Wire sizing

[Matt Benton]

I have a subpanel in my garage that is on a 50 amp circuit in the house. I will be running a new line from the garage panel to a building in the back to be a new shop (75 feet from garage panel to shop panel).

One man hobby shop, small space, 11x16. Tablesaw, dust collector and planer all 220v and 3 hp. BS and DP are 110v. Will only run the DC and one tool at a time, along with lights and possibly window unit A/C.

I'm thinking I'd like 60 amps, but my question is, can I replace the 50 amp breaker in the house with a 60 amp, and add a 60 amp breaker to the garage? I can size the new garage-to-shop circuit properly, but I'm worried about the circuit from the house to the garage, it may not be designed for 60 amps.

What do you think?

Thanks!

[Don Jarvie]

What you describe is exactly what I did. I installed a full panel with a 50 amp breaker in it and run this from a 50 amp breaker from my house panel. Part of the decision to have a 50 amp breaker compared to something higher was cost. 6 ga wire was 2.75 a foot and I needed 100ft of it so that 275.00 for wire then add in the conduit, panel, extra breakers, etc it was getting pricey.

Reason for the full panel was number of slots. The panel has 40 slots so I can run my heater, TS, lights, each section of the shop on different circuits and still have room to spare.

In my opinion 50 amp is plently for a 1 man shop. Add up the total number of amps from everything that may be on at one time. The decide what size overall breaker you need.

Finally, before you swap out the 50amp breaker for a 60 see what size the wire is from the house to the garage. If its 6g then stay with the 50 amps. The breaker and wire need to be sized together correctly.

[Steve Meliza]

What size of wire runs from the main panel to the sub panel in the garage? If it is NM-B 6/3 then you can use a 60A breaker as the wire is rated for 55A and you are allowed to up-size to the next largest standard size breaker. If you've got #8 wire then you're limited to 40A if the insulation is rated to 60C and as much as 60A if rated for 90C. Hard to say what you have based on breaker size because so many DIY homeowners make their own rules and you inherit the mess.

If you can up-size the sub panel in the garage to a 60A feeder breaker then I think you can add a 60A feeder breaker in it for the shop, but obviously you're not able to use the shop and garage to maximum load as the same time. If you don't have #6 wire feeding your garage sub panel you could replace it if the run isn't to long or challenging to access.

See NEC 2011 table 310.15(B)(16) for conductor ampacity rules.

[Matt Benton]

Thanks guys! I'll confirm the wire size from house to garage and go from there.

Assuming I can do a 60 amp circuit, can someone remind me of my wire options going from garage to shop? I'm planning to go straight from the garage panel to the outside, then run in conduit under the eaves until I get to the other side of the garage, then down the exterior wall and underground inside conduit to the shop. Would it just be 4 strands of #6?

Also, does the 75 feet from garage to shop not require me to go up in wire size?

Thanks again!

[Steve Meliza]

I'm not up to speed on the code requirements for out buildings so check with the local inspector. There are various factors pertaining to grounding and if you need to run a ground wire or not and if the panel in the shop needs a ground rod or not. Others will probably have relevant experience they can share.

You have to use UF cable or THHW wires in conduit if buried. Again, not sure what your local inspector will require. Your ground wire only needs to be #10 as it is good to 60A per NEC 2001 table 250.122. If you look at 6/3 cable you'll notice the ground wire is #10. Just be aware that you can't use NM cable in a wet location such as the run from your garage to the shop.

[Erik France]

60 Amps @ 220 would be a lot of power for a small one man shop. I ran a 40A line to my 10x12 shed and I don't have any 220v tools. I sized it for 60A @ 220, but I haven't needed to change out the 40A breaker in my main panel. I have a 15A lighting circuit, a couple 20A tool & DC circuits and a dedicated 20A circuit to my large window unit.

I think you're OK with #6 copper at 75' at the loads you are wanting. The distance from your main to your garage Sub-panel should be accounted for as well.

[Matt Benton]

Thanks for the replies!

Erik,

My TS, DC and planer all run on dedicated 20 amp circuits. Using two of those at a time is 40 amps, not including lights or a window unit. I know they don't pull the full 20 amps each when running, but I'd rather spend a little more on wire now knowing that I'm not going to have to dig it up later because I missed something....

[Daniel Berlin]

What size of wire runs from the main panel to the sub panel in the garage? If it is NM-B 6/3 then you can use a 60A breaker as the wire is rated for 55A and you are allowed to up-size to the next largest standard size breaker. If you've got #8 wire then you're limited to 40A if the insulation is rated to 60C and as much as 60A if rated for 90C. Hard to say what you have based on breaker size because so many DIY homeowners make their own rules and you inherit the mess.

As i'm sure you know, NM-B ampacity is always at 60C (334.80), and almost all breakers are only temp rated to 75C, so unless it's THHN-2 in conduit, and he has a 90C breaker, no way he'll get 60A on #8.

[Daniel Berlin]

Thanks for the replies!

Erik,

My TS, DC and planer all run on dedicated 20 amp circuits. Using two of those at a time is 40 amps, not including lights or a window unit. I know they don't pull the full 20 amps each when running, but I'd rather spend a little more on wire now knowing that I'm not going to have to dig it up later because I missed something....

Don't bury UF-B, bury conduit, pull wire, if in 3 years you decide it's not enough, pull different wire. My shed is 100 feet from the house. It took me one afternoon to heat bend all the conduits for some very weird shapes (i could have worked around it, but i had easy access to a proper heat bender), glue them together, and pull all the wire (it took longer to dig the trench, bury, and repair the grass :P). Just oversize the conduit (rigid PVC conduit is very cheap and good for this purpose. Under no circumstances should you use flex conduit) and pulling is easy. I buried 1.5" conduit, which cost maybe 25 bucks in PVC conduit from lowes, and ran 3 #6 conductors + 1 #8 ground in it (they were pre-lubricated). If you tried to do the same thing through 1" conduit, it would be much more painful.
I could technically upgrade all the way to 170 amp service (1/0) in the shed in this conduit (though i'd feel sorry for the guy who had to pull it). UF-B, i'd get to dig it all up and bury it all again.

For giggles, i had buried 1" empty conduit and a pull wire next to my shed conduit (since it was 20 bucks of PVC and the trencher made a 4 inch wide trench.
2 years later, i pulled cat-6 through it to my shed, and now I have a little self contained computer on the wall i use in there to bring up my sketchup and other plans, rather than printing them all out.

[Richard Jones]

.......There are various factors pertaining to grounding and if you need to run a ground wire or not and if the panel in the shop needs a ground rod or not.......

2008 NEC pretty much dictates 4 wire to an accessory building like this, with it's own ground rods. That's my interpretation.

I will ditto the conduit in lieu of UF, you just don't know what lies ahead for you and your endeavors.......... And burying two doesn't take twice as long as one...........cheap insurance.

R

[David Thompson 27577]

+1 on not using UF wire -- use conduit!

When I built my shop, I wired (using underground conduit) for a 60 Amp panel in the shop. A year and a half later, we decided to put in a hot tub, and decided to place it near the shop. By upgrading the house-panel breaker, the wire in the conduit, and my shop panel, I now have 125 Amps in the shop -- most of which goes to the 80 Amp hot tub. This was considerably less expensive than adding the 80 Amps with a new circuit from the house.

And if I want to beef-up some of my machines in the future, the wiring in my shop is already there.

And when selecting the conduit, go one size bigger than you think you need. It'll make pulling those bigger wires so much easier.