Overcurrent (Overload) Protection for Service Condutors and Equipment

[don_resqcapt19]

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?

It is not possible to provide short circuit and ground fault protection at the load end of a conductor. The only protection that can be provided for service conductors is overload protection as that can be at any point on the conductor and still provide the required protection. Most utility systems do not provide any protection for the service conductors. Short circuits and/or ground faults on service conductors typically have to "burn" open to clear the fault. The is one of the reasons for the more robust bonding that is required for service equipment and raceways.

 

[kwired]

Exactly...And the answer is Art. 215 already covers it. See my post #38 above.



Nothing in 240.21 relates the to minimum required overcurrent protection size for the tap. If I had a 90A continuous load that I wanted to tap from a 400A feeder, 240.21 would permit me to run #3 tap conductors to a 100A c/b. Art. 215 does not.



215.3 tells you the minimum allowable size for the protection of the tap conductor. It is a feeder conductor.

If you have a 90A continuous load you need a 112.5 amp minimum conductor regardless of whether it is a branch circuit, feeder or service conductor. Your overcurrent protection will be next size up which is 125 amps. 10 foot tap rule would require a 125 amp conductor for the tap conductors. 25 foot tap rule would require the tap conductor to be minimum of 133 amps. Outside tap rule would require tap conductors to be 125 amp. None of this is in 215.3 it is all in 240.21. 215.3 leaves us with 112.5 amp conductor on a 125 amp breaker being acceptable - which is what you will have on the load side of the breaker located at the end of the feeder tap.

 

[david luchini]

If you have a 90A continuous load you need a 112.5 amp minimum conductor regardless of whether it is a branch circuit, feeder or service conductor. Your overcurrent protection will be next size up which is 125 amps. 10 foot tap rule would require a 125 amp conductor for the tap conductors. 25 foot tap rule would require the tap conductor to be minimum of 133 amps. Outside tap rule would require tap conductors to be 125 amp. None of this is in 215.3 it is all in 240.21.

You'll have to point out the part of section 240.21 that says you need a 112.5 amp minimum conductor for a 90 amp continuous load for me. I read the entire section and couldn't find it.

 

[kwired]

You'll have to point out the part of section 240.21 that says you need a 112.5 amp minimum conductor for a 90 amp continuous load for me. I read the entire section and couldn't find it.

You missed "regardless of whether it is a branch circuit, feeder or service conductor."

With that example you must have at least 112.5 amp conductor, in all cases (unless 100% rated overcurrent devices are used) 240.21 may make you increase that for tap conductors and may be different result depending on which tap rule is being used.

 

[david luchini]

You missed "regardless of whether it is a branch circuit, feeder or service conductor."

No, I didn't...I highlighted it in the previous post.

With that example you must have at least 112.5 amp conductor

And where does that requirement come from?

 

[kwired]

No, I didn't...I highlighted it in the previous post.



And where does that requirement come from?

215, but that is the general starting point. The fact it is a feeder tap means 215 isn't necessarily the final answer, now you have to go to 240 and follow what is in there for this special condition.

 

[david luchini]

215, but that is the general starting point. The fact it is a feeder tap means 215 isn't necessarily the final answer, now you have to go to 240 and follow what is in there for this special condition.

So you have to find the minimum allowable size of your feeder conductors based on Art. 215, even for feeder tap conductors...seems like I've heard that suggested elsewhere in this thread.

The fact that it is a "feeder tap" has nothing to do with whether Art. 215 is the final answer...Art. 215 isn't the final answer for feeders either. Art. 240 doesn't tell you anything about sizing your conductors for the load. Art. 240 tells you how to protect your conductors against overcurrent. You have to go to Art. 240 for the overcurrent of conductors (and Art 310 for conductor ampacities) in all cases.



All of which is completely irrelevant to the OP's original question of why doesn't Art. 230 have similar requirements for OCPD sizing like 210.20 and 215.3. The answer to that question is, of course, that 215.3 covers it,
whether it's a main building OCPD within a panelboard, whether its a main building OCPD with a single feeder on the load side, or whether its a main building OCPD with feeder taps on the load side.

 

[Clark Mason @MTC]

Let me try it this way...

"The question is...why doesn't Art. 230 have OCPD sizing requirements the same way Art. 210 and Art. 215 do? After all, all 3 articles have the same requirements for conductor sizing."

This, sir, is exactly the question . Sorry I didn't state it that clearly.

 

[Clark Mason @MTC]

I add all the branch circuit loads for the entire building at the main panel (or main breaker), apply Art 220 demand factors, and size my main OCPD per 215.3.

I then go to Art 230 and size my service entrance conductors.....

Isn't this backwards?
230.42(A)(1) specifies conductor ampacity for service entrance conductors based on calculations and adjustments in 220 with 125%/100% applied for continuous/non-continuous loads, then 230.90(A) sets the overcurrent device rating or setting not higher than the allowable ampacity of the conductor selected in 230.42(A)(1).

 

[david luchini]

Isn't this backwards?
230.42(A)(1) specifies conductor ampacity for service entrance conductors based on calculations and adjustments in 220 with 125%/100% applied for continuous/non-continuous loads, then 230.90(A) sets the overcurrent device rating or setting not higher than the allowable ampacity of the conductor selected in 230.42(A)(1).

How do you mean backwards?

 

[Clark Mason @MTC]

I read your post as saying that you sized the OCPD, then sized the conductors - did I read it wrong?

 

[david luchini]

I read your post as saying that you sized the OCPD, then sized the conductors - did I read it wrong?

No, you read that correctly. Read it together with the next line...

What should be included in Art 230 for sizing the main OCPD (similar to Arts. 210 and 215) for the main OCPD that I just sized in step 3 above?

That post was an explanation of why there are no OCPD sizing requirement in Art. 230 similar to Arts. 210 & 215.

 

[Clark Mason @MTC]

That post was an explanation of why there are no OCPD sizing requirement in Art. 230 similar to Arts. 210 & 215.

In your example, you said you sized the OCPD based on 215.3, then selected conductors based on 230.90, correct?

My original question (that you reframed so eloquently) was about the fact that there are not similar requirements in 230 but there are, in fact, requirements in 230, they're just not explicitly spelled out like 210 and 215.

Indeed, the requirements are given in 230.90 and they are based on the conductor sizing specified in 230.42.

So wouldn't the method you indicate be "backwards" in the sense that you selected the OCPD first then the conductors?

 

[david luchini]

In your example, you said you sized the OCPD based on 215.3, then selecting conductors based on 230.90, correct?

My original question (that you reframed so eloquently) was about the fact that there are not similar requirements in 230 but there are, in fact, requirements in 230, they're just not explicitly spelled out like 210 and 215.

Indeed, the requirements are given in 230.90 and they are based on the conductor sizing specified in 230.42.

230.90 does not include minimum overcurrent protection device sizing requirements like 210.20 and 215.3. There is no need for OCPD sizing requirements in 230, because they are already given in 215.3.

210.19, 215.2 and 230.42 are there to make sure that the conductors are large enough to carry the load.

210.20 and 215.3 are there to make sure the overcurrent protective device are large enough to carry the load.

Art. 240 and Section 230.90 are there to make sure that the conductors are properly protected against overcurrent

So wouldn't the method you indicate be "backwards" in the sense that you selected the OCPD first then the conductors?

The method I indicated is the "forwards" method. You don't size your service conductors and OCPD first, and then set about designing a building with loads that match your service size...that would be the "backwards" method. You would figure out your loads first and then determine what service size you would need.

 

[Clark Mason @MTC]

The method I indicated is the "forwards" method. You don't size your service conductors and OCPD first, and then set about designing a building with loads that match your service size...that would be the "backwards" method. You would figure out your loads first and then determine what service size you would need.

Before we get any further lost in the weeds, this is not, by any stretch of the imagination, the method of sizing a service, its related equipment, or its conductors to which I was referring. The only even partially sane method of actually sizing a service would, of course, be to figure out the loads and then size the service.

Maybe we should take this one step at a time.... can we agree that 230.42 gives the method for sizing the service entrance conductor?

 

[david luchini]

can we agree that 230.42 gives the method for sizing the service entrance conductor?

Yes, it gives a method for determining the minimum allowable service entrance conductor size based on the loads.

 

[Clark Mason @MTC]

Great, can we now agree that, without considering the exceptions, 230.90(A), gives us a method for determining the size of the overcurrent (again, just overload) protective device based on the results of 230.42?

 

[jaggedben]

Great, can we now agree that, without considering the exceptions, 230.90(A), gives us a method for determining the size of the overcurrent (again, just overload) protective device based on the results of 230.42?

I don't know if it 'gives us a method.' It just says we have to do it somehow. Gotta look elsewhere for exact methods. There are several different code sections that could come into play. I think I agree with David as to the general principle he's arguing, although I would point out that the OCPD(s) may be protecting various things besides a 'feeder' on the load side. There might be a panelboard, a branch circuit, a inverter output circuit, or various combinations thereof. Each of those items is going to have a code section that is applicable to the load side. As far as I can tell, that's going to trump any calculation based on the service conductors every time.

 

[Clark Mason @MTC]

I don't know if it 'gives us a method.' It just says we have to do it somehow.

I think this may be at the heart of my original question. Unlike 210 and 215, 230 just says the OCPD can't be larger than the allowable ampacity of the conductors selected in 230.42, The "four corners" of 230.90, don't give us adequate instruction on how to size the device like 210 and 215 do.

BTW, I'm not buying the argument that 215.3 figures into this issue at all. 215.3 is for feeder overcurrent protection, not service conductor overcurrent protection. The mathematics may eventually relate 215.3 to 230.90 but there is no specific link either implied or stated in the Code that I am aware of between the two.

 

[david luchini]

Great, can we now agree that, without considering the exceptions, 230.90(A), gives us a method for determining the size of the overcurrent (again, just overload) protective device based on the results of 230.42?

You are conflating two separate issues.

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)

These sections specify a minimum conductor size based on the load.

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).

These sections specify a minimum OCPD size based on the load.

Section 230.90(A) relates to overcurrent protection of the conductors, not to minimum size. Just as Article 240 relates to overcurrent protection of feeder and branch circuit conductors, not to minimum size. Overcurrent protection of the conductor, and minimum size of the conductor are two separate issues. Both must be met in a proper installation.

As a for instance...let's say you had service conductors with a load of 200A continuous. If you look at 230.42, you find you need a minimum conductor size with an ampacity of 250A. If you look at 230.90(A), it says the OCPD must have a rating not greater than the ampacity of the conductor. That would be anything 250A or smaller, so a 225A c/b would comply with 230.90(A). A 175A c/b would comply with 230.90(A), etc.

There is nothing in Article 230, as you pointed out in your original question, that establishes a minimum OCPD size for the service entrance conductors. You have to look elsewhere in the Code for that.