neutral size rule - Page 2 - Electrician Talk - Professional Electrical Contractors Forum

electures · 10-23-2009, 11:40 AM

Quote:

Originally Posted by RICK BOYD

Where in the code do I justify the size of the neutral being smaller than the hots in a service
I know why I just can't find the rule

Rick

It can be found in 310.15(B)(6) last sentence. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

Please note that this can only be used on 120/240-volt, 3-wire, single-phase dwelling services and feeders. This is why I will only accept SE and SER cable for non-dwelling services if the contractor provides the calculations for the grounded and ungrounded conductors. 2/0 SE/SER cable has a 1/0 grounded conductor. 2/0AL is rated 135A. 1/0 AL is rated at 120A. 4/0 SE/SER cable has a 2/0 grounded conductor. 2/0 is rated at 135A. 4/0 SE/SER cable can only be used on non-dwelling 200A services and feeders if the calculated load does not exceed 180A on the ungrounded conductors and 135A on the grounded conductor.

The only way to undersize the neutral is if the calculated load is provided, otherwise fullsize. If there is no neutral load, then the grounded service conductor is the same size as the GEC per 250.24(C) (1) and 250.66. When the table max is reached then the grounded service conductor increases to 12.5% of the ungrounded conductors.

Hope this helps!

mcclary's electrical · 10-23-2009, 01:20 PM

Quote:

Originally Posted by electures

It can be found in 310.15(B)(6) last sentence. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

Please note that this can only be used on 120/240-volt, 3-wire, single-phase dwelling services and feeders. This is why I will only accept SE and SER cable for non-dwelling services if the contractor provides the calculations for the grounded and ungrounded conductors. 2/0 SE/SER cable has a 1/0 grounded conductor. 2/0AL is rated 135A. 1/0 AL is rated at 120A. 4/0 SE/SER cable has a 2/0 grounded conductor. 2/0 is rated at 135A. 4/0 SE/SER cable can only be used on non-dwelling 200A services and feeders if the calculated load does not exceed 180A on the ungrounded conductors and 135A on the grounded conductor.

The only way to undersize the neutral is if the calculated load is provided, otherwise fullsize. If there is no neutral load, then the grounded service conductor is the same size as the GEC per 250.24(C) (1) and 250.66. When the table max is reached then the grounded service conductor increases to 12.5% the ungrounded conductors.

Hope this helps!

4/0 SE/ SER cannot be used for a non dwelling 200 amp. 310.16 must be used.

electures · 10-23-2009, 02:12 PM

Quote:

Originally Posted by mcclary's electrical

4/0 SE/ SER cannot be used for a non dwelling 200 amp. 310.16 must be used.

230.42(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (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 busawy has been listed or labeled.

(1) The sum of the noncontinuous loads plus 125% of the continuous loads.

(2) The sum of the noncontinuous loads plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent devcie and its assembly are listed for operation at 100% of their rating.

230.79. Rating of Service Disconnecting Means. The sevice disconnecting means shall have a rating not less that the calculated load to be carried, determined in accordance with Part III, IV, or V of Article 220, as applicable.

240.4(B) Devices rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors protected are not part of a multioutlet branch circuit...,

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjuctments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

If you disagree with this, please provide the code section(s)...

mcclary's electrical · 11-01-2009, 08:53 PM

Quote:

Originally Posted by electures

230.42(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (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 busawy has been listed or labeled.

(1) The sum of the noncontinuous loads plus 125% of the continuous loads.

(2) The sum of the noncontinuous loads plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent devcie and its assembly are listed for operation at 100% of their rating.

230.79. Rating of Service Disconnecting Means. The sevice disconnecting means shall have a rating not less that the calculated load to be carried, determined in accordance with Part III, IV, or V of Article 220, as applicable.

240.4(B) Devices rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors protected are not part of a multioutlet branch circuit...,

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjuctments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

If you disagree with this, please provide the code section(s)...

I agree, But when I said a 200,,I meant a true 200

BuzzKill · 11-01-2009, 11:09 PM

Quote:

Originally Posted by Toronto Sparky

I've seen a lot of jobs lately that they spec a larger (or even doubled up) neutral for feeders.

Guess that's a harmonics thing.