Service Size And Continuous Loads

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
A 2017 NEC question, so that 2020 NEC issues of one service disconnect per enclosure and an outdoor emergency disconnect don't apply:

Looking through Articles 215, 220, and 230 for reference to 125% for continuous loads, I just find 215.2(A)(1)(a) (minimum feeder size), 215.3 (overcurrent protection), and 230.42(A)(1) (service-entrance conductors minimum size). In particular, I don't find an article 230 equivalent of 215.3, and 230 Part VII on OCPD doesn't mention size. 230.79 does say the service disconnecting means has to be rated for calculated load, but does not mention a 125% factor, so that would just be continuous + non-continuous loads.

Then say you have a residential underground service where the underground service conductors terminate directly on a meter/main panel, so there are no service-entrance conductors (to keep things simple). If that meter/main has just a single main breaker, then that breaker supplies a feeder subject to 215.2(A)(1)(a), and that OCPD is subject to 215.3 for sizing. That means the OCPD would be sized with a 125% factor for continuous loads, causing the meter/main to be similarly sized.

Now suppose you instead have two main breakers in your meter/main, supplying two separate feeders, one with a calculated load of 100A continuous, and one with a calculated load of 100A non-continuous. Then as I understand the above sections, you could use a 200A rated meter/main, with one 125A breaker for the continuous loads, and one 100A for the non-continuous loads. This effectively avoids the 125% factor for the service equipment.

Is that all correct?

Thanks,
Wayne
 

tortuga

Senior Member
Location
Oregon
Occupation
Electrical Design
In particular, I don't find an article 230 equivalent of 215.3, and 230 Part VII on OCPD doesn't mention size.
Here you go
2017 NEC said:
230.42 Minimum Size and Rating.

(A) General. Service-entrance conductors shall have an ampacity
of not less than the maximum load to be served. Conductors
shall be sized to carry not less than the largest of 230.42(A)(1) or
(A)(2). Loads shall be determined in accordance with Part III, IV,
or V of Article 220, as applicable. Ampacity shall be determined
from 310.15. The maximum allowable current of busways shall
be that value for which the busway has been listed or labeled.
(1) Where the service-entrance conductors supply continuous
loads or any combination of noncontinuous and continuous
loads, the minimum service-entrance conductor size
shall have an allowable ampacity not less than the sum of
the noncontinuous loads plus 125 percent of continuous
loads
Exception No. 1: Grounded conductors that are not connected
to an overcurrent device shall be permitted to be sized at 100 per-
cent of the sum of the continuous and noncontinuous load.
Exception No. 2: The sum of the noncontinuous load and the
continuous load if the service-entrance conductors terminate in
an overcurrent device where both the overcurrent device and its
assembly are listed for operation at 100 percent of their rating
shall be permitted.
(2) The minimum service-entrance conductor size shall have
an ampacity not less than the maximum load to be served
after the application of any adjustment or correction factors
(A) General. Service-entrance conductors shall have an ampacity
of not less than the maximum load to be served. Conductors
shall be sized to carry not less than the largest of 230.42(A)(1) or
(A)(2). Loads shall be determined in accordance with Part III, IV,
or V of Article 220, as applicable. Ampacity shall be determined
from 310.15. The maximum allowable current of busways shall
be that value for which the busway has been listed or labeled.
(1) Where the service-entrance conductors supply continuous
loads or any combination of noncontinuous and continuous
loads, the minimum service-entrance conductor size
shall have an allowable ampacity not less than the sum of
the noncontinuous loads plus 125 percent of continuous
loads
Exception No. 1: Grounded conductors that are not connected
to an overcurrent device shall be permitted to be sized at 100 per-
cent of the sum of the continuous and noncontinuous load.
Exception No. 2: The sum of the noncontinuous load and the
continuous load if the service-entrance conductors terminate in
an overcurrent device where both the overcurrent device and its
assembly are listed for operation at 100 percent of their rating
shall be permitted.
(2) The minimum service-entrance conductor size shall have
an ampacity not less than the maximum load to be served
after the application of any adjustment or correction factors
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Here you go
Yeah, I referred to that section in my OP. That's the 230 equivalent of 215.2(A)(1)(a) on sizing conductors. I don't see a 230 equivalent of 215.3 on sizing OCPD. And in my example, there are no service entrance conductors (just service conductors), so 230.42 has no bearing.

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Sure, where there is only one service OCPD, so you can refer to "the" service OCPD. But with multiple OCPD, as in my example, as under 408.36 Exception 1, that wouldn't apply, as far as I can see. So a 200A MLO service panel with 100A and 125A breakers, while if there were a single OCPD, it would have to be 225A, and the so the panel bus would have to be 225A instead of 200A.

Cheers, Wayne
 

kwired

Electron manager
Location
NE Nebraska
Sure, where there is only one service OCPD, so you can refer to "the" service OCPD. But with multiple OCPD, as in my example, as under 408.36 Exception 1, that wouldn't apply, as far as I can see. So a 200A MLO service panel with 100A and 125A breakers, while if there were a single OCPD, it would have to be 225A, and the so the panel bus would have to be 225A instead of 200A.

Cheers, Wayne
If total load calculation is 201- 225 then yes. Keep in mind any possible demand factors to apply could be different for individual feeders than it is for both together.
 

mwm1752

Senior Member
Location
Aspen, Colo
If total load calculation is 201- 225 then yes. Keep in mind any possible demand factors to apply could be different for individual feeders than it is for both together.
Agreed - keep in mind the service conductors shall carry the load also. In any case you have described if by some chance the feeders total load draws the 200 amps at least the Main breaker would protect equipment & conductors. Still the design is based upon load calculations prior to installation.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I think this would be a violation of 408.30 and 408.36.
408.36, I don't see any violation, the scenario complies with Exception 1.

408.30 is more interesting, thanks for pointing that out. The text is "All panelboards shall have a rating not less than the minimum feeder capacity required for the load calculated in accordance with Part III, IV, or V of Article 220, as applicable." Article 220 does not mention a 125% factor for continuous loads, that only comes in with Article 215 on feeders.

So one could say that the phrase "feeder capacity" implicitly invokes Article 215, and the 125% factor applies to the panelboard rating. Or one could say that as only Article 220 is referenced, there is no 125% continuous load factor for these busbars, so the scenario complies with 408.30.

My first thought is to take the latter position. After all, in the scenario, nothing is being loaded above its limits. MLO panel busbar ratings are continuous ratings, right?

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If total load calculation is 201- 225 then yes.
I don't follow.

In the scenario, the total load is 200 amps, of which 100 amps is continuous. Obviously any feeder, service entrance conductors, or OCPD carrying the full load would have to be sized at 225A to comply with 215.2(A)(1)(a), 230.42(A)(1) or 215.3, respectively. But in the example scenario there are none of those. The only conductors carrying the full load are service conductors before the service point (not subject to the NEC, I understand), and the MLO panel busbars. And as far as I can see, the MLO panel busbars only need to be 200A.

Cheers,
Wayne
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
408.36, I don't see any violation, the scenario complies with Exception 1.
I don't believe the exception removes the requirement to protect the panelboard at it's rating, it just removes the requirement for an individual ocpd to provide that protection. I don't see how 225A worth of overcurrent protection is protecting a 200A panel bus.

408.30 is more interesting, thanks for pointing that out. The text is "All panelboards shall have a rating not less than the minimum feeder capacity required for the load calculated in accordance with Part III, IV, or V of Article 220, as applicable." Article 220 does not mention a 125% factor for continuous loads, that only comes in with Article 215 on feeders.

So one could say that the phrase "feeder capacity" implicitly invokes Article 215, and the 125% factor applies to the panelboard rating. Or one could say that as only Article 220 is referenced, there is no 125% continuous load factor for these busbars, so the scenario complies with 408.30.
The reference to Article 220 is for the "load calculated in accordance with." Since Article 220 doesn't say anything about "feeder capacity," I would say it necessarily invokes Article 215.

If 408.30 said "all panelboards shall have a rating not less than the load calculated in accordance with Part III, IV, or V of Article 220, as applicable," then I think a 200A bus would have been compliant for the situation you describe.
 

tortuga

Senior Member
Location
Oregon
Occupation
Electrical Design
two main breakers in your meter/main, supplying two separate feeders, one with a calculated load of 100A continuous, and one with a calculated load of 100A non-continuous. Then as I understand the above sections, you could use a 200A rated meter/main, with one 125A breaker for the continuous loads, and one 100A for the non-continuous loads. This effectively avoids the 125% factor for the service equipment.
so your talking about this set up:
meter_w_6main-n.png

I think 230.90 Exception 3 covers you with this.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
I don't believe the exception removes the requirement to protect the panelboard at it's rating, it just removes the requirement for an individual ocpd to provide that protection. I don't see how 225A worth of overcurrent protection is protecting a 200A panel bus.
OK, this is more subtle than I realized, and really isn't clear to me.

408.36 starts off saying "in addition to the requirement of 408.30, a panelboard shall be protected . . ." and describes protection that could reasonably be called individual protection. So when the exceptions say "Individual protection shall not be required . . ." in certain conditions, does that mean that instead (a) the requirement of 408.30 are sufficient or (b) no, OCPD protection is still required, but it doesn't have to be individual (add up the ratings of multiple OCPD).

If that were the whole story, then I would say that (a) is correct, as inferring a procedure of adding is too much, when 408.36 starts off by telling us what the baseline protection is. But Exception 1 says in the second sentence "In panelboards protected by three or more main circuit breakers or sets of fuses." Now we definitely have the idea of protection by multiple OCPD, and certainly for Exception 1 it seems reasonable to say that we must take interpretation (b) above.

Then the question is whether that clarifies the meaning of "individual protection shall not be required" in all cases to mean (b) above, or if that only applies to Exception 1. I guess I lean towards the former, but I can see it either way.

The upshot is that my scenario is only plausibly compliant with 408.36 if it meets Exception 3 (existing panelboard) and we interpret that exception to mean 408.30 protection is sufficient.

As to 408.30, I still disagree that the phrase "feeder capacity required for the load calculated in accordance with Part III, IV, or V of Article 220, as applicable" means we are referred to the requirements of 215.2(A)(1)(a). Both feeder and capacity and terms used in Parts III, IV, and V of 220, and "feeder capacity" is not an Article 100 defined term. So I see no reason to look farther than Parts III, IV, or V of Article 220, and nothing there refers to 125% or refers us to Article 215.

Cheers, Wayne
 

tortuga

Senior Member
Location
Oregon
Occupation
Electrical Design
But your example has two mains and is per 230.71 right?

meter_w_2main.png



Now suppose you instead have two main breakers in your meter/main, supplying two separate feeders, one with a calculated load of 100A continuous, and one with a calculated load of 100A non-continuous.
2017 NEC said:
408.36 Exception No.1
Individual protection shall not be required for a panelboard used as service equipment with multiple dis-
connecting means in accordance with 230.71.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
But your example has two mains and is per 230.71 right?
Yes. So 230.90 Exception 3 says that only 200A service conductors are required.

But in the example, I understand the service point to be the termination of the underground service conductors on the meter/main terminals. That means the only service conductors are either (a) before the service point (and not subject to the NEC) or (b) the panel busbars.

So unless 230.90 Exception 3 somehow overrules Article 408, we still need to determine if Article 408 allows busbars rated at 200A, or if it requires 225A rated busbars.

Cheers, Wayne
 

tortuga

Senior Member
Location
Oregon
Occupation
Electrical Design
Yes. So 230.90 Exception 3 says that only 200A service conductors are required.

But in the example, I understand the service point to be the termination of the underground service conductors on the meter/main terminals. That means the only service conductors are either (a) before the service point (and not subject to the NEC) or (b) the panel busbars.

So unless 230.90 Exception 3 somehow overrules Article 408, we still need to determine if Article 408 allows busbars rated at 200A, or if it requires 225A rated busbars.

Cheers, Wayne
Gotchya yeah this is a good discussion, I run into allot of these in existing residential and light commercial installations.
That wording in 408.30 'feeder capacity' to me is what it all hinges on no?

2017 NEC said:
408.30 General. All panelboards shall have a rating not less
than the minimum feeder capacity required for the load calculated
in accordance with Part III, IV, or V of Article 220, as applicable.
There is no feeder supplying the service panelboard in your scenario so 408.30 is moot.
And I would do an article 220 load for the service, a single calc combining everything on both feeders, using demand etc.. I would not add feeder A + Feeder B load calc.
 

kwired

Electron manager
Location
NE Nebraska
I don't follow.

In the scenario, the total load is 200 amps, of which 100 amps is continuous. Obviously any feeder, service entrance conductors, or OCPD carrying the full load would have to be sized at 225A to comply with 215.2(A)(1)(a), 230.42(A)(1) or 215.3, respectively. But in the example scenario there are none of those. The only conductors carrying the full load are service conductors before the service point (not subject to the NEC, I understand), and the MLO panel busbars. And as far as I can see, the MLO panel busbars only need to be 200A.

Cheers,
Wayne
If the load was calculated using art 220, particularly when using any allowable demand factors, is there any continuous load in there that you have to add an additional 25% to?

Straight continuous @ 125% plus 100% of non continuous yes the original scenario would need 225 amp supply/overcurrent device
 

tortuga

Senior Member
Location
Oregon
Occupation
Electrical Design
OK
408.30 applies to panelboards supplied by both feeders and service conductors.
David will all due respect, there is no feeder (an NEC defined term) in the context, they are service conductors.
Either way I argue:
  • 'feeder capacity' is undefined in the NEC
  • 'feeder capacity' is synonymous with the defined term "ampacity" when there is no feeder in the context.
The text of 408.30 is posted above, and it does not reference 215.

meter_w_2main-s2.png

Straight continuous @ 125% plus 100% of non continuous yes the original scenario would need 225 amp supply/overcurrent device
Yeah but were discussing what code section requires the added 25% in the above scenario.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
OK

David will all due respect, there is no feeder (an NEC defined term) in the context, they are service conductors.
Which is completely irrelevant. 408.30 says "ALL panelboards shall have a rating not less than..."

Unless the NEC has a definition of "All" which is different than the dictionary definition of "All"



Either way I argue:
  • 'feeder capacity' is undefined in the NEC
  • 'feeder capacity' is synonymous with the defined term "ampacity" when there is no feeder in the context.
A lot of things are undefined in the NEC, including 'feeder capacity'. Perhaps because the authors think that the term has a plain meaning that doesn't require a special definition.

For instance, leaving the issues raised in the OP behind, if I were to ask what is the minimum feeder capacity for a 100A continuous load or what is the minimum feeder capacity for a 100A non-continuous load, I would imagine you could provide an answer. I'm sure most people on this forum could provide an answer based on the NEC, without needing a specific definition.
 
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