May I use 500 MCM Aluminum Feeders for 400 amp service? - Electrician Talk - Professional Electrical Contractors Forum
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Old 07-27-2018, 08:53 PM   #1
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Default May I use 500 MCM Aluminum Feeders for 400 amp service?

I am going to put in a 400 amp service for a residence. I will have a 15' conduit for my feeders for the meterbase. May I use 500 MCM XHHW aluminum conductors for the feed to the meterbase, even though they are rated at 350 amps? I thought I read in the NEC (can't find it) that the feeders may be sized to the load, not necessarily to the service ampacity. Am I correct? My inspector told me one time that those ampacity charts are figured on a 100' or longer run; shorter runs cans be smaller wire. help me out here.
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Old 07-27-2018, 08:58 PM   #2
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315.10 B 7 or thereabouts.

also 230.23

what code cycle are you on.
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Old 07-27-2018, 09:33 PM   #3
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Ampacity charts are based on amperes. There is no length consideration. Resistance (technically impedance) in a cable is in ohms per foot. Cable heating is proportional to I*I*R per foot. Cooling is also per foot. Of course more cable means more voltage drop so that increases with length but current is current. The inspector is just plain wrong.

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Old 07-30-2018, 07:04 PM   #4
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No according to 310.15 B in 2014 you’d need 600 MCM XHHW AL for 83% of desired ampacity
Using 350 MCM copper for a 400 amp next week
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Old 07-30-2018, 08:52 PM   #5
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I've used parallel 4/0 Al on one 400amp service job and the next job, parallel AL 250s.
I'm not the engineer
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Old 07-30-2018, 09:31 PM   #6
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We use 500mcm AL all the time for 320/400A residential services.
320 continuous is less than 350.
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