What Exactly Is The "Service Rating"?

[david]

I'll get back to you

 

[kwired]

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

The 83% can only apply to the supply conductors to an individual dwelling, and for cases with an calculated ampacity of 100 to 400 amps.

If you have multifamily any common supply conductors can not use the 83% has to be the conductors feeding an individual unit to use the 83% allowance and must be supplying entire load of an individual unit. Sub feeders within a dwelling can't use the 83% rule either.

 

[kwired]

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.

I thought the 2014 change was just fine. There was need to throw in the adjustments, say you had a high ambient temp or for whatever reason multiple feeders in same raceway or wireway.

If no adjustments were needed the straight 83% yielded same minimum conductor size as the previous table allowed anyway.

 

[david]

The 83% can only apply to the supply conductors to an individual dwelling, and for cases with an calculated ampacity of 100 to 400 amps.

If you have multifamily any common supply conductors can not use the 83% has to be the conductors feeding an individual unit to use the 83% allowance and must be supplying entire load of an individual unit. Sub feeders within a dwelling can't use the 83% rule either.

I'm sorry I have to disagree at least for the moment. I am however willing to yield

310.12 for one family dwellings service and feeder conductors supplied

It also includes for service and feeder conductors for individual dwelling units of two family dwellings


And also includes services and feeders for individual dwelling units of multifamily dwellings

310.12 (A) clarifies this services rated 100 through 400 amps

Services conductors supplying the entire load associated with one family dwellings

Service conductors associated with individual dwelling units of two family or multi family dwellings

I read it not to say it has to be the service supply conductors to an individual dwelling unit.

By the definitions of single family, two family dwellings and multi family dwellings

A service is not to an individual dwelling unit it is a supply to a building associated with a individual dwelling unit

it has to be service conductors supplying the entire load associated with

 

[david]

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

And if my service conductors from the service drop to the two gang meter socket are oversized because that's what I had in my truck

Class 200 amp meters will fit class 100 or class 200 meter sockets. So I use a two gang class 200 two gang were I could have used a class 100 two gang meter socket

I'm still thinking the rating would be based on the only constant in building a service with a two gang meter socket. That constant being the calculated load

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.

That's not necessary true. In my example. The common service entrance conductors For A are supplied by meter A they enter a through (A junction point) and are "tapped to three separate service conductors supplying three separate service disconnect / distribution panels for unit A

And meter B and associated service contuctors are set up the same way for unit B

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.


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

Cheers, Wayne

So to clarify the service conductors identified as (1) supply the service conductors for the entire building. Service conductors associated with the entire supply for both dwelling units (A) and (B)

the service conductors identified as (2) lets call the first set (2a)supply the entire load associated with unit (A)

Let's call the second set (2b) supplied from location identified as (2). Supplying the total load associated with unit (B)

What i am saying is the the rating of the service is based on the calculated load for locations 1 and 2

Location 3 the 83% dosn't apply to. For that portion dosn't supply the entire load associated with dwelling unit (A) or dwelling unit (B)

 

[wwhitney]

I'm still thinking the rating would be based on the only constant in building a service with a two gang meter socket. That constant being the calculated load

Nope, the calculated load is a function of the downstream installed loads. The service rating is a function of the wiring and equipment installed at the service. The NEC requires that calculated load <= service rating. If I add a load to the dwelling unit, that doesn't change the service rating. It just increases the calculated load, and I need to confirm that the new calculated load is still less than the service rating.

That's not necessary true. In my example. The common service entrance conductors For A are supplied by meter A they enter a through (A junction point) and are "tapped to three separate service conductors supplying three separate service disconnect / distribution panels for unit A

Ah, OK, I misread that part. Anyway, 310.12 applies to the service conductors between the dwelling unit A meter socket and the point of division among the various dwelling A service disconnects.

Cheers, Wayne

 

[david]

Nope, the calculated load is a function of the downstream installed loads. The service rating is a function of the wiring and equipment installed at the service. The NEC requires that calculated load <= service rating. If I add a load to the dwelling unit, that doesn't change the service rating. It just increases the calculated load, and I need to confirm that the new calculated load is still less than the service rating.

I didn't say the calculated load establishes the service rating. I said the service rating is based on the calculated load when there are two to six service disconnects.

There is nothing else that establishes the min size the service conductors have to be.

the code says when you you have two to six service disconnects, add ampacity of the overcurrent devices protecting the service conductors, they must meet or exceed the calculated load.

Adding the ampacity of the two to six service disconnects together does not establish the service rating for the building

looking at service conductors ampacity size does not establish the the service rating for a building having two to six service disconnects

And yet you're saying the calculated load has no bearing in establishing the service rating for a building with two to six service disconnects

I don't drink but to you this night I say cheers

 

[wwhitney]

I didn't say the calculated load establishes the service rating. I said the service rating is based on the calculated load when there are two to six service disconnects.

The calculated load determines the minimum service rating. The actual service rating depends on the service conductors and equipment installed.

Same as with a feeder--if the calculated load is 100A, I need to install at least a 100A feeder. If I choose to install conductors with a 115A ampacity, protected by a 125A circuit breaker, that's a 115A feeder. As 100A (calculated load) < 115A (feeder ampacity) all is well.

There is nothing else that establishes the min size the service conductors have to be.

Correct. But the actual service rating installed depends on the conductor size chosen, and may be greater than the calculated load.

the code says when you you have two to six service disconnects, add ampacity of the overcurrent devices protecting the service conductors, they must meet or exceed the calculated load.

Presumably you mean rating of the OCPDs, not ampacity. While that is generally true, there are possible corner cases.

For example, it has been suggested on this forum that the wording of 220.82 means that a service or feeder supplying the full load of a dwelling unit may use the optional calculation, but that one supplies the full load of two dwelling units may not. And that for two dwelling units, you must use the standard calculation, or use 220.85.

If that's true, then you can come up with an example where the service for a duplex would have a calculated load of, say, 220A, but the calculated load for each dwelling unit is only 95A. So then you'd need to have a service rated at least 220A, but it could supply two 100A service disconnects, one for each dwelling unit. And 100A + 100A < 220A, meaning your statement would not be true.

However, the idea of needing more than 190A of service conductors is such a scenario is stupid, so I adopt the position that you can obviously just add up the 95A load for each dwelling unit and get a total load of only 190A. In which case your statement would be true.

looking at service conductors ampacity size does not establish the the service rating for a building having two to six service disconnects

Yes, it does (with the modification provided by 310.12). It tells you the largest possible load those service conductors may supply.

Cheers, Wayne

 

[jaggedben]

I thought the 2014 change was just fine. There was need to throw in the adjustments, say you had a high ambient temp or for whatever reason multiple feeders in same raceway or wireway.

If no adjustments were needed the straight 83% yielded same minimum conductor size as the previous table allowed anyway.

As Wayne pointed out, a panelboard busbar can be just as much a service conductor or feeder as a wire-type conductor. Do you think it was their intention to allow a 150A breaker (say, in a meter main) to protect a 125A subpanel with all the loads downstream? 125A is 83% of 150A. I don't think they intended that.

I'm not sure how they ended up going down the road they did. But if they wanted the table to apply prior to adjustment and correction, it would have only taken a sentence to say so.

 

[jaggedben]

Also kwired, do you think that if my calculated load for my dwelling is 90A, that the 2014 change intended to raise my required conductor size from 4awg to 3awg, on the same 100A breaker?

 

[tortuga]

Interesting discussion but I think the OP left the building.

The calculated load determines the minimum service rating. The actual service rating depends on the service conductors and equipment installed.

I think that summs it up, you all kinda are saying similar things.

If that's true, then you can come up with an example where the service for a duplex would have a calculated load of, say, 220A, but the calculated load for each dwelling unit is only 95A. So then you'd need to have a service rated at least 220A

That could never happen if you use 220.85 and do a correct two dwelling unit calc.
You figure both units together using the multifamily method 220.84(C) and you get 190A ( you don't do per unit in multifamily but just for this example its 95A per unit)
190A X 1.5 = 285A ( to make three identical units)
T220.84(B) demand factor for 3 units = 45%
285 * .45 = 128A

 

[wwhitney]

That could never happen if you use 220.85 and do a correct two dwelling unit calc.
You figure both units together using the multifamily method 220.84(C) and you get 190A ( you don't do per unit in multifamily but just for this example its 95A per unit)

You don't get 190A, actually. If I back calculate from 95A under the optional procedure with an assumed HVAC load of say 15A (mild climate), I get a "total connected load" of 41.7A (first 10 kVA) + 38.3A / 0.4 (40% of the rest) + 15A (HVAC) = 152A. Under 220.85, that would give you a 2-unit load of 152*3*0.45 = 205A.

Which isn't quite the 220A I assumed in my example, but is still greater than 200A, so it works for my example. And I still say a 190A service for such a duplex would suffice, it makes zero sense to require more than a 200A supply to a load consisting of (2) 100A breakers.

Cheers, Wayne

 

[tortuga]

You don't get 190A, actually. If I back calculate from 95A under the optional procedure

You use 220.84 not 220.82 you can't use the 220.82 at all in a 220.85 calc, becasue 220.85 says to use 220.84 and 220.82 says its only for a individual dwelling unit.
say you have two dwelling units the combined floor area is 1200 SQFT, each has a 10kw electric range, a 4.5kw resistance electric water heater, and a 3.6kw heat pump.
To calculate the service for the two dwelling units you simply add everything up according to 220.84(C) ~190A
Then you multiply by 1.5 to create 3 equal units
Then apply the demand factor for 3 units T220.84(B)
You'd get ~129A for the service entrance
Then I'd re-run those numbers in the 220.82 for the feeders to the individual dwelling units.
which would end up at 72A per feeder.

 

[wwhitney]

You use 220.84 not 220.82 you can't use the 220.82 at all in a 220.85 calc, becasue 220.85 says to use 220.84 and 220.82 says its only for a individual dwelling unit.

I didn't mix 220.82 and 220.85; actually you did. You took the 220.82 individual dwelling unit load, and used it as the "total connected load" for your 220.85 calculation. But the 220.82 load already has the "40% after the first 10 kVA" adjustment, so that's not the input to the 220.85 calculation.

All I did was figure out that if the 220.82 load is 95A, and the HVAC load is 15A, then the "total connected load" is 152A. Which is the input you need to use for the 220.85 calculation, not the 95A figure. And then the 220.85 calculation gives you 205A for 2 dwelling units.

Which is a non-sensical result, that (2) dwelling units with a 95A load each (say on a 100A breaker) require a service size of 205A (say on a 225A breaker).

Cheers, Wayne

 

[wwhitney]

say you have two dwelling units . . .
You'd get ~129A for the service entrance

I didn't check your computations, but based on the 220.85 result you get, I infer the unit "total connected load" is 129A/0.45/3 = 95.5A. And I believe you are correct that you don't have the disparity I mentioned for this case.

Then I'd re-run those numbers in the 220.82 for the feeders to the individual dwelling units.
which would end up at 72A per feeder.

If you're trying to flesh out my example, you need to end up with 95A here, not 72A. The larger load will result in the disparity I mentioned.

Throw in some more fixed appliances with a total nameplate load of (95-72)/.4 = 57.5A. The 220.82 load for each dwelling unit will become 95A. The 220.85 calculation for both units will go up by 57.5*3*.45 = 77.6A to a total of about 205A, as I indicated.

Cheers, Wayne

 

[NoahsArc]

Let me interject an absolute rookie question here:
For 310.12B, "feeder rated 100 amps+", would this include say a 100A subpanel feed fed from a 100A 2p breaker in a main panel? Would this include any 100A motor feeds?

 

[wwhitney]

Let me interject an absolute rookie question here:
For 310.12B, "feeder rated 100 amps+", would this include say a 100A subpanel feed fed from a 100A 2p breaker in a main panel?

Only if that feeder supplies the entire load of a dwelling unit. E.g. if you had a 200A service panel for a duplex, with (2) 100A breakers, each one supplying the entire load of one of the dwelling units.

Would this include any 100A motor feeds?

No, that would never be the entire load of a dwelling unit.

Cheers, Wayne

 

[tortuga]

The 220.85 calculation for both units will go up by 57.5*3*.45 = 77.6A to a total of about 205A, as I indicated.

Wyane I am sorry I have to disagree thats still not how you do a 220.85 Two dwelling calc.
To arrive at your 95A optional take the following:

​	Single dwelling unit feeder 220.82​	​	​	VA​
Line​	Description​	VA​	QTY​	VA​	Neutral​
1​	General Lighting Load​	​	​	​	​
2​	Apartment square Feet​	3​	1200​	3600​	3600​
3​	Small Appliance​	1500​	2​	3000​	3000​
4​	Laundry​	1500​	1​	1500​	1500​
5​	Water heater​	4500​	1​	4500​	0​
6​	Dishwasher​	960​	1​	960​	960​
7​	Electric Clothes Dryer​	5000​	1​	5000​	3500​
8​	Range​	11000​	1​	11000​	7700​
9​	​	​	​	​	​
10​	​	​	​	​	​
11​	Sub Total​	​	​	29560​	20260​
12​	​	​	​	​	​
13​	First 10 kva​	​	​	10000​	​
14​	Remainder​	​	​	19560​	​
15​	40% of remainder​	​	​	7824​	​
16​	​	​	​	​	​
17​	​	​	​	​	​
18​	Ductless Heatpump RLA​	3000​	1​	3000​	0​
19​	Electric resistance heater​	2000​	1​	2000​	0​
20​	​	​	​	5000​	​
21​	Total​	​	​	22824​	​
22​	Amps​	​	​	95​	​

 

[tortuga]

Then a service to supply two of those units I'd calculate like so:

​	Two dwelling units NEC 220.85​	​	​	​	​	​	​
​	220.84 Multi family​	​	​	​	​	VA​
Line​	Description​	VA​	QTY​	NEC Section​	​	Phase​	Neutral​
1​	General Lighting Load​	​	​	​	​	​	​
2​	Total square Feet​	3​	2400​	220.84(C)(1)​	​	7200​	7200​
3​	Small Appliance​	1500​	4​	220.84(C)(2)​	​	6000​	6000​
4​	Laundry​	1500​	2​	220.84(C)(2)​	​	3000​	3000​
5​	Water heater​	4500​	2​	220.84(C)(3)​	​	9000​	0​
6​	Dishwasher​	960​	2​	220.84(C)(3)​	​	1920​	1920​
7​	Electric Clothes Dryers​	5000​	2​	220.84(C)(3)​	​	10000​	7000​
8​	Main House Range​	11000​	2​	220.84(C)(3)​	​	22000​	15400​
9​	Ductless Heatpump RLA​	3000​	2​	220.84(C)(5)​	​	6000​	0​
10​	Electric resistance heater​	2000​	2​	220.84(C)(5)​	​	4000​	0​
11​	Sub Total​	​	​	220.84(C) 1-5​	​	69120​	40520​
12​	​	​	​	​	​	​	​
13​	Multiply Line 11 by 150% to make 3 'dummy’ units​	​	1.5​	220.85​	​	103680​	60780​
14​	​	​	​	​	​	​	​
15​	Apply 45% Demand Factor For Total Load​	​	​	T220.84​	​	46656​	27351​
16​	​	​	​	​	​	​	​
17​	Amps​	​	​	​	​	194​	114​

 

[NoahsArc]

Only if that feeder supplies the entire load of a dwelling unit. E.g. if you had a 200A service panel for a duplex, with (2) 100A breakers, each one supplying the entire load of one of the dwelling units.

I can see this interpretation, but the "For.... X, Y, OR Z" can also be read as "For X, for Y, OR for Z" rather than "For all X that are Y or Z."