What Exactly Is The "Service Rating"?

[wwhitney]

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?

75C ampacity of 3/0 Al = 155A, so the rating is 155A / 0.83 = 186A. Please note that nothing in 310.12 says that the service rating has to match a standard OCPD size.

Cheers, Wayne

 

[gene6]

Thanks for the example
I would have said the service rating of the 3/0 AL is 175A.
Now we're going look at installing ESS and a ATS at that townhouse and the calculated load is 180A.
The ATS will act as the service main ahead of the wireway. The 125A disconnects will now be down stream (neutrals floated).
What should the service rating of the ATS be? Can the 3/0 service entrance remain?

 

[wwhitney]

Now we're going look at installing ESS and a ATS at that townhouse and the calculated load is 180A.
The ATS will act as the service main ahead of the wireway. The 125A disconnects will now be down stream (neutrals floated).
What should the service rating of the ATS be? Can the 3/0 service entrance remain?

So the ATS will now have the singular service OCPD? Then you'd need to get a non-standard 180A or 185A OCPD (if that's even possible) to be able to continue to use the 3/0 Al service conductors. As the standard size 175A is too small for the calculated load, while the standard size of 200A is too large to protect the 3/0 Al even under 240.4(H).

Cheers, Wayne

 

[gene6]

So the ATS will now have the singular service OCPD? Then you'd need to get a non-standard 180A or 185A OCPD (if that's even possible) to be able to continue to use the 3/0 Al service conductors. As the standard size 175A is too small for the calculated load, while the standard size of 200A is too large to protect the 3/0 Al even under 240.4(H).

Cheers, Wayne

Yeah I agree in principal however we are not on the 2023 NEC here no 240.4(H)
The "rating of the service" is determined by the calculated load (230.23 & 230.79)

 

[jaggedben]

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

Wow, they really messed that up then.

 

[LarryFine]

I based my 200a answer on:

Existing 200A MLO residential panel
Service entrance conductors are 4/0 AL
Calculated load is 180A

 

[Strathead]

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

Thank you for saving me research time. That is what I said earlier without the code references.

 

[david]

If I have 4/0 alum service conductors as the supply to a class 200 meter socket supplying 6ea 100 amp panels grouped in my basement, what is my service rating?

 

[david]

If I have 4/0 alum service conductors as the supply to a class 200 meter socket supplying 6ea 100 amp panels grouped in my basement, what is my service rating?

If I build a 500 amp service can the 83 percent be applied to my service conductors? By t he code language I need to know what the service rating is.

To know the service rating I need to know what calculated load is the service being built for.

So the service rating would be the minimum size service needed to supply the calculated load.

Now the example of the 500 amp service above may not be what was intended by the parameters of 100 amp through 400 amp.

An electrical inspector would have to know what the service rating is

 

[kwired]

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

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?

I'd say NEC basically limits this to 187 amps being the design capacity after factoring in the 83% rule even though overcurrent devices will allow 250 amps to flow with no interruptions.

 

[kwired]

Thanks for the example
I would have said the service rating of the 3/0 AL is 175A.
Now we're going look at installing ESS and a ATS at that townhouse and the calculated load is 180A.
The ATS will act as the service main ahead of the wireway. The 125A disconnects will now be down stream (neutrals floated).
What should the service rating of the ATS be? Can the 3/0 service entrance remain?

Based on what? Outside the 83% allowance for individual dwelling supply conductors it can have overcurrent protection of 175 amps (next size up rule) but still can not have a load calculation of more than 155.

NEC has this somewhat strange rule when there is more than one service disconnect however, the conductor ampacity on supply side of multiple service disconnects only needs to be equal or greater than the load calculation. You can have six 100 amp main breakers as the service disconnect yet supply those with a conductor sized per the actual art 220 load calculations, if that happens to be only 150 amps you can run 150 amp conductor.

 

[david]

Let's say it's a multi family dwelling and the feeders with individual dwelling units having a calculated load of 60 amps. I know the 83% can not apply to a 60 amp rated feeder.

But what if I up size the feeder to 100 amps even though the calculated load is 60 amps. Can I apply the 83% to the 100 amp feeder?

Here we kind of say an inspector dosnt decide what he/she believes the intent to be but what the language says

But I think we are in an area where an inspector is going to have to decide what the intent is

 

[gene6]

Based on what? Outside the 83% allowance for individual dwelling supply conductors it can have overcurrent protection of 175 amps (next size up rule) but still can not have a load calculation of more than 155.

OK good info, I must admit I use a 2008 codebook in my truck and it still has the residential ampacity table that shows 3/0AL as 175A, so I thought it could carry 175A in residential.

NEC has this somewhat strange rule when there is more than one service disconnect however, the conductor ampacity on supply side of multiple service disconnects only needs to be equal or greater than the load calculation.

Yeah agreed loac calc is service rating, and thats not required to be marked on the service like fault current is, you need to get in there an look at the service entrance conductors and hopefully you can read the wire size.

 

[gene6]

If I build a 500 amp service can the 83 percent be applied to my service conductors?

The 83% rule appears to end at 401 amps. So for a 500A rated service (calculated load) you just use the regular table.

Can I apply the 83% to the 100 amp feeder?

For a 60A calculated dwelling unit load, if you use a 100A feeder protected by a 100A OCPD you could use #4 CU or #2AL, with a 60A breaker you'd need to use #6CU or 4AL (regular tables)

 

[david]

The 83% rule appears to end at 401 amps. So for a 500A rated service (calculated load) you just use the regular table.

For a 60A calculated dwelling unit load, if you use a 100A feeder protected by a 100A OCPD you could use #4 CU or #2AL, with a 60A breaker you'd need to use #6CU or 4AL (regular tables)

That's exactly my point when we have two to six service disconnects. And looking at the service rating high side 400 amps. The calculated load is the min size my service has to be.

But cannot be the only factor in determing what the service or feeder rating is. So if I have a service that could have a rating of 400 amps or less but I build a service greater than that in my example 500 amps. Now what's the service rating calculated load or overcurrent protection?


Looking at the low end dwelling units with with a calculated load of 60 amps or less. They are required by code to have a min of 60 amp supply. Now I supply each dwelling unit upgraded to 100 amp overcurrent. What's the rating?

So when I have two to six disconnects grouped at on location, the rating is the calculated load


But when the feeder or service supply is to single overcurrent protection the rating is the size of the overcurrent protection.


Pretty dificult to put that into a ddefinition when it's a combination of both

 

[jaggedben]

My comments in this reply might be moot for now in as much as the code now says what it says, but I believe the original intent of this section has been substantially bastardized if we were to follow the strict reading Wayne laid out in post #40.

If you go back to the 2011 version of this section (then 310.15(B)(7)) it applied only to 120/240 volt and simply permitted what was shown in the table. Implied were two things: (a) that the section actually modified the NEC ampacity of the conductors, and (b) that all other components in the service would have the rating for the applicable row in the table. I believe that the idea is that a 120/240 feeder really only has two current carrying conductors, and thus an increased ampacity is justified (as compared to the regular table for up to 3 CCCs ). (Someone asked a question about this on this forum in recent years, but I haven't dug through the revision documentation on the change to include 208V to verify. It just makes sense to me.) In any case, the idea that under this section one could protect 4awg cu at 100A, but not use it on a service with a calculated load of greater than the otherwise applicable 310 ampacity, had no support in the way things were written in 2011. Also, the broad permission to use the table implied that adjustment and correction could be ignored. For what it's worth, as far as I can tell this rule had not substantively changed for decades prior.

2014 gave us the revision to "an ampacity not less than 83% of the service rating" and deleted the table. I believe the motivation here was to remove the implication that adjustment and correction could be ignored. I also believe there was no intention to otherwise substantively change the rule. However the way they went about it introduced two new ways of interpreting the section that I believe were unintentional and make no sense from a physics point of view. The first is that the 83% adjustment doesn't apply to the calculated load. The second is that other service conductor components not mentioned in the table, such as meters or MLO panel bussing, also get to take advantage of the 83%. Explaining why they make no sense would make this reply too long, but to me they make no sense.

2017 gave us the inclusion of 3-wire 120/208 feeders. I'd have to look at the substantation, but I believe this was the result of lack of institutional memory, i.e. everyone completely forgetting why 120/240 circuits should get a special allowance here (i.e. one less current carrying conductor).

In 2020 the table was put back in with the caveat that adjustment and correction factors not be applicable. (This is also when it was relocated to 310.12). The phrasing "Table 310.12 shall be permitted to be applied" is sufficiently vague that one could ask whether, in this situation, we are modifying the NEC ampacity of the conductors again. Whatever. It's a mess.

The solution would be to re-write the section so that the ampacities in the table are permitted to be used as the starting point before adjustment and correction factors, and to otherwise restore the section to more or less the way it was in 2011. In other words, make it plain that the NEC ampacity of these conductors is modified in this situation. The table would no longer refer to rating of the service, but just conductor size and ampacity, like any other table. And the phrase 'service rating' and similar would disappear. Which would, blessedly, also make the section shorter again.

 

[wwhitney]

Pretty dificult to put that into a ddefinition when it's a combination of both

For a service that supplies only one dwelling unit, every conductor or piece of equipment that carries the full dwelling unit load has a rating. The service rating will be the minimum of those ratings.

When there is a single OCPD rated 100A to 400A that carries the full dwelling unit load, the OCPD sizing rules require that the conductor rating(s) be at least the OCPD rating, so the OCPD rating will be the minimum of those ratings and be the service rating. If you have a single OCPD outside that range, then if 240.4(B) is used, the conductor rating may be less than the OCPD rating, in which case the conductor rating will be the service rating.

When there is no OCPD that carries the full load of the dwelling unit, the OCPD sizes are irrelevant to the determining the service rating. Likewise, the results of the load calculation have no bearing on the service rating. If the result of the load calculation exceeds the service rating, that just means you have an NEC violation.

Cheers, Wayne

 

[gene6]

That's exactly my point when we have two to six service disconnects. And looking at the service rating high side 400 amps. The calculated load is the min size my service has to be.

But cannot be the only factor in determing what the service or feeder rating is. So if I have a service that could have a rating of 400 amps or less but I build a service greater than that in my example 500 amps.

look at the examples the OP quoted in annex D If say you have calculated a load for service entrance conductors that carry the entire load of one dwelling and come up with 401 Amps you simply can't use the section.
If its 400A then you can, and your allowed to protect 400kCMIL copper with a 400A breaker.
If your 400kCMIL copper service entrance conductors go to a tap box and you have six 200A disconnects you still have a 400A rated service per 230.79 & 230.80

Looking at the low end dwelling units with with a calculated load of 60 amps or less. They are required by code to have a min of 60 amp supply. Now I supply each dwelling unit upgraded to 100 amp overcurrent. What's the rating?

If your service entrance is existing I'd say you go off the wire size, so if its a #4 CU then it has a 100A rating. Even if it lands in a 200a MLO panel with 6 mains.

The OP is in the ATS business and I think this is something they run into pretty often where an ATS becomes a single main disconnect on existing service entrance conductors.
I know I have run into old split buss or 6 main service panels on Generator ATS installs.
I have to rate the service based on the existing service entrance conductors.
And often they have the wrong size ATS like my 3/0 AL into a MLO 200A panel with 6 mains example. That needed a 175A ATS. Not a 200A.

 

[david]

When there is no OCPD that carries the full load of the dwelling unit, the OCPD sizes are irrelevant to the determining the service rating.

Likewise, the results of the load calculation have no bearing on the service rating.

if i have a two family dwelling unit with six service disconnects grouped at one location in a common basement area.

I have service conductors from the service drop to a two gang meter socket

(1) What is the service rating?

When I go from my two gang meter socket with two service entrance conductors supplying one tdwelling A's tap location.

The second service entrance conductors supplying dwelling B's tap location

(2) What's the service rating?

from dwelling (A's) tap location i have three individual taps supplying three service disconnects for dwelling (A).

And the same set up supplying dwelling (B)

(3) whats the service rating?

In one 1, and 2 above dosn't the calculated load have bearing on the service rating?

If the result of the load calculation exceeds the service rating, that just means you have an NEC violation.

Cheers, Wayne

 

[wwhitney]

if i have a two family dwelling unit with six service disconnects grouped at one location in a common basement area.

I have service conductors from the service drop to a two gang meter socket

(1) What is the service rating?

The minimum of the ampacity of those service conductors and the line side rating of the two gang meter socket.

When I go from my two gang meter socket with two service entrance conductors supplying one tdwelling A's tap location.

The second service entrance conductors supplying dwelling B's tap location

(2) What's the service rating?

Not following why you use the word "tap". If the two meters are to meter dwelling A and dwelling B separately, then within the two gang meter socket the supply conductors for dwelling A will separate from the supply conductors for dwelling B. Each meter will supply two ungrounded service conductors for one of the dwellings. I.e. on the load side of the two gang meter socket there will be no conductors common to both dwelling units.

Regardless, after the point at which the supply to dwelling A separates from the supply to dwelling B, we are back in the "service for a single dwelling unit" category; see my previous post.

In one 1, and 2 above dosn't the calculated load have bearing on the service rating?

It has no bearing in determining the service rating for a given installation. It is a minimum for the required service rating.

Cheers, Wayne