Wire size for distance

rat423June 4, 2007

I recently discovered that a sub-panel in my garage that feeds a 220v 50 amp plug (5hp air compressor or welder, whatever I am using) has been daisy chained off my dryer wiring in my attic.

The breaker at my main box is a 50 amp. This does not seem safe to me so I would like to replace the 50 amp breaker with a 30 amp and run a new wire from the main box to the dryer with no interuptions. The distance from box to dryer is about 70 feet and all indoors, the origonal wire that I want to replace is aluminum and in bad shape at the point where it was spliced. What size copper wire should I use for 70 feet and 30amps?

Also I am going to put the garage sub-panel on its own 50 amp breaker. The distance to the sub-panel is 95 feet from the main box and I would like to know what size wire would be sufficient for 220v 50 amps at 95 feet, also indoors. I would like to run both wires without conduit through the attic if it can be done safely with these amps and distances. Any advice you can give me would be a great help, I don't like trusting the the guys who sell this stuff retail when a bad piece af advice might mean a fire! Thank you for your time.

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Ron Natalie

You're right you have a problem. The existing wiring sounds extremely bogus.

For the 30A dryer circuit, 10g will probably suffice. I'd pull 10-3 (older circuits may have been grandfathered into not having a separate ground and neutral), new wiring should use a four wire (3 + ground).

For the 50A 95' run, you'll need three current carrying conductors plus ground. You will need either #8 (if you use 75 degree rated cables or beter like USE or THHN/THWN) or #5 if you use 60 degree rated (NM).

In both cases these are the minimum cable size. However when I compute the voltage drop at the full rated current you gave and the lengths given it's not more than 3% in the worst case so I don't seen any need to "Size up".

    Bookmark   June 4, 2007 at 10:58AM
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"You will need either #8 (if you use 75 degree rated cables or beter like USE or THHN/THWN) or #5 if you use 60 degree rated (NM)."

#6 wire. I have never heard of or seen #5 gauge wire.

    Bookmark   June 4, 2007 at 11:33AM
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Thank you for responding to my post, you are right about the wiring in my house being old (1965) all the 220 wiring is aluminum 3 wire (2 110 legs and neutral/ground) I would prefer upgrading both the 30 amp and the 50 amp circuit to 4 wire. If I understand this correctly I would connect the black & red wires to their respective 110 poles and the white & bare wires the the nuetral/ground rail. Also to make sure I understand the proper wire to purchase, I want 10/3 for the 30 amp run & #6 60NM for the 50 amp run? Thanks Again!

    Bookmark   June 4, 2007 at 9:20PM
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You do not want to bond the neutral & ground wires together in the garage subpanel box.

    Bookmark   June 5, 2007 at 7:47AM
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Ron Natalie

The #5 was a typo, that should be #6.

Must be the freaking honey bunches of oats wearing me down.

    Bookmark   June 5, 2007 at 2:09PM
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I assumed the #5 was a typo, thanks for the info ronnatalie! Thank you too "Mike 13" I noticed in my garage subpanel that a ground post has never been configured, only the neutral bar. The incoming power is old 10/2 romex with the ground on the neutral bar but the short wire from subpanel to receptacle is 10/3 romex and both the ground and white wire are on the neutral bar. If I understand you correctly this is not proper at a subpanel? So I need to run a ground wire to a post in the panel to keep them separate? Or is keeping them separate only apply to the incoming power to the subpanel, not the receptacles wired out of the breakers? Sorry for my ignorance of the 4 wire system, the only house wiring I have had any exp with has been REALLY old. Thanks again!

    Bookmark   June 5, 2007 at 10:30PM
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Refer to the FAQ (link @ top of page of electrical forum) describing the installation of a subpanel by Tom O. That is an excellent write-up & explains it in more detail than I want to type.

I don't think the FAQ refers to receptacles so the short answer is you should not have the neutral & ground wires connected anywhere but at the main panel box. They should not be bonded together in a receptacle box. There are some exceptions dealing with detached structure's subpanel boxes but that is not the case here.

    Bookmark   June 5, 2007 at 10:45PM
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Thanks for the link Mike, that explained it quite well. The more I learn the more I see how crappy a job this current circuit is, amazing it has not melted down. In Tom O's tutorial he stated "For a sub-panel in the same dwelling there is no need for additional ground rods or connections to metal piping" So if I understand this correctly I can install an equipment ground bar that contacts my subpanel box to keep the ground and neutral wires separate without connecting the new ground bar to a ground post or water pipe? That sounded kind of odd since the subpanel is mounted to wood studs. But if that is acceptable practice it seems easy enough. Does it sound like I have interpreted the info correctly? Thanks Again!

    Bookmark   June 5, 2007 at 11:34PM
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I reread Tom O's FAQ & he states at the beginning it is written for "remote buildings" but at the end he will list items that can be omitted if the subpanel box is in the same building as the service entrance/main panel box which is your case assuming you have an attached garage.

Bottom line, the neutral bus bar in the subpanel box should be electrically isolated from the subpanel box enclosure & a ground bus bar should be electrically bonded with the subpanel box enclosure. There is no need to electrically bond the ground bus bar to "earth" ground either via ground wires or via metal cold water pipes, again assuming this is an attached garage to a residence that contains the service entrance/main panel box.

Your comment of "That sounded kind of odd since the subpanel is mounted to wood studs. " concerns me to the point that I should really recommend you retain an electrician to fix these problems.

    Bookmark   June 6, 2007 at 12:34AM
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I only meant that the emphasis on a separate ground bar that contacts a subpanel with no earth ground does not seem like a ground. Your "bottom line" paragraph explained the connection as I interpreted Tom O's tutorial so I will proceed with the project. Thank you for your time.

    Bookmark   June 6, 2007 at 12:51AM
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