Minimum Overcurrent Protection (OCP) on Service

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lmcgill

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Hello all, I would like your opinion on the following: Consider an existing commercial building that is undergoing an extensive remodel - practically all of the existing electrical installations are to be removed and new to be installed. The intent, however, is for the existing 120/240V, 1PH service to remain. Tabulating the building's electrical loads per article 220 III, IV & V yields 450A.

The existing service entrance conductors are 3#750MCM copper THWN (good for 475A per table 310.15 of the 2014 NEC).
The existing main circuit breaker is a 2P-400A, 80% rated circuit breaker in an i-line distribution panel - this is where I'm a bit confused.

My conductors can handle my load, so I'm ok per NEC 230.42 and NEC230.90(A) only sets a MAXIMUM of the OCP on a service.
I do see that NEC 210.20 sets a minimum for OCP on branch circuits and NEC 215.3 sets a minimum for OCP on feeders, but I don't see anything that forces my OCP on my service to have a minimum rating. Thoughts?
 
230.79 does apply to the circuit breaker, but it sets a minimum of 60A. Strangely enough, the code does not explicitly require that the OCPD on a service be sized for the calculated load, AFAIK.
 
jaggedben said:
230.79 does apply to the circuit breaker, but it sets a minimum of 60A. Strangely enough, the code does not explicitly require that the OCPD on a service be sized for the calculated load, AFAIK.
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Did you mean to say that 230.79 applies to the overcurrent protective device? If so that is not correct
 
Dennis Alwon said:
Did you mean to say that 230.79 applies to the overcurrent protective device? If so that is not correct
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Only if the disconnecting means is a circuit breaker, then it ends up applying to the OCPD rating as well since they are the same. If the disconnecting means is a fused disco then you can put in smaller fuses if you want as long as the switch rating complies. I suppose theoretically a breaker with trip settings could be set lower than the 230.79 minimums although that doesn't seem like a real-world situation.

I have to wonder how intentional this is but that's how the code is written.
 
jaggedben said:
Only if the disconnecting means is a circuit breaker, then it ends up applying to the OCPD rating as well since they are the same. If the disconnecting means is a fused disco then you can put in smaller fuses if you want as long as the switch rating complies. I suppose theoretically a breaker with trip settings could be set lower than the 230.79 minimums although that doesn't seem like a real-world situation.

I have to wonder how intentional this is but that's how the code is written.
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I have asked eveyone I can think of to explain it and I have had no help in understanding this... This is the same issue with 225.39.. I can run 10/3 to a detached structure such as a garage install a 30 amp breaker at the house and install a 60 amp disconnect at the garage and it is compliant.. :?
 
There is this to consider as well.


IV. Service-Entrance Conductors

230.42 Minimum Size and Rating.

(B) Specific Installations. In addition to the requirements
of 230.42(A), the minimum ampacity for ungrounded conductors
for specific installations shall not be less than the
rating of the service disconnecting means specified in
230.79(A) through (D).
Click to expand...
 
Dennis Alwon said:
I have asked eveyone I can think of to explain it and I have had no help in understanding this... This is the same issue with 225.39.. I can run 10/3 to a detached structure such as a garage install a 30 amp breaker at the house and install a 60 amp disconnect at the garage and it is compliant.. :?
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The 10/3 may or may not be compliant depending on the load, but I agree with your point. Odd as it is, I have not found any requirement in Chapter 2 that an OCPD be sized in accordance with a calculated load.
 
The OP said that the existing service had an existing 400 amp main circuit breaker. I inferred that that circuit breaker was the building's require disconnecting means. I have almost always used a main circuit breaker on the main service panel to serve that function. If that is the case, then my response citing 230.79 was the answer that the OP sought. So tell us, lmcgill, does the main circuit breaker serve as the required disconnecting means?
 
jaggedben said:
The 10/3 may or may not be compliant depending on the load, but I agree with your point. Odd as it is, I have not found any requirement in Chapter 2 that an OCPD be sized in accordance with a calculated load.
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Actually for services there is something 230.42 as Bob stated however that is for conductor ampacity but this does not exist for outside feeders

(B) Specific Installations. In addition to the requirements
of 230.42(A), the minimum ampacity for ungrounded conductors
for specific installations shall not be less than the
rating of the service disconnecting means specified in
230.79(A) through (D).
Click to expand...
 
lmcgill said:
Hello all, I would like your opinion on the following: Consider an existing commercial building that is undergoing an extensive remodel - practically all of the existing electrical installations are to be removed and new to be installed. The intent, however, is for the existing 120/240V, 1PH service to remain. Tabulating the building's electrical loads per article 220 III, IV & V yields 450A.

The existing service entrance conductors are 3#750MCM copper THWN (good for 475A per table 310.15 of the 2014 NEC).
The existing main circuit breaker is a 2P-400A, 80% rated circuit breaker in an i-line distribution panel - this is where I'm a bit confused.

My conductors can handle my load, so I'm ok per NEC 230.42 and NEC230.90(A) only sets a MAXIMUM of the OCP on a service.
I do see that NEC 210.20 sets a minimum for OCP on branch circuits and NEC 215.3 sets a minimum for OCP on feeders, but I don't see anything that forces my OCP on my service to have a minimum rating. Thoughts?
Click to expand...

I have to agree with Jag on the specific answer -- The existing main circuit breaker is a 2P-400A(which appears to be the rating of the existing service disconnecting means) though it is true nothing "forces"/ requires the OCPD in this case to be a minimum the NEC only states it's size cannot be less than 60 amps. Certainly unrealistic to do -- but the conductors can carry the load presumed even though you can choose the OCPD that would not hold. I'm sure I would have a long discussion with owner/contractor about their consequences if I were to inspect such a outrageous service.
 
Please allow me to reiterate my point.
mwm1752 said:
. . . though it is true nothing "forces"/ requires the OCPD in this case to be a minimum the NEC only states it's size cannot be less than 60 amps.
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If, in fact (and this has not yet been confirmed by the OP), the main circuit breaker in the main switchgear is the building's required disconnecting means, then yes the NEC does force it to be, as a minimum, the 450 amps that represents the calculated load. Straight to the heart of the matter: you would have an existing violation on your hands.

 
mwm1752 said:
I have to agree with Jag on the specific answer -- The existing main circuit breaker is a 2P-400A(which appears to be the rating of the existing service disconnecting means) though it is true nothing "forces"/ requires the OCPD in this case to be a minimum the NEC only states it's size cannot be less than 60 amps. Certainly unrealistic to do -- but the conductors can carry the load presumed even though you can choose the OCPD that would not hold. I'm sure I would have a long discussion with owner/contractor about their consequences if I were to inspect such a outrageous service.
Click to expand...
Would you pass an inspection for OP if it had a 60 amp main breaker but 450 amp conductors, or would you be looking harder for something that seems logical would require at least 450 amp overcurrent protection?

charlie b said:
Please allow me to reiterate my point. If, in fact (and this has not yet been confirmed by the OP), the main circuit breaker in the main switchgear is the building's required disconnecting means, then yes the NEC does force it to be, as a minimum, the 450 amps that represents the calculated load. Straight to the heart of the matter: you would have an existing violation on your hands.

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I agree. Haven't found the code to cite just yet, but am pretty certain it is in there.

I guess with a circuit breaker with a fixed setting - that setting is the rating.

With a fused disconnect - you probably will not find a 450 amp rated disconnect so a 600 amp switch with 450 amp fuses would be what is likely used.
 
charlie b said:
Please allow me to reiterate my point. If, in fact (and this has not yet been confirmed by the OP), the main circuit breaker in the main switchgear is the building's required disconnecting means, then yes the NEC does force it to be, as a minimum, the 450 amps that represents the calculated load. Straight to the heart of the matter: you would have an existing violation on your hands.

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Charlie-I stand corrected with your logic per charging statement of 230.79 nice
Kwire -code is minimum and I do not use the I told you so rule, so sometimes discussion is required prior to decision as stated.
 
Dennis Alwon said:
Actually for services there is something 230.42 as Bob stated however that is for conductor ampacity but this does not exist for outside feeders
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OCPD rating is not conductor ampacity. We're looking for a section that explicitly addresses OCPD rating.

charlie b said:
Please allow me to reiterate my point. If, in fact (and this has not yet been confirmed by the OP), the main circuit breaker in the main switchgear is the building's required disconnecting means, then yes the NEC does force it to be, as a minimum, the 450 amps that represents the calculated load. Straight to the heart of the matter: you would have an existing violation on your hands.
Click to expand...

What code section is being violated?

kwired said:
Would you pass an inspection for OP if it had a 60 amp main breaker but 450 amp conductors, or would you be looking harder for something that seems logical would require at least 450 amp overcurrent protection?
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I'm not an inspector so fortunately there's no real consequence to whatever I answer. ;) For the sake of argument, I would say yes, I would be obligated to pass that installation, unless someone can show me the specific code section that is being violated.

I agree. Haven't found the code to cite just yet, but am pretty certain it is in there.
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Half a dozen people on this thread and no one can find it. I wait with anticipation. :D

I guess with a circuit breaker with a fixed setting - that setting is the rating.
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I believe I've seen this point argued, but I agree with you.

***

This a perhaps a matter of real consequence to me, since in the solar work I do we frequently downsize main breakers. I've been asked by some AHJs to provide load calculations. And I'm wondering if I can't make an argument that it doesn't matter. I wouldn't seriously undersize a breaker to the point that I know it would nuisance trip, but in borderline cases...
 
kwired said:
I agree. Haven't found the code to cite just yet, but am pretty certain it is in there.
Click to expand...
jaggedben said:
OCPD rating is not conductor ampacity. We're looking for a section that explicitly addresses OCPD rating.
Click to expand...
jaggedben said:
What code section is being violated?
Click to expand...
jaggedben said:
Half a dozen people on this thread and no one can find it. I wait with anticipation.
Click to expand...
I will respond to all of these by pointing to my original comment. See post #2.

To put it another way, it is not the OCPD that has a minimum requirement. It is the disconnecting means. But we can, and very often do, use an OCPD (specifically the main breaker on the main switchboard) to serve the role of disconnecting means. Thus, if the main breaker is the disconnecting means, then 230.79 tells us that it must have a rating not less than the calculated load to be carried.
 
charlie b said:




I will respond to all of these by pointing to my original comment. See post #2.

To put it another way, it is not the OCPD that has a minimum requirement. It is the disconnecting means. But we can, and very often do, use an OCPD (specifically the main breaker on the main switchboard) to serve the role of disconnecting means. Thus, if the main breaker is the disconnecting means, then 230.79 tells us that it must have a rating not less than the calculated load to be carried.
Click to expand...

It seems to me this thread is one of those 'the grass is green, no the sky is blue' arguments.

Charlie I do not think anyone is disputing your point about what a disconnecting means is vs a OCPD. I think they are interested in the OPs question of what is the minimum size OCPD allowed for a service?
 
jaggedben said:
OCPD rating is not conductor ampacity. We're looking for a section that explicitly addresses OCPD rating.
.
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I agree but It is unlikely the overcurrent protective device will be sized less than the conductors especially based on 230.79 where the largest would be 100 amp. I also bet that most authority having jurisdiction's will read this section and require 100 amp overcurrent protective device for a dwelling service. If the disconnect and conductors must be 100 amp why would you install a smaller overcurrent protective device.

I have tried to write a change for this section but they insist it is the way they want it.... I don't think they understand what they had written because it makes no sense.
 
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