Run 240v to my garage

Hi All,

I'm hoping to run a 240v circuit out to my garage. There is an existing 120v circuit that runs out there now, but it is not sufficient for the load.

Here is my existing panel in my house:


Here is the run exiting the back of my house and going underground:


Coming back up and going into my garage:


And the inside of my garage to the disconnect switch:


So I know I'll need a new panel in my garage to go from 240 back down to 120. Can I use the existing wire at 240v? If not, can I use it to pull a new wire through the existing conduit? I also know I'll need to change the connection in my existing panel.

I'm an electrical engineer, so I can follow most of the theory, but it's a super different thing than an electrician... who actually know how to apply all this stuff!

Thanks very much!
Timothy

 

That looks like indoor cable only so it is useless except for a pull string.

What size circuit do you need out at the shed? You are going to be limited by the conduit size without making other changes.

 

I agree. You only have what looks to be a 1/2" conduit and it may take a 1" to properly run a circuit of THWN (Red/Black/White/Green), depending on how much power is needed. Is the garage attached or detached? What sort of loads will you encounter in the garage? Always better to go bigger and not need it than to be in your situation.

 

Assuming a detached structure the 120v feed will have to be abandoned. You can not have two feeds to one structure. Also the existing run does not have enough wires. You need for wires for a 120/240 feed to a subpanel. How many amps does the 240v device draw? What other significant loads will you have now or reasonably expect in the future?

 

Hi All and thanks for the replies!

I haven't calculated the maximum potential load out at the (detached) garage, but it will be something like:
a 1-2 kW heater
some fluorescent lights for lighting... maybe a few hundred watts here?
1-2 pieces of woodworking machinery, like a table saw + a shop vac. This probably works out to be another 2kW?

Is that nuts? I would step back down to 110v at the subpanel in the garage to power the various equipment.

I can get the ID of the conduit if it looks like it's going to be borderline. I think it is 1/2 inch as you guessed.

 

Nominal voltages are 120v and 240. You would not really be stepping down voltages. For 120 you would use one leg of the 240 and the neutral to derive a 120 volts. Using 240 for machinery cuts the amps in half and allows for smaller wire which in a way is an advantage.

From the figures you give you could probably get by with a 30 amp feed but I would suggest a 40 amp feed to give a bit of future proofing. For that you would need two #8 black, one #8 white, and one #10 green THWN wires.

I'd suggest a 12 space 100 amp main breaker panel at the garage so you have plenty of spaces for breakers and a main disconnect (the main breaker) that is code required if you ever exceed six breakers in the panel. Another advantage is that main breaker panel kits which include some branch circuit breakers are usually the cheapest way to go.

You will need to buy a ground bar to add to the subpanel and you will need one or two ½"x8' ground rods. Your EGC (green wire) from the house, #6 GEC (from the ground rod[s]) and all branch circuit grounds go to the ground bar. The neutral will be isolated (no bonding strip or screw used). No ground wires allowed on the neutral bar.

At the house you will use a 40 amp 2 pole breaker. You will need a minimum of ¾" PVC conduit but 1" will make pulling easier.

 

Super helpful. Thank you so much! I will follow up if I have additional questions.

 

Unless this garage is super insulated AND practically air tight you are kidding yourself with only 2kw of heat. I have TWO electric heaters in my garage, a 5.6 and a 6.0 kw. With fairly mild temperatures today, mid forties, it took over an hour to reach 60 degrees which I find the minimum temperature to be comfortable when working.

I would suggest no less than an 80 ampere feeder to the garage.

 

Furd I missed the location as Minnesota. I too was a little concerned about heat. One reason why I said 40a instead of 30 amp, my original thought. If you can't run gas to the garage I agree 80 amps. Gas though might be better or at least cheaper to operate in most areas.

 

hmm... my plan is to insulate my garage, but I was only going to heat it to about 40. I grew up in ND and live in MN now. I typically just put on an extra layer or two.

Are these dual gang breakers rated for the sum of the two, or the individual? (Do i have 200 amps coming in and 60 amps to the house?)

 

Whatever is one the handle of the breaker is the rating. It is not doubled for a 2 pole breaker.

 

Heya,

So I must be planning something wrong. If I'm looking at about an 80 amp breaker, then according to this chart: Ampacity Charts

I should be looking for about #3 or #4 THWN 4 conductor wire.

So I went to the Home Depot (my home improvement store of choice) website, and they don't have anything like this. They have like #10 & #12 in 4 conductor THWN:

THWN-2 - 4 - Wire - The Home Depot

So am I significantly miscalculating or something?

Thanks kindly!
Timothy

 

You need #4 THHN/THWN in single conductors, not a cable. You can get one long piece and cut it into four pieces. Go to electrical supply store, not HD.

 

Actually cut into three pieces of #4 plus a #8 green THWN might be better. One of the #4s will need to be marked on both ends with bands of white tape.

You will need a minimum of 1" conduit but 1¼" will make pulling easier.

 

Yes, using #8 for the ground will save you some money instead of a piece of #4. If using #8 it will have to be GREEN because wire #6 and smaller is not to be remarked with tape or marker to identify.

 

Sorry if I missed it, but are you planning to dig up and replace the conduit? If not, the maximum feeder you can run to the garage in 1/2" pipe is 30A (7200W) using #10 copper wire.

As a fellow north latitude dweller I'd strongly recommend any heating fuel other than electric in a workshop if you can do it. Even propane will be cheaper than electric.

 

You have to balance the capital cost of the heating system against the expected operating cost. In my case the cost of purchasing and installing a natural gas heater would have been well in excess of $1,000. I have about $350 invested in heaters, wiring and a kwh meter (the latter not yet connected) and I calculate it will cost about $1.20 an hour of operating time when I use the heaters.

Of course, my electrical cost of about ten cents per kwh is significantly lower than the national average AND my normal temperatures are a lot warmer than in the Midwest. In addition, I won't be using the heat all that much, yesterday the heaters were cycling, being off about the same amount of time as they were on so the average cost for several hours of heat was about sixty cents per hour. Insulating the garage (project for this summer) and providing an outside combustion air source for my home furnace and water heater should significantly drop the heating requirements.

 

Hi ibpooks,

I don't think you missed it, but yea I am going to have to dig up the conduit unfortunately. ( )