Subpanel advise

[Tonkaman]

I’ve always planned to put a subpanel in my detached garage so that I could Enjoy using more than one wimpy circuit.

The plan has always been to trench my way from the meter all the way around the house and over to the shop. Recently I had an idea to just run the wire through my attic instead of trenching.

Here is a birds eye view of my house. The meter is on the top left and the shop at the bottom.

I’ll just mimic what the solar conduits are doing. Up the wall, into attic, out the other end, down the wall, short trench to shop, and up into panel.






I figure about 105’ of wire minimum to get there. The house has a 100 amp main breaker, and I’d like to put the biggest breaker possible in the new subpanel.


Is this a viable option to run through the attic instead of trenching?

 

[mbryson]

I'm far from an electrician but have been working with them of recent. From what I've seen building my house, I think you'd be fine with running the wire through the attic? We ran 200 amp to my house, 100 from the main box to a 100 amp box in the garage in the same way you're planning to in the attic (we went through my basement ceiling). The wire was NOT cheap but you'd have to pay for that whether in conduit or otherwise.

 

[Corban_White]

That should work just fine. You always need more wire than you measure. Open the door of the panel next to the meter and snap a picture and we can help with that part of the planning. And yes, the wire will NOT be cheap.

 

[frieed]

..... The house has a 100 amp main breaker, and I’d like to put the biggest breaker possible in the new subpanel.

Is this a viable option to run through the attic instead of trenching?

You really only have 100A service? I would have guessed 200A
I put a 100A sub panel in my garage but was quite lucky. The meter is on the exterior of the garage and I put the new panel on the opposite side of the wall inside the garage. Took all of 1 foot of wire.

 

[Tonkaman]

Ok so here is the pictures of my panel by the meter.

Assuming I use 150’ of wire and 100 amp subpanel what would my wire gauge be?

Does in need to be in conduit inside the attic? If so what type?

Can I go the entire length in 1 run or do I need junction boxes?

The biggest reason I’m curious about this now is because I’m getting my attic blown in to R38 and don’t want to go back up there after that’s done.

 

[mbryson]

The electricians did my 65-70’ run in one piece. I would assume you’d want it all in one piece as every termination adds resistance creating heat and loss.

I’m sure there’s some calculator about the wire gauge you need. In this situation, I’d likely enlist a professional to at least do the termination on each end. They can tell you what gauge wire they would use, and how they’d make the run.

 

[nnnnnate]

You really only have 100A service? I would have guessed 200A
I put a 100A sub panel in my garage but was quite lucky. The meter is on the exterior of the garage and I put the new panel on the opposite side of the wall inside the garage. Took all of 1 foot of wire.

I did the same at my last house. I bought like 4 feet just in case and have like 3 feet extra that I didn't toss. If someone can do the same I'd be happy to donate it to the cause.

 

[frieed]

Ok so here is the pictures of my panel by the meter.

sorry, didn't mean to come off as doubting, just a bit surprised.

 

[Tonkaman]

sorry, didn't mean to come off as doubting, just a bit surprised.

No worries at all! I was hoping you knew something I didn’t.

The electricians did my 65-70’ run in one piece. I would assume you’d want it all in one piece as every termination adds resistance creating heat and loss.

I’m sure there’s some calculator about the wire gauge you need. In this situation, I’d likely enlist a professional to at least do the termination on each end. They can tell you what gauge wire they would use, and how they’d make the run.

I’m looking at the charts online, but I’m a little lost when it comes to the temperature ratings of conductor. Is this maximum temperature? In this chart would I choose the 75C column?

 

[shortstraw8]

I believe for 100' run you can get away with 1/0 service cable in aluminum for 100amp. It has been years since I did electrical and I can not find my uglys book. If you are going to be drawing on that 100amp constantly I would suggest 2/0 aluminum service.
With 1/0 aluminum you will need 1"1/2 conduit I would suggest 2" though
Just make sure to nolox your connections to copper, and that you snug it good and not break the lug

 

[Tonkaman]

Tell me if I’m reading this right. 100amp at 75C will need 3AWG copper bumped up one size to 2AWG to accommodate The long run. Or the equivalent aluminum is 1AWG bumped up to 1/O for the long run.

 

[jeeper]

I used a piece of left over dryer cord to run my 220 plug for my welder, and have like 18 plugs on 1 30 amp breaker.. so I don't think I have anything smart to add to this conversation

But when you are ready to run it, I'm there to help!

 

[shortstraw8]

After reading your box door knowing that it is good for 75c, I would say that is enough. There is some de-rating per foot though I think you would still be fine.
Also if you are going to use a constant load of (I think) 70% you need to step up a size.

 

[nnnnnate]

I don't think you should need conduit in the attic space. Sure, getting from the box to the attic and from the attic down to the trench but not up in the space.

What are you wanting to run in the garage? 50 amp welder? Monster compressor? Just a bunch of 110 outlets?

I have a mess of new breakers and I'll toss a few your way if you end up needing them. I know you can get a box+breaker combo from the big box store but if it works out different let me know and I'll check and let you know what I have. (There are styles of breakers so you need to match them to the right style box.)

 

[Tonkaman]

I don't think you should need conduit in the attic space. Sure, getting from the box to the attic and from the attic down to the trench but not up in the space.

What are you wanting to run in the garage? 50 amp welder? Monster compressor? Just a bunch of 110 outlets?

I have a mess of new breakers and I'll toss a few your way if you end up needing them. I know you can get a box+breaker combo from the big box store but if it works out different let me know and I'll check and let you know what I have. (There are styles of breakers so you need to match them to the right style box.)

I don’t see why I’d need conduit in the attic either, but I could see it as a possibility.

As far as what I plan to power, the sky is the limit! At a minimum, 220 welder, 220 air compressor, 220 heater, 220 CNC router, 20amp receptacles, 15 amp lighting.

In my future I’d like the possibility of adding things like lathe, mill, vapor hone cabinet etc.

I know that most of those things won’t run simultaneously so they won’t draw huge amounts of power. It is likely to run heater/air compressor/plus one other piece of equipment at the same time though.

 

[shortstraw8]

If you buy the cable with no outer jacket you will need to protect it in the attic with conduit. If it does not have that outer shield the conduit needs to be a continues unbroken run junction box to junction box. At least that is code.

 

[gijohn40]

I hate to burst your bubble but that main panel doesn't have room for another 2 breakers. so you would need to upgrade that panel before you could even do this. You will need a 100amp dual breaker in the main panel and another in the sub panel. I would go with the 1/0 aluminum and thru the attic you do not need the conduit but outside you will. 1 1/2 inch conduit will be fine.

But like I said your main panel is not set up for any more circuits.

 

[Tonkaman]

I hate to burst your bubble but that main panel doesn't have room for another 2 breakers. so you would need to upgrade that panel before you could even do this. You will need a 100amp dual breaker in the main panel and another in the sub panel. I would go with the 1/0 aluminum and thru the attic you do not need the conduit but outside you will. 1 1/2 inch conduit will be fine.

But like I said your main panel is not set up for any more circuits.

I had my doubts since I didn’t see any more knockouts in there. Honestly it would still be worth replacing that panel to get power over to the shop.

Thank you for your input!

 

[Tonkaman]

I got the wire pulled through the attic, and dug out the trench today.

 

[jeeper]

That’s a big wire! How’d the arm feel after dragging it though the attic?