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secant 07-06-2012 12:45 PM

need help on service to new detached garage
Hello all, First post ever here but I have been reading for some time. I need to bring electricity to a new garage I have built on a lot where I presently live in a camper (house comes later). The camper has 100 amp service that consists of a pole mounted meter base, small 125 amp panel beneath it and a underground wire that runs underground to a 100amp breaker box next to the camper (where it runs the camper and well etc). I have built a garage that I need to bring electricity to and I am interested in any comments regarding a new service layout. What I have planned is as follows: I am thinking about installing a 200 amp meter main with pass through lugs at the pole in place of the meter base and detached panel (I am thinking of either of these ..."QO" type: QC816F200C or the "Homeline" type: RC816F200CH). From this panel I would install a 100 amp breaker to feed the camper with the existing wiring to it. to the garage ( about 200ft away) I wanted to use the pass through lugs to run 4/0 4/0 4/0 #2 ground aluminum in 2" conduit underground to a 100amp sub panel in the garage. At a later date I might upgrade the garage to 200 amps. I am wondering if this is a sound plan and also if a later upgrade to 200 amps in the garage (welder etc.) is possible with that 4/0 AL wire and the 200 amp main panel using the pass through lugs. Thank you and any and all comments are greatly appreciated. Brian

zappa 07-06-2012 02:32 PM

I like meter mains because they are so universal. Everything seems fine with your plan except for a few things. 200' may be a difficult pull in 2" conduit and I'm not sure about the % fill with 4/0. Also, 200 amps with 4/0 AL is a service rating but since you are using it as a seperate run to the barn you would have to downgrade the amps slightly and take into account any voltage drop. I would go ahead and install a 200 amp panel in the barn. They may be cheaper than a 100 amp panel and you can put any size breaker in it.

Added info....I'm not sure if you can use the feed through and 200 amp breaker in the meter main for the barn feed because of the derating that I mentioned above. Hopefully some pros will come along and fill in the details.

secant 07-06-2012 07:00 PM

Thanks Zappa,

It looks more like 150 feet to the garage if that might affect things. Hopefully I can get some other input before a final plan is formed.

Thank you again, Brian

a_lost_shadow 07-06-2012 08:38 PM

For residential applications, the NEC recommends (but doesn't require) no more than a 5% drop to the furthers outlet, with no more than 3% of drop coming from either the feeders or the branch circuit. There are a number of calculators on the web to help with figuring out voltage drop. Playing around with some, I found that you're looking at a 2.2% drop on 240V using 150' of 250 mil, and 2.6% drop using 4/0.

The other part that might come up is whether you'll be allowed to use 4/0 for 200 to the garage & #2 to the mobile home if that's what you have. NEC 310.15(B)(7) allows the use of 4/0 Al for 200 amp & #2 Al for 100 amp service on feeders to a residential panel if it's handling 100% of the load. When you're branching off to go to a subpanel that isn't using 100% of the load, you have to use Table 310.15(B)(16), which limits 4/0 Al to 180 amps, and #2 to 90 Amps. Now I've seen arguments that other articles could apply in your case causing an exception to allow you to use 310.15(B)(7). However, I've seen a lot of confusion on this. So I'd suggest talking with your inspector about it.

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