230.23, 230.31, and 230.42 have no exceptions that reference the 310.12 "rating," the calculated load on the conductors must not exceed their ampacity, and the 83% factor does not enter into that check.

Cheers, Wayne