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# Feeding a subpanel

#1
10-19-05, 01:12 PM
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Feeding a subpanel

I am finishing space above an attached garage, and plan to feed a subpanel in the finished space using a circuit breaker in the main panel.

I have calculated the service needed for the finished space is 78amps (it will be a furnished mother-in-law apt.). I have been eyeballing 4/3 NMB wire from Dale Electric to provide the main-to-subpanel feed. However, in one conductor sizing chart I see that 4 guage wire is sufficient for only 70 amps as a "branch or feeder" and sufficient for 100 amps as a "service conductor". The distance between the main and subpanel is 101 feet through the basement and up two floors to the attached apartment space.

1. Should I use 4/3 NMB or use heavier guage 3/3?
2. Does it matter whether I use NMB stapled to the floor joists in the basement, or get THHN and run it through conduit (argh)?
3. Are there any other considerations I should have in mind due to the relatively long run of 101 feet?

Thanks!
Hugh
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#2
10-19-05, 01:25 PM
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You cannot use Table 310.15(B)(6) for this. That's the table that would have given you the 100-amp rating.

So you've got to use Table 310.16. If you use NM-B, you are restricted to the 70-amp rating. If you use individual conductors in conduit, and if your panel and breaker are both rated for 75-degree terminations, then you can use the 75-degree column, where #4 is rated for 85 amps. Or you can achieve the 85-amp rating with #3 NM-B.

78 amps over 101 feet of #4 copper loses only 2% of 240 volts, or 1.6% if using #3. 101 feet isn't that long for a 240-volt feeder.

If you use the NM-B, cable this heavy can be directly stapled to the bottom of the floor joists.

#3
10-19-05, 01:46 PM
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Thank you for that explanation. I'm going to look for #3 NM-B as a matter of simplicity vs. running conduit.

#4
10-19-05, 02:16 PM
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Is it permissible to use 3-3-3-5 SER copper wire for this purpose?

#5
10-19-05, 02:40 PM
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Originally Posted by hdiy
Is it permissible to use 3-3-3-5 SER copper wire for this purpose?
3/3 NM-B is somewhat rare. Cerrowire for example, doesn't even make a 3/3 NM cable. 3-3-3-5 SER copper is acceptable for your installation. It can be stapled directly to the bottom of joists in a basement if needed. Your feeder breaker can be up to 85A for this cable.