What Exactly Is The "Service Rating"?

[VItamaltz]

I've found a befuddling little wormhole. 310.12 states:

For a service rated 100 amperes through 400 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.

That's all well and good, but it bothers me that "service rating" is not defined anywhere in the Code. We all probably think we know what the service rating is, but what is it really? A few options:

Maybe it's the calculated load from 220? I don't really believe that since the service can be bigger than the calculated load. However, this is supported by an example in Informative Annex D, where the service rating is equated with the calculated load:
Example D2(b) from 2023:
Calculated Load for Service
29,200 VA ÷ 240 V = 122 A (service rating)

Maybe it's the minimum conductor ampacity required by 230.23, as informed by 310? If that's the case, Table 310.12(A) should really reference the calculated load instead of the "Service or Feeder Rating". Otherwise we have a circular logic situation where 310.12 is saying the ampacity of the service is 83% of the ampacity of the service.

Maybe it's the service disconnect OCPD rating? This is what most people would say and what I think is likely the correct answer, but it still seems murky and could be made more explicit. Undefined terms like "service rating" should either be defined or discarded.

Maybe it's whatever the utility says the service is or whatever the available increments of utility service are? That interpretation is kind of supported by looking at Example D7 in Informative Annex D:
"If a 175-ampere service rating is selected, a service conductor is then sized as follows... 175 amperes × 0.83 = 145.25 amperes per 310.1"

A service rating is "selected", not "calculated", so perhaps it's selected based on the utility's stated service ampacity?

I'm interested to read what other people think. Maybe there's something obvious that I'm overlooking.

 

[Strathead]

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

 

[LarryFine]

Maybe it's the service disconnect OCPD rating?

That's what most, if not all of us use.

 

[VItamaltz]

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

Thanks. I think this is likely the case. It'd be nice if the Code said that explicitly. I've always assumed this, but it should be written down somewhere.

Service equipment is defined as The necessary equipment, consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the serving utility and intended to constitute the main control and disconnect of the serving utility. So, if the Code referenced "service equipment rating" instead of "service rating," I would know exactly what was meant.

 

[LarryFine]

So, if the Code referenced "service equipment rating" instead of "service rating," I would know exactly what was meant.

What if they used 225a equipment with a 200 breaker?

Do you want to know the bus rating or the service rating?

That's why we thin the breaker's rating is more pertinent.

 

[VItamaltz]

Don't get me wrong, I agree. A bus that is upsized relative to the service disconnect breaker is no different than a conductor that is upsized. The breaker still sets the ampacity of the service. For that matter, the breaker bus is on the load side of the service point so definitely isn't relevant to the service rating size. My complaint is not so much about actually being confused as it is about not having a clear indication in the Code about what is meant by "service rating." Seems like we all know what it means... so much so that the Code forgot to explain what it means.

 

[gene6]

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.

 

[GoldDigger]

Technically, the service conductors end at the service disconnect, or at least the OCPD associated with the service disconnect. But the conductors up to the service point are the exclusive purview of the utility and their sizing is not determined by the NEC.
If POCO sets the service point farther upstream, then there will be service conductors which need to be sized according to the NEC.
In some cases POCO requires the customer's electrician to install service conductors which are upstream of the service point, but that alone does not make their size subject to the NEC>

 

[jaggedben]

I think it's a great question and the language should probably be different. But practically speaking it's what Strathead said. One could say the service rating is the weakest link of the OCPD and any other component in the service conductor path, which would normally include a meter socket and might possibly include other stuff. But since the OCPD has to protect all those components from overload, it amounts to the same thing in a compliant installation.

 

[wwhitney]

So if I have a single dwelling unit with an overhead service, 500 kcm Al riser conductors, a CL320 meter/main with (2) 200A breakers only, then my service rating is 373A? (310A 75C ampacity / 0.83) Compliant if the single dwelling unit load is 373A or less?

Now if I add an ADU (accessory dwelling unit) and one 200A breakeru supplies 2/0 Cu for all the main house loads, and the other 200A breaker supplies 2/0 Cu to the ADU for all its loads, each feeder is rated 200A, but the service is now rated 310A, so the sum of the two dwelling unit loads has to be 310A or less?

Cheers, Wayne

 

[Eddie702]

It's just like the "rating of a branch circuit" it's what is stamped on the breaker or fuse. Wire size has nothing to do with it.

 

[electrofelon]

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

It's just like the "rating of a branch circuit" it's what is stamped on the breaker or fuse. Wire size has nothing to do with it.

I disagree it is that simple. For one thing, As has been mentioned, what happens when you have multiple service disconnects?

 

[jim dungar]

...As has been mentioned, what happens when you have multiple service disconnects?

Typically it will be the size of the POCO mete,, or what they list on their billing.

 

[Strathead]

I disagree it is that simple. For one thing, As has been mentioned, what happens when you have multiple service disconnects?

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.

 

[jaggedben]

So if I have a single dwelling unit with an overhead service, 500 kcm Al riser conductors, a CL320 meter/main with (2) 200A breakers only, then my service rating is 373A? (310A 75C ampacity / 0.83) Compliant if the single dwelling unit load is 373A or less?

Now if I add an ADU (accessory dwelling unit) and one 200A breakeru supplies 2/0 Cu for all the main house loads, and the other 200A breaker supplies 2/0 Cu to the ADU for all its loads, each feeder is rated 200A, but the service is now rated 310A, so the sum of the two dwelling unit loads has to be 310A or less?

Cheers, Wayne

Lol
Yup I think you're right.

 

[VItamaltz]

Typically it will be the size of the POCO mete,, or what they list on their billing.

I've seen cases where the utility equipment and conductors do not get changed during a service upgrade from 100 to 200 amps. In that case, I think from the utility side it's more about updating the load calculations for their distribution circuit and updating the billing software.

 

[jaggedben]

I've seen cases where the utility equipment and conductors do not get changed during a service upgrade from 100 to 200 amps. In that case, I think from the utility side it's more about updating the load calculations for their distribution circuit and updating the billing software.

The utility side of the service point is under totally different rules.

 

[VItamaltz]

The utility side of the service point is under totally different rules.

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

 

[jaggedben]

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

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

 

[Eddie702]

At least one of the POCOs around here when you upgrade a service from 100-150 or 200 will ask on their forms "any added load?" I have seen many 100-200 amp service changes where the utility never upgrades the service drop.