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Old 10-15-2012, 09:33 PM   #1
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New Subpanel in Garage


The plan is to put a new sub-panel in the attached garage so I can add 20 amp plugs and some extra lighting and run a 5 HP 240V compressor. I've purchased the EATON box (it can handle 125AMP) and has copper bus bars.
The distance of the wire run is just under 100'. I was going to use copper but I am reading that aluminum may work just as well. The wire path will go up through wall and into the attic and across the entire living section to the garage. It seems like #2 wire will do the job for each leg. My question is: Does the Wire gauge need to change to use Aluminum (THHN or THWN) and does the ground and common need to be the same size (#2). Best if someone could just say the correct sizes to insure I get the right type and size. Also trying to decide whether to use EMT or PVC.
Suggestions?
Thanks

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Old 10-16-2012, 03:41 AM   #2
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New Subpanel in Garage


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Originally Posted by niceez View Post
The plan is to put a new sub-panel in the attached garage so I can add 20 amp plugs and some extra lighting and run a 5 HP 240V compressor. I've purchased the EATON box (it can handle 125AMP) and has copper bus bars.
Use a ground bar kit. Do not bond the neutral bar. It must "float."
The distance of the wire run is just under 100'. I was going to use copper but I am reading that aluminum may work just as well. Copper is forgiving. Al is not. Carefully torque your connections to specs. Use noalox per manufacturers instructions. The wire path will go up through wall and into the attic and across the entire living section to the garage. It seems like #2 wire will do the job for each leg. #2 cu is 95 Amps for 60 degree C terminations My question is: Does the Wire gauge need to change to use Aluminum Yes. (THHN or THWN) and does the ground and common need to be the same size (#2). No Best if someone could just say the correct sizes to insure I get the right type and size. Also trying to decide whether to use EMT or PVC.
Suggestions?
Thanks
From your choice of wire size, I guess you want to run 125A. This amperage is way oversized for your described needs. You are not forced to feed your panel it's rating, less is fine. A 5 hp motor is 28 amps at 240 volts, "by the book." Another 20 or 30 amps will light your basement and run power tools besides. Remember, each leg is 120 volts. So, 20 added amps of feeder is 20 amps on one 120 v circuit and 20 amps on another 120 v circuit. Whole houses used to be fed with 60 amp services.

What exactly will be all on at the same time? If you have ten power tools but only one person, then only one will be on at a time. Or do you want to stick with 125 amps?

What is the temperature rating for the terminations/lugs at each end of the feeder?

I need to know the attic temperature range. What is your location?

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Last edited by Glennsparky; 10-16-2012 at 03:47 AM.
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Old 10-16-2012, 01:40 PM   #3
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New Subpanel in Garage


You Say: From your choice of wire size, I guess you want to run 125A. This amperage is way over-sized for your described needs. You are not forced to feed your panel it's rating, less is fine.

True, It is a 125A box and I am using a 100Amp breaker at the main panel to feed the sub-panel with its own 100 amp breaker.

You say: A 5 hp motor is 28 amps at 240 volts, "by the book." Another 20 or 30 amps will light your basement and run power tools besides. Remember, each leg is 120 volts. So, 20 added amps of feeder is 20 amps on one 120 v circuit and 20 amps on another 120 v circuit. Whole houses used to be fed with 60 amp services.
What exactly will be all on at the same time? If you have ten power tools but only one person, then only one will be on at a time. Or do you want to stick with 125 amps?

I run a vacuum system all the time and a 120V router, at the same time there are lights running as well as a 240v electric heater or air conditioner. Most of the time we use 1850 watt heat guns for 2 to 4 hours while using power tools (ie.. Bandsaws, and such) We will have some devices recharging batteries, and some power tools that are 120V in use. At times there will be the compressor, a 3HP 240V table-saw, and a vacuum system running with all the lights on with the heater and recharging batteries. At times there could be a Travel trailer plugged into a service outlet that this box will feed and of course the trailer has air conditioning adding to the load. The objective is not to over load the sub panel.

---
You Ask: What is the temperature rating for the terminations/lugs at each end of the feeder?
I need to find that out.... Is it written on the box? I am not 100% sure I understand the terms used in the question but will try to learn and understand it. Guessing the feeder is the main box? It is being changed out also.

You Ask: I need to know the attic temperature range. What is your location?
I would have to estimate the Attic temps here in Lewiston, Idaho. Winter Mean temp would be 25 to 40 Summer, guessing 70 to 120.

Just for clarification: The main panel is in the basement, the garage is 18 inches below the main house floor height. (Split entry home).
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Old 10-17-2012, 12:25 AM   #4
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Wow Now I'm worried you don't have enough amps. I've never done an industrial load calc. Someone else may have a better answer. Here's one answer (of a few) that's code, for your 100A breaker.
75 degree C lugs, 4 wires:
Copper, #2 black, #2 black, #3 neutral, #6 ground. EMT or PVC sch. 40, minimum 1 1/4 inches.
Aluminum, 1/0 black, 1/0 black, #1 neutral, #4 ground. EMT or PVC sch. 40, minimum 1 1/2 inches.
For pulling wire, bigger conduit is easier, fewest 90 degree sweeps is best. Mixing some Al and some Cu wire is OK.
The feeder is the wire run between the two panel boxes. Your 125A box has a label inside the front cover. Somewhere on it, or on the main breaker, should read something like this:

Line terminals A,B & N
Suitable for 60/75 C conductors
Wire size copper #14-2/0 AWG. Aluminum #12-2/0 AWG
Torque terminals to 45 Lb.-ins.

On your 100A breaker in the main panel, it will be more cryptic, something like:

Use 60/75 C
CU/AL wire
AWG 4-4/0
In. Lbs 45

Try to balance your 120V loads. Esp. that travel trailer. It can overload one of the 120V feeder legs pretty fast.

Duct seal the conduit ends. Hot and cold air meeting and mixing in the panels can cause condensation.

Last edited by Glennsparky; 10-17-2012 at 03:27 AM.
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Old 10-17-2012, 12:57 PM   #5
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Sparky,
I made a trip to see an electrician who works for Agent Orange (Home Depot) and after we chatted for a minute he said I was required to connect the grounds together at each box.... I figured that actually. He showed me a cable with, Alu 1/0, 1/0, 1/0, 2/0 all inside of it's own sleeve and it meets code. The cost is about 1/3 of Cu cable. The way I spread the load at the plugs it to run two legs to each box, each box has 2 plug units, each plug is on different legs and EMT is used as the ground leg. This way when I plug tools in, I am not straining one breaker and leg. I in turn use a dual breaker so it is single point should off. Looks like I will have enough even if I need 125 amps. BTW, I install plugs at every light so the lights plug in the same way as tool so the lights use both legs to balance the amp usage. Appreciate your insight and willingness to help.
When the big cable enters the garage it will be inside conduit to the Sub panel.
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Old 10-18-2012, 12:57 AM   #6
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Cool beans. Tell your Orange friend that a 125A breaker at the main panel is definitely illegal for SE(R) in this case. Table 310.15(B)(7) only applies when servicing an entire residence. Violations would be 310.15(B)(2) and 338.10(4)(a) "installed in thermal insulation" in the 2011 code.

If you have a pull down ladder to your attic, the code is, don't run the cable within six feet of the hole, and protect the cable from physical damage. So, if you can walk, even a crouch, nail 2x4s on end on either side of the cable. If you have to crawl, no 2x4s necessary.

Yeah, EMT is a legal ground. So you can skip the ground wire in my previous post if you use it. Just make damn sure your connections are tight and your connectors are listed for use. And if threads from your Male Adapter stick into the panel a bond bushing jumped to the ground bar wouldn't hurt.
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Old 10-18-2012, 01:43 AM   #7
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BTW, I'm not worried about "one breaker and one leg" of your Multi Wire Branch Circuits. I'm worried about one leg of your feeder. With your system you're leaving things to chance. If everyone is running equipment that just happens to go back to the half of the double-pole breakers powered from one panel buss. And your 50A single pole breaker to your 50A 120V travel trailer plug is also on the same buss. And your 240V loads are pulling from both busses. Bad things will happen.
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Old 10-18-2012, 02:55 AM   #8
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Originally Posted by Glennsparky View Post
If you have a pull down ladder to your attic, the code is, don't run the cable within six feet of the hole, and protect the cable from physical damage. So, if you can walk, even a crouch, nail 2x4s on end on either side of the cable. If you have to crawl, no 2x4s necessary.
Whoa The '08 and '11 codes are much stricter, if you have a pull down ladder or stairs. Any time cable runs across joists it needs "substantial guard strips that are at least as high as the cable", ie 2x4s secured on each side. That's any place you can walk, crawl or reach. If you run across rafters or studs, you still need the guard strips. Unless the cable is higher than seven feet from the joists. What a pain. [338.10(B)(4)(a) to 334.23 to 320.23(A)]

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