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Discussion Starter · #1 ·
I have a 400 amp 3 wire 208v disconnect. The disconnect will be feeding a transformer.

Please correct me if I am wrong.
I am looking at table 310.15(b)(7) and this table is only for feeders coming into the home, this making this table useless to me.

I turn to table 310.15(b)(20), and find that this temperature rating is suitable for the equipment. And it says that 400mcm will be suitable for about 500amps. Am I correct in saying that 400mcm is suitable for my 400 amp service?
 

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RIP 1959-2015
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39,618 Posts
I have a 400 amp 3 wire 208v disconnect. The disconnect will be feeding a transformer.

Please correct me if I am wrong.
I am looking at table 310.15(b)(7) and this table is only for feeders coming into the home, this making this table useless to me.

I turn to table 310.15(b)(20), and find that this temperature rating is suitable for the equipment. And it says that 400mcm will be suitable for about 500amps. Am I correct in saying that 400mcm is suitable for my 400 amp service?

No.

Look at table 310.15(B)(16)


You can use conductors in parallel 1/0 or larger too.
 

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RIP 1959-2015
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Also table 310.15(B)(7) has been deleted in the 2014 NEC:eek:



310.15(b)(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted be sized in accordance with 310.15(B)(7)(1) through (4).
(1) 
For a service rated 100 through 400 A, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.
(2) 
For a feeder rated 100 through 400 A, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling, unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.
(3) 
In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).
(4) 
Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, provided that the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.
Informational Note No. 1:  The conductor ampacity may require other correction or adjustment factors applicable to the conductor installation.
Informational Note No. 2:  See Example D7 in Annex D.
 

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Discussion Starter · #7 ·
seabee41 said:
Are your conductors going to be supported by a messenger?
No but thanks. I overlooked that wording and now understand the differences between the two.
 

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Discussion Starter · #8 ·
HARRY304E said:
No. Look at table 310.15(B)(16) You can use conductors in parallel 1/0 or larger too.
Got it. So 500 mcm will work for me.

Also how does 1/0 work if it is rated for less?
 

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Discussion Starter · #10 ·
HARRY304E said:
Read 310.10 (H) It's Long...:eek::laughing:
Well... I didn't read I asked my boss and he explained it, but I will read when I am free. Thank you very for your help gentlemen
 
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