ground wire size

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Old 10-02-09, 07:43 PM
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ground wire size

I'm in the process of running electrical service to a small garage detached from my house, and I think I have most everything accounted for except for the wiring of a few 240v circuits.

This is the situation: I am running some 240v circuits for heavy appliances - a welder, compressor, and one to be determined at a later date (possibly a plasma cutter). The welder and compressor are strictly 240v, and only use three wires (hot, hot, ground - no neutral). I have used a 4 conductor nm cable for each of these - partly because I wanted to plan ahead in case I ever needed to use these circuits for 120v/240v applications, and partly because I had some 4 conductor cable on hand.

For the compressor, I ran 10/3 nm cable and I will be using the red and black hot wires, and the bare ground wire. the white (neutral) wire will not be connected to anything at the current time. This all seems fine to me since all four wires are 10ga, which will be overkill, because even the largest compressor I am likely to ever own will draw less than 20a.

For the welder, I ran 6/3 nm cable for a 50a circuit. This is where I am starting to be uncertain. I will be using the red and black hot wires, but for my ground wire, the bare ground wire in the bundle is a 10ga wire. Is it correct to use this 10ga ground wire in this instance, or do I need to use the 6ga white wire (normally the neutral) and attach that to the ground terminals at the sub panel and receptacle, and not use the bare ground wire at all.

I will be the first to admit I don't fully understand the differences between the ground and neutral. This is the first time I have ever had a sub-panel, so in the past any circuits were connected to the main panel, where the neutral and ground are bonded together. I realize there is a difference between them, but for all intents and purposes, ground and neutral go to the same place in a main panel.
 
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Old 10-02-09, 08:01 PM
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As a side note, Before I registered and posted here, I had done a Google search for my qeustion and found a link to a calculator for sizing grounding conductors, based off table 250-122. According to that calculator, a 10ga ground would be acceptable for 50a. I just wanted to make sure I could apply that to my situation.
 
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Old 10-02-09, 08:06 PM
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Sorry, post deleted. Wish you had mentioned the subpanel first.
 

Last edited by ray2047; 10-02-09 at 09:29 PM.
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Old 10-02-09, 09:02 PM
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I was initially trying to keep things simple by just trying to determine whether I could use the 10ga ground wire in a 6/3 nm cable for a 50a 240v circuit, but here's the whole novel - no cliff notes...

The main house has a 200a service. Along with the normal selection of high draw electric appliances (water heater, range, dryer), the house initially had electric heat with several 30a and 20a breakers. In this part of the country, electric heat went out of style a few decades ago, based on cost alone, and the heating system was replaced by a hot air furnace that probably draws about as much as my coffee pot (probably less, cause I drink a lot of coffee). I'm assuming that if this all worked with the electric heat cranking away in the dead of winter, I won't have any trouble occasionally striking an arc with whatever is leftover.

To connect the garage, I am putting a 100a breaker in the main panel in the house. I am running 4 individual wires in 2" conduit from the main panel in the house all the way to the sub panel in the garage. The wire conductors are 1/0-1/0-1/0-2 aluminum. I don't have the wire in front of me, so I am unsure of the exact type, but I purchased the wire from a local electrical supply house after giving them the details that it would be approximately 180' from panel to panel. I believe they said it was good for 100a up to 190'. I've shaved a few feet off my run with the placement of the conduit, so it may be more like 170' when finished.

In the garage I have installed a new panel with a 100a main breaker. Along with the 4 conductors coming from the main panel in the house, I have grounded the panel with 6ga stranded bare copper wire and two 8' copper clad rods placed approximately 8' apart.

Along with normal lights and receptacles, I am running three extra circuits as described in the previous post. I cannot and will not bother to give any rating plate information on the equipment, because equipment can and will change frequently, so I have sized the branch circuits for what I consider the worst case loads.

10ga for a 240v compressor more than covers it, since my current one runs off a 15a 120v circuit. I have looked at larger ones, and what I consider to be a likely replacement is only 17a 240v. a far cry from the capacity of 10ga wire.

The same goes for the welder and extra circuit that wil most likely run a plasma cutter. My current welder is an old stick welder that has a 50a plug on it, but I haven't checked for any rating plate. I have looked at newer welders and plasma cutters, and the only ones I see as feasable for my use draw less than 30a at 230v.

My intent is to use the 6ga and 8ga cables I have run as the worst case scenario, but I want to make sure they will perform as I intend them to. I am not sure about how the code applies to the use of 6/3 and 8/3 cable where 6/2 and 8/2 is needed, but to me it was the choice of common sense. The use of x/3 cable as opposed to x/2 cable is determined by whether it is 120v/240v, or strictly 240v. Even though the devices I will be powering are strictly 240v, I worked on the assumption that maybe something exists with a digital control panel and requires 120v/240v. If I had run x/2 wire in the walls, I would be out of luck. By running x/3 cable, I end up with a currnetly unused connector in the walls and boxes. surely there is some way to account for this.
 
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Old 10-02-09, 09:18 PM
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Lost, you are doing everything correctly. I don't know why Ray wrote that condescending reply, maybe he just read your initial post quickly and reacted as he did because all too often people don't take the care that you obviously have in your installation.

A bit more information is that often a motor circuit (your air compressor) can have a larger circuit breaker than would normally be installed for a particular size of wire. The number 10 conductors can supply a true five-horsepower motor and for some reason unknown to me (but probably purely advertising) home-shop air compressor motors are vastly over-rated in their horsepower.

The oversizing of circuit breakers in relation to wire sizes is also common with arc welders, the key factor is the duty cycle of the welder.

In my opinion you are in fine shape for what you have described as your project.
 
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Old 10-02-09, 09:24 PM
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I forgot to answer your original question. Yes, a number ten equipment ground is fine for your number six conductors. Equipment grounding conductors are based upon the rating of the circuit breaker or fuse feeding the circuit. Here is an abbreviated chart using copper wire.

15, 20 and 30 ampere circuits require the same size of equipment grounding conductors as the supply conductors, i.e. #14, #12 and #10.

40, 50 and 60 ampere circuits all require a #10 equipment grounding conductor.

100 ampere circuits require a #8 equipment grounding conductor.

If the supply conductors are upsized to compensate for voltage drop the equipment grounding conductor must also be upsized.
 
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Old 10-03-09, 04:48 AM
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Thanks, Furd. As I said before, I'll be the first to admit that I don't fully understand grounds and neutrals. My only other experience has been with automotive electrical systems, and in the cases where something isn't just grounded to the chassis, both wires are generally the same size going to the positive and negative side of a battery.

It just seemed funny to me to wire a 3 conductor 50a receptacle with two 6ga hot wires and then only a single 10ga ground, but that's the conclusion that everything I found led me to - even before your reply.

Oh yeah, one last question: As I stated before, I've used x/3 cable in all three of my 240v circuits. Is there a correct/preferred way to "store" the unused neutral wire? I assume that in the panel I can hook it to the neutral bar as you normally would - I can't imagine it's acceptable to have loose wires dangling inside the panel. But at the receptacle, what should I do? Can I just wire nut and tape over the end of the neutral wire and stuff it in the back of the box, or is there another way?

Thanks again for your help.
 
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Old 10-03-09, 12:08 PM
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Originally Posted by lost_cause View Post
Oh yeah, one last question: As I stated before, I've used x/3 cable in all three of my 240v circuits. Is there a correct/preferred way to "store" the unused neutral wire? I assume that in the panel I can hook it to the neutral bar as you normally would - I can't imagine it's acceptable to have loose wires dangling inside the panel. But at the receptacle, what should I do? Can I just wire nut and tape over the end of the neutral wire and stuff it in the back of the box, or is there another way?
Wire nut it and stuff it in the box/panel. The wire will then be insulated from ever touching an energized part and be perfectly safe.

The difference between a neutral and the ground is the neutral carries current when there is a load (when it is used) and the ground only carries current during a fault. They are connected to the same place in the main panel. (and only the main panel)
 
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