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fburke 07-26-2007 10:58 PM

Wire size for distance Question
 
I just bought a new house that has a shed in the back yard. It is wired for lights but thre is no electrical feed to itÖI think the homeowner ran and extension cored to itÖ

I want to install a small sub panel (110v) I donít want to run much just some lights, small power tools and maybe a small air compressor no welders or big machinery.

Itís about 75 feet to from the house to the shed and of cours the panel is on the other side of the house so I will need to go another 30 feet or so so about a 110 feet to be safe.

What size pipe and wire should I run and what should I size the breaker at the main panel and at the sup panel.

Itís a 200 amp service with 8 open slots home has electric hot water, stove and dryer, heat is oil.


Thanks




SecretSquirrel 07-27-2007 08:23 AM

You could get by with a 4 circuit panel with a 30amp feeder. One 15 amp circuit for lights, one 15 amp for the power tools, and another 15 amp for the air compressor as it could possible run while you're using another tool. Split the last two circuits between power legs... one on L1 and the other on L2 and that will distribute the load across your 240 volts.

The distance between your house and shed isn't out of the ordinary so you could run 4 #10's in a 3/4" conduit, 2 hots, a neutral, and a ground. You'd need a 30 amp 2-pole breaker at the house panel and no main breaker is required at the sub panel.

If you have the smallest inkling that you would need more power in the future, then now is the time to consider that by bumping up the feeder size. A 40 amp would require #8 wire and a 1" conduit and a 60 amp would require #6 wire and could be in a 1" conduit also. I'm assuming THW wire in all cases. And of course the feeder breaker size would have to be adjusted as well. If you go with a larger feeder I would bump the subpanel up to a 6 circuit panel. Anything larger than a 6 circuit would require a main breaker in the sub. Someone correct me on that if I'm wrong... running on memory here. Also I'm not clear if the receptacles need to be GFI's... need advice on that as well.

dmaceld 07-28-2007 12:47 AM

I would say 110 ft is long enough you should go up one size to avoid excessive voltage drop. For 30 amp feeder that would be #8 rather than #10.

Be sure to run a 4 wire feeder, hot, hot, neutral, & equipment ground. In the subpanel the equipment ground and neutral conductors have to be on separate terminal bars. Also, to be fully code compliant you will need to install a ground rod at the shed and connect the equipment grounds to it.

SecretSquirrel 07-28-2007 09:39 AM

[quote=dmaceld;54887]I would say 110 ft is long enough you should go up one size to avoid excessive voltage drop. For 30 amp feeder that would be #8 rather than #10.
quote]

The voltage drop for #10 wire @ 100' @ 20 amp load for 240 volts is 4.8V.
The voltage drop for #8 wire @ 100' @ 20 amp load for 240 volts is 3.0V.

See Voltage Drop Calculator to "play" with the numbers.

dmaceld, is there a reference either in the code book or some other document that recommends when to bump up conductor size or when voltage drop becomes excessive? And please don't think I'm being a SA. It's a serious question and I would like to know because the last thing I want to do is to hand out "uninformed" advice. Thanks!!!

My apology fburke for getting off topic here (minor hijack) but I felt it would be pertinate to your question. I'll start another thread if you wish.

dmaceld 07-28-2007 10:29 AM

Quote:

Originally Posted by SecretSquirrel (Post 54902)
The voltage drop for #10 wire @ 100' @ 20 amp load for 240 volts is 4.8V.
The voltage drop for #8 wire @ 100' @ 20 amp load for 240 volts is 3.0V.

See Voltage Drop Calculator to "play" with the numbers.

dmaceld, is there a reference either in the code book or some other document that recommends when to bump up conductor size or when voltage drop becomes excessive? And please don't think I'm being a SA. It's a serious question and I would like to know because the last thing I want to do is to hand out "uninformed" advice. Thanks!!!

I didn't find any kind of table in the residential portion of the NEC that I have. I was looking at a table I got at JLC Online, which comes from NFPA I believe, about extension cord lengths, loads, and recommended wire size. I extrapolated from that.

I was sure there was a calculator online but I didn't feel like looking for one! Thanks for the link. If you put in 20 amps at 120V for 110 feet with 10 AWG, the worst case for the proposed subpanel, the drop is 5.3 V. The additional 10 feet adds .5 V! The drop is 4.4%. At 30 amp load it's 7.9 V & 6.6% A local power company rep said their guideline is no more than something like 3% voltage drop in the service line.

With the power company allowed to go to something like 110v on their service, and you subtract 5 from that, the subpanel could potentially go as low as 105 volts. Not likely, but could.

It's all a matter of risk assessment. Make a guess what the worst case scenario might be, what is the probability it will occur, and make a choice about what you're comfortable living with.

Looking at the numbers above, I wouldn't criticize fburke if he opted for 10 AWG, especially considering the cost of copper!


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