Detached Garage and a 7-1/2 HP compressor

[metal4130]

Currently I rent a small ranch that has a decent detached garage. I just bought a 7.5HP compressor with a full load amp draw of 31 amps. I am new to wiring and I just want to throw some questions out there. The main panel to the house is 100A being fed with AWG 2 aluminum wire from the street. The garage has a small 100 amp panel that is fed with AWG 6 aluminum 3 wire. Garage gets its power from a 60 amp breaker in the house.

I want to determine the amp draw that is safe for the #6 wire that is running to the garage, can it actually handle 60A? When I look at a wire chart I see different temps and ampacities ranging from 40-60 amps. What number do I use?

Ideally I would like to have the full 60 amps available to the garage and I am just looking for options to get this done. It might mean I need to go to AWG 6 copper rather than AL. The wires are buried in the yard and seem to be in conduit.

Thanks for the help guys.

 

[wyliesdiesels]

First off, most wire is sized based on the 75c deg column. The 90c deg column is for derating purposes and most terminals such as breaker lugs are not rated for 90c deg. With that being said, the wire feeding your garage could be older wire with a lower rated insulation and thus should be sized on the 60c deg column. Having said that, it should NOT have a 60a breaker protecting it. It should have at most a 50a breaker feeding it!

A few questions for you:
Is there any metalic paths between the garage and the house such as water pipe? If there is, the feeder to your garage has to be 4-wire. If a new feeder is put in, it should be 4-wire as well.
What is the wire distance from house to garage? Voltage drop can play a factor in the amount of amps u can put on the feeder before the voltage will sag too much.
What tools other than the compressor do u plan on using?

The compressor is considered a continous load and should be factored at 125% of full load current. Since the wire SHOULD be protected at 50a, a continous load of no more than 40a should be run on that line!

 

[metal4130]

Thanks for the reply. After reading what you said and also doing some more reading it is clear the current set-up is not really safe. You are correct, the wire is older so I think at best it should have an ampacity of 40A. I think the best thing I can do now is to pull a new 4 wire feed to the garage. The wire distance from the house to the garage would be roughly 75 feet.

The larger loads I know I will have and possibly on at the same time.

31 FLA compressor motor
Window air conditioner (not sure of amperage draw)
8 banks of florescent lights
22A welder (would not use at same time as compressor)

All the other typical little things like a parts washer, power tool chargers, bench grinder etc. that would not all be on at the same time.

Since I’m a renter I would like to do this cheaply which sounds like that means AL wire. The current conduit would not be big enough for 4 wire so I would just go the direct burial route. Seems like a #6 copper or a #4 aluminum would be what I need (65A ampacity). Does that sound correct? What style of wire should I be looking for (THHN etc.)?

Also, I shouldn’t be running 60A continuous load on a 60A breakerm, correct? How do I determine what continuous load is suitable for a 60A breaker?

Sorry for all the questions!!!

 

[metal4130]

I also have Square-D QO panels. Is that of decent quality?

 

[metal4130]

Not too much interest in this I guess. I did more reading and I have decided to just go with 2-2-2-4 mobile home feeder. The stuff is fairly inexpensive, available at the home store, and seems to be real popular for this task. I will still run it off a 60A breaker in the house so it will be plenty oversize. The run goes through the attic for about 15’ so I plan to run 2” PVC conduit the entire run. If I read right MHF can be used inside but it needs to be in a conduit or other raceway. Any comments on the plan?

 

[wyliesdiesels]

Sorry been busy. I'll respond a little later today!

 

[wyliesdiesels]

I must start off by saying that since youre a renter, u should NOT be doing electrical on the place your rent. If something goes wrong, u could be held liable.

As far as continous loads go- you multiply them by 125% to figure out the feeder and breaker size. So a 48a continous load is the max u could run on a 60a circuit/wire! But if youre gonna go to the trouble of running a new feeder, why not breaker it at 90a? And yes MHF can be direct buried but needs to be in conduit inside because it does NOT have an outer sheath!

 

[metal4130]

I guess there is no reason I couldn't breaker it at 90A

Can I just do the labor of digging the trench and running the wire then just let the landlord hook it up?


Is the Square D QO a decent load center or would it be worth changing?

 

[rlitman]

Square D QO is excellent.

Just to clear up the continuous load. The breaker should be 125% of the maximum continuous load.
That means that the maximum continuous load is 80% of the breaker rating.

 

[VHF]

Square D QO is condered one of the best. It is frequently used for commercial installations.

 

[metal4130]

http://ecmweb.com/basics/sizing-circuit-breaker


Thanks guys. This was a really good read and it cleared up a lot of questions.

 

[metal4130]

"CBs that are 100%-rated are permitted to be loaded continuously at their full rating as long as the assembly is listed and conductors are properly connected."


I quoted that from the article linked. What CB's are rated for 100% continuous loads?

 

[rlitman]

They're out there, but not generally used in a residential panel. I have a few at work, but we're talking 1000A breakers with computer controls here.

 

[metal4130]

So the landlord gave me the okay to dig the trench and run the conduit so I have a couple questions.

Do I have to use some kind of glue on the PVC conduit?

I'm going to use 2-2-2-4 Mobile Home Feeder so can I get away with 1-1/2 conduit or is 2" the minimum?

 

[Charles (in GA)]

2" will be a much easier pull. Use the same glue that you use on PVC plumbing.

Charles

 

[wyliesdiesels]

So the landlord gave me the okay to dig the trench and run the conduit so I have a couple questions.

Do I have to use some kind of glue on the PVC conduit?

I'm going to use 2-2-2-4 Mobile Home Feeder so can I get away with 1-1/2 conduit or is 2" the minimum?

Since youre renting, i would get a letter from the landlord stating that he is ok with you doing this work and save the letter in case some issue arises down the road and the landlord denies he gave u permission to do the work. I would also hire a trusted electrician to look over your work before energizing anything! You and the landlord may have issues with the work because youre doing it without a permit!

 

[metal4130]

I got the supplies this past weekend and started out by digging out the old feed. I was surprised to see that it was buried only about 5 inches deep. The feed had no less than 12 splices underground and was made of several different gauges and styles of wire. Originally I bought 2" PVC but after seeing the 2-2-2-4 MHF and how small it actually was I decided to go with 1-1/2" which I believe will do the job just fine.

 

[metal4130]

This is the panel inside the house. The only thing I've done is disconnect and remove the old feed. I'm gonna see if the power company can shut the power off so I can tidy up and combine some of the ground wires. All the ground lugs are about filled so I need to make some more space. If I had it my way I would replace it with a bigger load center but I really don't have that option.

I just wanted to post some pictures to see if you guys saw any issues. I'd like to bring the new wires in through the top center of the load center, is there any problems with that? Thanks guys.

 

[pattenp]

All I can say is it's going to be one royal pain to get conduit to the top of the panel to run the MHF into the panel. Looks like the main service wires are going to be in the way.

 

[metal4130]

No doubt about that! Do you think I would be better off running 2 wires through one of the smaller holes on the right and then 2 though another? I need to wire the neutral and ground of the MHF to the panel ground bar correct?

 

[pattenp]

The MHF needs to enter through one conduit into the panel. You do realize the conduit needs to connect to the panel? And the ground and neutral of the MHF do need to connect to the neutral/ground bar in the main panel. The ground and neutrals are to be separate in the sub-panel, not bonded.

 

[metal4130]

A problem I ran into is how can I bring the service into the wall and then go right to the panel within the same wall stud. I use an LB on the outside but then how do I make another 90 degree change in the wall? It is like I need a 90 degree access pull but I can not find one in 1-1/2"

Also are there any requirements for the height of the LB when entering the garage or the panel height in the garage?

Thank you.

 

[Aceman]

I'd swap your house panel out for a 30+ space QO panel. Then I'd take your 20 space house panel and use it in the garage if you wanted to save a little money.

Your house panel has 24 wires on 18 breaker spaces. Plus your neutral bar is stuffed. Tidying up the wires would look a lot better, but your still lacking on breaker space and neutral/ground holes. You could add a separate ground bar to give yourself more neutral space, but a new bigger panel would be a lot cleaner.

If you are already thinking of having the power company disconnect power to clean up the wiring you're already halfway to a panel swap anyhow!

 

[metal4130]

While I agree, replacing the house panel just isn't an option. Its a small ranch house so I really have no idea why there are so many circuits needed. If it was my home I would be doing exactly as you stated. If I buy the separate ground bar how would I bond it to the existing ground/neutral? Also, do you need to get behind the panel to screw the ground bar in?

 

[pattenp]

There is no requirement on placement height of an LB. There is a height/placement requirement for the panel. See this......
http://www.jlconline.com/panels/q-a--electrical-panel-location-in-kitchen.aspx

 

[metal4130]

Ha, that's great. The garage panel will be just fine but according to the article the panel in the house would fail an inspection. It is located inside an empty kitchen cabinet above the washing machine. How about this one?

A problem I ran into is how can I bring the service into the wall and then go right to the panel within the same wall stud. I use an LB on the outside but then how do I make another 90 degree change in the wall? It is like I need a 90 degree access pull but I can not find one in 1-1/2"

Would my only option be to put 2 LB's back to back?

 

[pattenp]

You can use 2 LB's back to back but the inside LB needs to be accessible. You can't sheet rock over it. Well actually you could sheet rock over it but it wouldn't meet code.

 

[metal4130]

You can use 2 LB's back to back but the inside LB needs to be accessible. You can't sheet rock over it. Well actually you could sheet rock over it but it wouldn't meet code.

Got it, thats what I will have to do. I will leave it exposed. To install the additional ground/neutral bars in the panel do I need to get to the back side to screw them in?

 

[pattenp]

The ground/neutral bar is screwed in from the inside. Look and see if you can find some predrilled holes to use. The add-on bar kit should come with mounting screws.

Edit: the holes are threaded so the screws are not self tapping. Thanks Charles for the correction.

 

[Charles (in GA)]

The ground/neutral bar is screwed in from the inside. Look and see if you can find some predrilled holes to use. The add-on bar kit should come with self tapping screws.

Self threading actually. The holes are punched so on the outside on the back of the panel they look like a volcano sticking up. The screw is a MACHINE thread screw with a slightly tapered point and v cuts on the sides of it to clear the chips. Makes its own machine threads 6-32 or 8-32 or 10-32 on the way thru.

Be sure and buy the same brand of ground bar as the panel is, otherwise (1) the hole spacing won't be correct, and (2) you would violate the listing approvals to use a different brand in most cases.

You may also need a lug to attach the heavier wire to. Most ground bars have holes that accept up to #6 wire. The lug will attach to the ground bar, either sitting on top of it and screwing into it in the place of an existing screw in the bar, or have two tabs that poke into the holes in the ground bar, and are secured by the ground bar screws.

This pic shows the lugs in use at the red arrow on the left and partially covered by the red circle on the right.

Charles

 

[pattenp]

Yeah, I thought about that later that they are regular threaded, but it was late and I said oh the hell with it and didn't sign back on to fix it. Thanks for keeping me straight.

Self threading actually.

 

[Charles (in GA)]

Yeah, I thought about that later that they are regular threaded, but it was late and I said oh the hell with it and didn't sign back on to fix it. Thanks for keeping me straight.

Should have added a smiley face to my post. I knew what you were referring to, just wanted to clarify it for the rest of the world.

Charles

 

[Aceman]

Why can't you run up the wall outside and LB into the back of the panel?

 

[metal4130]

Why can't you run up the wall outside and LB into the back of the panel?

I can't run up the wall on that side of the house because that is where the driveway is which buts right up to the house. To put conduit under the driveway would be an extreme job. Also, directly on the backside of the panel is where the meter is so I'm not sure there would be room.

My plan right now is to come into the panel from the top right. There is a larger knockout there, I believe 1 inch, which I will use. To do so I will have to reduce my 1-1/2” conduit down to an inch. I have good access from above in the attic to push the wire through so I think it can be done. It will be tight but I believe it is possible. I still need to take a length of MHF to the home store to confirm that it will fit into 1” conduit. I still plan to have the power disconnected to move some wires around inside the load center.

I went to Lowes last night and bought a separate grounding bar for both panels. Luckily the house panel does have the holes drilled for it. My question is on the house panel can I just connect this ground bar to the main ground/neutral bars with a length of #10 copper wire?

 

[metal4130]

I was pretty busy this weekend working on this project but it seems to all be coming together. I was able to finish up 90% of the conduit work and fished the cable through it. I ended up doing the 2 back to back LB's and it turned out well. I got a majority of the trench filled in too. I thought I was just going to be able to push the wire through the conduit but I was shut down pretty quick on that one. I had to get a fish tape reel and some pull rope to get it done and it was still not easy at all. It was a two person job. I also finished mounting the garage panel as well as getting the main feed hooked up to it. I bought a set of Greenlee 727 cable cutters to do that job and they worked excellent. They cut the cable with ease and also stripped cable like a dream. I would recommend them to anyone doing work like this. I'm gonna get the power shut down early this week and hope to have the project wrapped up soon after. Thanks again for all the help and guidance, I really appreciate it!

 

[metal4130]

Guys I'm in a serious pinch. How do I size the wire that goes to the additional ground bar?? Help needed!!!

 

[pattenp]

Technically if the second ground bar is properly attached with machine screws and engage the enclosure with no less than 2 threads you don't need a jumper.

Edit: If you do add a jumper use at least a #8 copper.

 

[metal4130]

I just talked to an electrician friend of mine and he told me the neutral wires and ground wires should not be on the same bar together. He said neutral wires go to the neutral bar of the panel and the ground should go the the seperate ground bar (no jumper required). Is he correct in saying this? I am getting conflicting stories on this one!

 

[Norcal]

I just talked to an electrician friend of mine and he told me the neutral wires and ground wires should not be on the same bar together. He said neutral wires go to the neutral bar of the panel and the ground should go the the seperate ground bar (no jumper required). Is he correct in saying this? I am getting conflicting stories on this one!

Which panel are you asking about? It does make a difference.

 

[wyliesdiesels]

I just talked to an electrician friend of mine and he told me the neutral wires and ground wires should not be on the same bar together. He said neutral wires go to the neutral bar of the panel and the ground should go the the seperate ground bar (no jumper required). Is he correct in saying this? I am getting conflicting stories on this one!

Just as NORCAL has pointed out, the panel you're referring to matters! If you're talking about the main service panel, the neutral and ground bus bars SHOULD BE bonded. In a subpanel, they SHOULD NOT be bonded and the neutral bar needs to be isolated/insulated from the panel enclosure!

EDIT: SORRY NORCAL!