Running power to new detached garage

[Ciescokid]

Hi my name is Bob and I am building a new detached garage for my woodshop. I was thinking of running power from my current 200a service box in my house to a 100a subpanel in the garage which would be an approximate 150' run. I plan on burying the wire in 1.5" conduit. Does anyone have advice on wire size/type and grounding methods. I also had to ground the rebar in my footings. Does that become my ground at the service panel or do I need additional grounding rods. Any advice would be appreciated.

 

[matt151617]

Go with #2 mobile home feeder. You may have to contend with some voltage drop, but chances are you'll never need 100 amps. Remember you need 4 wire. Neutral and ground need to be separated in the subpanel.

The ufer ground will suffice, no rods needed.

 

[wyliesdiesels]

#2 AL MHF is rated for 90 amps max! Just make sure to feed it with a 90a breaker!

 

[pattenp]

Ciescokid, where do you live. What country/state. This will make a difference with info you need and products that may be available.

 

[camaro0991]

why use conduit. #2 aluminum direct burial cable is what I used for the two hots and neutral, used a single conductor #6 for ground. Dug 18 inches down n just laid it in.

 

[VHF]

Note that you can still use a 100A subpanel in your garage even if you feed it with #2 AL wire with a 90A circuit breaker in your main panel. Many people have done this and found 90A is plenty (exceptions would be electric heat or running multiple large tools at the same time.)

If you use copper wire you would need three (3) conductors of #3 or #2 CU and are looking at $800 to $1,000 based on your distance. This would allow a "full" 100A service. The ground wire can be smaller #6 or maybe even #8 (check with inspector/electrician.)

 

[wyliesdiesels]

#8 cu or #6 al EGC is for 100a branch circuit! And #3 cu 90C deg. conductors are the smallest u could use to keep the voltage drop down to 3% as recommended by the NEC!

 

[pattenp]

For 100A @ 150' using copper you'd need #2 to keep it at no more than a 3% voltage drop.

 

[wyliesdiesels]

Maybe my math is wrong but I came up with #3.

Vd= (2K*L*I)/CM (2*12.9)*150'*100= 387000/52620=7.35volts! 7.35v/240v=.03

K=resistance L=length I=Amps CM=Circular mills

 

[pattenp]

I use a voltage drop calculator which comes up with #2 Cu with a 5.7 volt drop or 2.375%.

#3 is 7.5 volt drop or 3.125%, so #3 is okay. I just used the #2 because that's what the calculator suggested.

Here's another way to calculate...

CM = (2 X K X I X D) / VD

VD is 7.2V or 3% of 240V

(2 X 12.9 X 100 X 150) / 7.2 = 53750 CM, the next size up is is 66360 which is #2. #3 is 52620, so it's right on the border line.

 

[Ciescokid]

Thanks all for the info. Pattenp, I am in MN to answer your question of where I live. It sounds like running 100a may be over kill and expensive! Maybe I can run 60a to get by. My shop will be used for making acoustic guitars thus I will be closely controlling the temp. And humidity . I will be running AC in summer and heating (radiant in floor powered by LP boiler). Worst case is I could have dust collection (1 1/2 hp),220 v (5 hp) planer, and ceiling air filtration system running together along with lights on. Would 60a be adequate? If so, what wire type for that?

 

[Ciescokid]

I forgot to mention in my last post I could also be running humidifier along with heat in winter.

 

[cj7jeep81]

personally, i'd run the #2 AL mobile home feeder like others have mentioned. Its very cost effective. Breaker it at 90 amp and your fine.

 

[wyliesdiesels]

Thanks all for the info. Pattenp, I am in MN to answer your question of where I live. It sounds like running 100a may be over kill and expensive! Maybe I can run 60a to get by. My shop will be used for making acoustic guitars thus I will be closely controlling the temp. And humidity. I will be running AC in summer and heating (radiant in floor powered by LP boiler). Worst case is I could have dust collection (1 1/2 hp),220 v (5 hp) planer, and ceiling air filtration system running together along with lights on. Would 60a be adequate? If so, what wire type for that?

What is the rated load of everything that will be on @ the same time? With all that on @ once 60a might be borderline but we need to know the load ratings to know for sure!

 

[dankicksass]

Why not get the garage on its own meter? It'll cost you a few hundred more now for the meter and run install, and about three bucks extra a month, but if you ever need to have a real business out there it makes that simpler and realistically it'll probably be a wash in cost anyway.

 

[TRC51]

If you are going to just run it off the house, just get the #2AL and leave the 100A in the garage box, but breaker it at 80A in the house. I actually used a 60A breaker on #2AL and this works well. Basically the 100A in the panel in the garage works as a full panel switch only because the 60A in the panel in the house is max. I had trouble finding an 80A breaker for my panel or I would have bought that.... but the 60A has been more than fine.

The only downside to #2AL is getting it throught the "L's" and 90's in the conduit is a little tougher. But it's not THAT bad. I am back and forth about direct burial cable. It would have been much easier to use direct burial, but I am very glad I put it in conduit (based on the fact that I always seem to be changing my mind about things and dig in my yard a lot). Peace of mind is good.

EDIT: BTW, if you use #2AL, be sure to get at LEAST 2" conduit.

 

[pattenp]

The #2 Al at 150' at a 100% load of 90A will have 3.6% voltage drop which is not too bad. I doubt you'll ever max it out to the 90A. At a 100% load of 75A it will be right at 3%. 3% VD is the recommended max by the NEC for feeders.

 

[cj7jeep81]

Why not get the garage on its own meter? It'll cost you a few hundred more now for the meter and run install, and about three bucks extra a month, but if you ever need to have a real business out there it makes that simpler and realistically it'll probably be a wash in cost anyway.

that will depend on his local electric company. where I am, it's more than $3 a month, and the electric used is billed at a higher rate.

 

[BIMMERBOYZ]

How big is your house? It's probably not any concern but worth asking.