Overcurrent (Overload) Protection for Service Condutors and Equipment

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Clark Mason @MTC

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Harrisonburg, VA, USA
I'm preparing a class concerning Branch Circuits, Feeders, and Services. In an effort to illustrate the similarities and differences among the three, I've either missed something (most likely), I don't quite understand something (also fairly likely) or the code has a hole in it.

Concerning conductor sizing, the following indicate that conductors shall have an ampacity of 125% of continuous and 100% of non-continuous loads:
210.19(A)(1)(a)
215.2(A)(1)(a)
230.19(A)(1) (service entrance conductors)

The code specifies similar requirements for overcurrent protection for conductors and equipment in:
210.20
215.3
But I can't find any such requirement for service equipment in Article 230 Part VII (or part VI for that matter even that is only for disconnect rating).

I understand that 230 Part VII protection is for the OVERLOAD portion of overcurrent protection only but that seems most applicable to the requirements of 210.20 and 215.3 since g.f. and s.c. currents would be greatly in excess of the 125% and 100% ratings required.

What am I missing?
 
Dennis Alwon said:
Service conductors are generally unprotected conductors. Maybe that is why you are having an issue
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I think I figured it out; although I'm still a little confused why the Code doesn't spell it out a little clearer.

I understand that service conductors are not protected from short circuit and ground fault overloads like branch circuits and feeders are but Part VII of Article 230 is dedicated to the overcurrent (overload only in this case) protection of Service Equipment and Conductors.

Specifically, 230.90 (A) "Where Required. Each ungrounded service conductor shall have overload protection. Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor."

If we go look at 230.42(A)(1), the conductors are sized according to the 125%/100% rule and since the overcurrent device is selected based on the conductor size, indirectly the OCD is sized using the 125%/100% method.

Does that sound about right?
 
Clark Mason @MTC said:
I think I figured it out; although I'm still a little confused why the Code doesn't spell it out a little clearer.

I understand that service conductors are not protected from short circuit and ground fault overloads like branch circuits and feeders are but Part VII of Article 230 is dedicated to the overcurrent (overload only in this case) protection of Service Equipment and Conductors.

Specifically, 230.90 (A) "Where Required. Each ungrounded service conductor shall have overload protection. Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor."

If we go look at 230.42(A)(1), the conductors are sized according to the 125%/100% rule and since the overcurrent device is selected based on the conductor size, indirectly the OCD is sized using the 125%/100% method.

Does that sound about right?
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It sounds fine to me. Just so you recognize that the OCPD in question is part of or adjacent to the service disconnect and thus protects the service wires from a downstream fault just as the downstream OCPD protects tap conductors. There is no useful overcurrent protection at the POCO end of the service conductors.
 
I think the answer is that the best we can do is overload protection for service conductors (and note where there are multiple service disconnets fed by common conductors, the sum of the ratings of the OCPD's can exceed the ampacity of the SEC's). Short circuit and ground fault protection is provided by the short length and other restrictions on SEC's.
 
electrofelon said:
I think the answer is that the best we can do is overload protection for service conductors (and note where there are multiple service disconnets fed by common conductors, the sum of the ratings of the OCPD's can exceed the ampacity of the SEC's). Short circuit and ground fault protection is provided by the short length and other restrictions on SEC's.
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I think that is the reason there is no rule like OP is searching for - the only time you do provide overload protection at conductor ampacity is for an individual service disconnecting means. If you have multiple disconnecting means with a common supply conductor the rules change. You can have six 100 amp main breakers and supply them with a 200 amp conductor if the calculated load is 200 amps or less.

I can think of one installation where I have a Main lugs I line panel as the service equipment, service disconnecting means are two 400 amp breakers and two 125 amp breakers. Service conductors are three 350 aluminum conductors - which is only 750 amps worth of supply conductor, but total load was under that.
 
kwired said:
I think that is the reason there is no rule like OP is searching for - the only time you do provide overload protection at conductor ampacity is for an individual service disconnecting means. If you have multiple disconnecting means with a common supply conductor the rules change. You can have six 100 amp main breakers and supply them with a 200 amp conductor if the calculated load is 200 amps or less.

I can think of one installation where I have a Main lugs I line panel as the service equipment, service disconnecting means are two 400 amp breakers and two 125 amp breakers. Service conductors are three 350 aluminum conductors - which is only 750 amps worth of supply conductor, but total load was under that.
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Why does the NEC let us do that? Its a bit surprising considering how conservative it is with most things. Perhaps interestingly, where multiple OCPD's serve as transformer secondary protection, that sum cant exceed the rating of that of a single (which is 125% of nameplate for most transformers).
 
electrofelon said:
Why does the NEC let us do that? Its a bit surprising considering how conservative it is with most things. Perhaps interestingly, where multiple OCPD's serve as transformer secondary protection, that sum cant exceed the rating of that of a single (which is 125% of nameplate for most transformers).
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IDK, but comes in handy sometimes. The I line I mentioned saved needing a 600 or 800 amp main breaker, plus larger conductor had I used an 800 amp main breaker. I'd bet that application would never tripped a 400 amp main breaker, but we were also considering a possible future load that might easily put it to needing at least a 600 amp main.
 
kwired said:
IDK, but comes in handy sometimes. The I line I mentioned saved needing a 600 or 800 amp main breaker, plus larger conductor had I used an 800 amp main breaker. I'd bet that application would never tripped a 400 amp main breaker, but we were also considering a possible future load that might easily put it to needing at least a 600 amp main.
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Yeah I love that rule, and use it frequently myself. One upgrade I did, I went off utility data. Max demand was about 450 amps. put in a new MLO with 6 discos, that totaled about 1300 amps, ampacity of SEC's was 1000. Utility was serving it with 150 KVA. Kinda a text book example really of utility maxing out their transformers, and NEC service size being 2.5 times actual demand.

Another one I did was 760 amps of SEC's, sum of mains was 1200.
 
Clark Mason @MTC said:
I think I figured it out; although I'm still a little confused why the Code doesn't spell it out a little clearer.

I understand that service conductors are not protected from short circuit and ground fault overloads like branch circuits and feeders are but Part VII of Article 230 is dedicated to the overcurrent (overload only in this case) protection of Service Equipment and Conductors.

Specifically, 230.90 (A) "Where Required. Each ungrounded service conductor shall have overload protection. Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor."

If we go look at 230.42(A)(1), the conductors are sized according to the 125%/100% rule and since the overcurrent device is selected based on the conductor size, indirectly the OCD is sized using the 125%/100% method.

Does that sound about right?
Click to expand...

Sounds about right. I was going to say something like 'doesn't it say somewhere that the service conductors have to be sized to the calculated load, and doesn't it say somewhere else that they need overcurrent protection?'
 
Clark Mason @MTC said:
Concerning conductor sizing, the following indicate that conductors shall have an ampacity of 125% of continuous and 100% of non-continuous loads:
210.19(A)(1)(a)
215.2(A)(1)(a)
230.19(A)(1) (service entrance conductors)

The code specifies similar requirements for overcurrent protection for conductors and equipment in:
210.20
215.3
But I can't find any such requirement for service equipment in Article 230 Part VII (or part VI for that matter even that is only for disconnect rating).
Click to expand...

There is nothing in Article 230 regarding the overcurrent protection sizing because 215.3 already covers it.
 
Clark Mason @MTC said:
How so? Isn't 215.3 just concerned with feeders, not service entrance conductors? Is there an intervening step that I'm missing that links feeder protection with SE protection?
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A quick scenario:

You have a load which is 100A continuous plus 100A non-continuous. You have service entrance conductors come to your main service disconnect, and then feeder conductors run from the service disconnect to the main panelboard.

230.42 tells you that your service entrance conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.2 tells you that your feeder conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.3 tells you that the feeder conductors must be protected against overcurrent at not less than 100A+1.25*100A= 225A.

The overcurrent protection for the feeder will be at the main service disconnect. There is no need to have anything in Art. 230 about sizing the overcurrent protection for the service conductors because 215.3 already covered the sizing of that overcurrent protection device.
 
david luchini said:
A quick scenario:

You have a load which is 100A continuous plus 100A non-continuous. You have service entrance conductors come to your main service disconnect, and then feeder conductors run from the service disconnect to the main panelboard.

230.42 tells you that your service entrance conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.2 tells you that your feeder conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.3 tells you that the feeder conductors must be protected against overcurrent at not less than 100A+1.25*100A= 225A.

The overcurrent protection for the feeder will be at the main service disconnect. There is no need to have anything in Art. 230 about sizing the overcurrent protection for the service conductors because 215.3 already covered the sizing of that overcurrent protection device.
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What if there is no feeder? What if all the loads are connected to a panelboard that also contains the service disconnect?
 
david luchini said:
A quick scenario:

You have a load which is 100A continuous plus 100A non-continuous. You have service entrance conductors come to your main service disconnect, and then feeder conductors run from the service disconnect to the main panelboard.

230.42 tells you that your service entrance conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.2 tells you that your feeder conductors must have an ampacity that is not less than 100A+1.25*100A= 225A.

215.3 tells you that the feeder conductors must be protected against overcurrent at not less than 100A+1.25*100A= 225A.

The overcurrent protection for the feeder will be at the main service disconnect. There is no need to have anything in Art. 230 about sizing the overcurrent protection for the service conductors because 215.3 already covered the sizing of that overcurrent protection device.
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I still disagree 215.3 is for overcurrent protection of a feeder.

Overcurrent protection of a service conductor is covered in 230.90. But as has already been mentioned, with 2-6 service disconnecting means you can have a common supply side conductor that doesn't have overload protection at the conductor's ampacity. Individual conductors to each disconnecting means must have ampacity in accordance with the overcurrent protection level of the particular disconnecting means however.

If I have 4 - 100 amp service disconnecting means supplied by a 4/0 copper common supply conductor - the total calculated load can't exceed the ampacity of the 4/0. Taps off that 4/0 to each 100 amp disconnect must be 100 amp conductors though. However for dwellings you can apply 310.15(B)(6) ampacity allowances.
 
david luchini said:
The overcurrent protection for the feeder will be at the main service disconnect. There is no need to have anything in Art. 230 about sizing the overcurrent protection for the service conductors because 215.3 already covered the sizing of that overcurrent protection device.
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I think it's sloppy to say it that way. Rather I would say the same OCPD is meeting both the 230 and 215 requirements.
 
electrofelon said:
I think it's sloppy to say it that way. Rather I would say the same OCPD is meeting both the 230 and 215 requirements.
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I agree.

Is also possible to have a service disconnect that provides overcurrent protection for the service conductors, and then the load side of service disconnect is all feeder taps to smaller overcurrent devices. 215.3 doesn't have anything to do with the service conductors in such a installation.
 
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