I built a 12x18 studio and now want to supply the feed for [2] 6' electric baseboard heaters wired on 1 thermostat [1500 watts, ~ 6 amps draw each] and 2 circuits for lights, stereo system , etc. I figure they will be on [2] 15 amp breakers.
The studio is about 175' from the main house. The main house has a 200 amp service and panel. Plenty of space in the box.
What gauge service wire can I get by with to supply the above? Would inside 10-3
run in conduit 18" below the ground work?
I need only enough room in the new studio box for a main breaker, a double breaker [probably 30 amp?] for the heaters, and [2] 15 amps for the duplex/lights. I can only find a small 100 amp box that will allow me the required space. So do I have to use 100 amp service wire to feed that?
Thanks for any help.

 

No. Conduit outside is considered a wet location. If you meant NM-b (AKA Romex) it is not rated for wet locations. If you meant UF-b best practice is to not run cable in conduit except for short conduit sleeves to protect it when needed such as when it enters and leaves the ground. Because of voltage drop at 175 feet #10 would not be large enough for a 30 amp feed which is probably the minimum you need.

I'd suggest a 40 amp feed using #6. Either 6-3 UF-b or conduit with two black #6 THWN, one white #6 THWN, and one #8 green THWN.

Cheapest way to go is usually a 100 amp main breaker kit. It will include some branch circuit breakers and the 100 main amp breaker will serve as your disconnect. You will also need a ground bar for the subpanel and an 8' ground rod at the studio.

 

In addition to what Ray posted your heaters total only 12 amperes so a double pole 15 ampere circuit breaker (not a 30) is the correct choice.

I also strongly suggest that you get a copy of the book Wiring Simplified and read it cover-to-cover. The book is available at many on-line sources as well as the big box mega-mart homecenters and many neighborhood hardware stores. The cost is less than $10. In my area the big box stores have it in the electrical aisle and not the books and magazine sections.

 

Thanks Ray.
OK - no conduit. Just curious, is that because they [code guys] assume the conduit will leak and present a danger?
I will be using NM-b 10-3 for another application and was hoping I could buy a big roll to do both - guess not.
It's really interesting to get the take on this. I posted in a couple DIY forums and get varying opinions.
Here is what one guy said:

"Baseboard heaters @ 6.3 amp each =12.6 amps (needs 20 amp breaker).
Two 15 amp for lights and plugs sounds good.
175 ft sub panel feed will need to be derated for voltage drop @ that distance. 2 pole 40 amp, for example might need a supply feeder of #6. So a 6-3 with ground would work. 18 in. Trench with 1" or 1-1/4" PVC."

So, he agrees with your assessment of using 6-3, but says conduit is a yes.

Is it common practice to run a supply feed [UF-b] without conduit - is it safe?

Yeah, I was thinking I would use a double 40 amp breaker at the main, and a matching double 40 at the new box. Does that sound right?

The small 100 amp box I bought came with a double 100, double 30 and 2 single 15's.
So I could use the 15's for the duplexes. 30's I could use for the heaters. Hmm- you say the 100 amp could serve as the disconnect. How would that work?

Cost is adding up on this - 6-3 is not cheap. How do you think that would compare to
"two black #6 THWN, one white #6 THWN, and one #8 green THWN." ?

Oh yeah - And just to make it interesting - I was thinking of having a "switch" in the main house so I could turn everything off in the studio without trekking out there or down to the basement where the main panel is- i figured i'd get a small box with an appropriate breaker.Your thoughts?

Well, many thanks for your input. Much appreciated.

 

I didn't write no conduit. That in fact is probably the best way. It's just that even if well sealed condensate from moisture in the air could fill it.Yes. That is what it is made for but because it is run unprotected minimum depth is 24" as opposed to conduit that is only 18". It is not against code to use UF-b in conduit but it is difficult to pull. You do still need conduit to protect it where it enters and leaves the ground.Yes and No. Yes 40 amp at the house panel but no not 40 at the subpanel. It comes with a 100 amp main breaker and that is fine* because it is only a disconnect not protection. The protection is the 40 amp at the house.You'll will have to run the numbers yourself based on local cost.You could use an unfused toggle type unfused 60 amp air conditioner disconnect but I don't see any reason to do that. You can always turn it off at the house breaker box.

*Verify that the minimum size wire that the 100 amp breaker in the subpanel is #6 or smaller. If #6 i too small pigtail larger wires to it.

 

Thanks Ray. So - a double 40 at the main and use the supplied 100 double at the studio box?
Not sure what you are saying about verifying the minimum size wire...can you rephrase/dumb it down?

 

Either on the label inside the panel door or on the little paper tag on the 100 amp main breaker you should find the minimum and maximum wire sizes the 100 amp breaker's lugs will accept. Since you are using #6 wire, I think I'd bump my feed breaker at the main panel up to either 50 or 60 amps. It wouldn't mean you would use any more power.

 

That is correct.The terminals of the breaker have a minimum and maximum wire size. They are are not rated to properly grip a wire that is too small. Usually that will be in the breaker specs. If for instance the specs for the 100 amp breaker said #4 was the smallest size it could clamp reliably you would wire nut a short length of #4 to the #6 and put the #4 into the breaker. Neutral and ground will fit without a problem because the ground and neutral bars take a much larger range of sizes.

I know the theory but the pros know the hands on. If you have specific questions about your main breaker wire size they can advise you about that.

 

Joe's suggestion of increasing the breaker size at the house is fine but because of your distance just remember if your total load at any time starts to exceeds 40 amps you may see some voltage drop.

 

Thanks guys - I sure appreciate the advice and info!