Next size up rule

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mbrooke

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Where does the next size up rule apply? Would it be proper to assume it would work for services under 800amps?
 

Carultch

Senior Member
Location
Massachusetts
Where does the next size up rule apply? Would it be proper to assume it would work for services under 800amps?

It doesn't apply to service conductors. Other rules allow you to use lower ampacity wire than the OCPD for service conductors
It doesn't apply to multi-receptacle branch circuits.
It doesn't apply to taps or transformer secondary conductors.
It doesn't apply to any circuit with an OCPD larger than 800A.

It applies to feeders, and any circuit type not mentioned above. Generally, any circuit which has overcurrent protection from all possible sources, and cannot be overloaded by the nature of the loads.
 

Dennis Alwon

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Where is the article that forbids the use of the next size on service conductors? Art. 230.90(A) exception 2 appears to allow it.

I have been incorrect so much lately I am afraid to even post this. :D

230.90(A) 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. A set
of fuses shall be considered all the fuses required to protect
all the ungrounded conductors of a circuit. Single-pole circuit
breakers, grouped in accordance with 230.71(B), shall
be considered as one protective device.
Exception No. 1: For motor-starting currents, ratings that
comply with 430.52, 430.62, and 430.63 shall be permitted.
Exception No. 2: Fuses and circuit breakers with a rating
or setting that complies with 240.4(B) or (C) and 240.6
shall be permitted.
 

mbrooke

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Ok, so I guess all services have to have the wire increased accordingly, even if 5 or 10 amps lower than the OCPD.
 

don_resqcapt19

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Illinois
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retired electrician
I don't agree with Dennis.
230.90(A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with ea ch ungrounded service
conductor that has a rating or setting not higher than the allowable ampacity of the conductor. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit. Single-pole circuit breakers, grouped in accordance with 230.71(B), shall be considered as one protective device.
The wording in bold says you can't use the "round-up" rule. This is the exact same wording that was used in 240.21 a number of code cycles and the change to the wording that now says "The provisions of 240.4(B) shall not be permitted for tap conductors", was made because code users did not understand the old wording. Remember that the rule in 240.4(B) does not change the ampacity of the conductor. It just permits the use of an OCPD with a rating that exceeds the ampacity of the conductor.

Note that this only applies where you have a single service overcurrent protective device. Where you have multiple service overcurrent protective devices, the service conductors only have to be sized for the calculated load.
So if you are caught on the rule for a single service OCPD, just as a small second service disconnect and as long as your conductors have an ampacity equal to or greater than the calculated load you are good to go:)

Of course you could apply Exception #2 that specifically says you can use the provisions of 240.4(B). (I have to remember to read the exceptions before posting, especially when I an not agreeing with people like David)
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I don't agree with Dennis.

The wording in bold says you can't use the "round-up" rule. This is the exact same wording that was used in 240.21 a number of code cycles and the change to the wording that now says "The provisions of 240.4(B) shall not be permitted for tap conductors", was made because code users did not understand the old wording. Remember that the rule in 240.4(B) does not change the ampacity of the conductor. It just permits the use of an OCPD with a rating that exceeds the ampacity of the conductor.

Note that this only applies where you have a single service overcurrent protective device. Where you have multiple service overcurrent protective devices, the service conductors only have to be sized for the calculated load.
So if you are caught on the rule for a single service OCPD, just as a small second service disconnect and as long as your conductors have an ampacity equal to or greater than the calculated load you are good to go:)

Of course you could apply Exception #2 that specifically says you can use the provisions of 240.4(B). (I have to remember to read the exceptions before posting, especially when I an not agreeing with people like David)

Zing...
 

david luchini

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Location
Connecticut
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Engineer
Of course you could apply Exception #2 that specifically says you can use the provisions of 240.4(B). (I have to remember to read the exceptions before posting, especially when I an not agreeing with people like David)

That's too funny because I did the same thing a while back. I read 230.90 and provided an answer that the next size up rule didn't apply to services. And of course, it was Dennis who politely pointed me to Exception No. 2.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
That's too funny because I did the same thing a while back. I read 230.90 and provided an answer that the next size up rule didn't apply to services. And of course, it was Dennis who politely pointed me to Exception No. 2.

That is the great thing about this place. We all learn from each other :thumbsup:
 

mbrooke

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Location
United States
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Technician
So it is allowed then as long as the device is under 800amps? Re-reading Dennis does have a point. But Don got me thinking...;)
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
So it is allowed then as long as the device is under 800amps? Re-reading Dennis does have a point. But Don got me thinking...;)


You do need to make sure the calculated load is equal to or less than the ampacity of the conductor. For instance I can have a #6/3 nm cable rated 55 amps at 60C but I can install a 60 overcurrent protective device as long as the calculated load is not greater than 55 amps.
 

mbrooke

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Location
United States
Occupation
Technician
You do need to make sure the calculated load is equal to or less than the ampacity of the conductor. For instance I can have a #6/3 nm cable rated 55 amps at 60C but I can install a 60 overcurrent protective device as long as the calculated load is not greater than 55 amps.

Ok that makes sense. :)
 
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