Subpanel wire requirements

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  #1  
Old 06-08-07, 07:37 PM
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Subpanel wire requirements

I am remodeling a kitchen and would like to add a subpanel in the basement below the kitchen. The old double oven used a 6 ga. aluminum SE cable (2 insulated connectors, one braided ground) on a 50 amp 240 circuit, it would be convenient to use this to power the subpanel. I would like to reuse the 50 amp breakers. The planned load is well below this capacity.

Question 1: I get the impression that I cannot use this to power a subpanel that I want to use for a 240V circuit and several 120V circuits, can this SE (2 insulated conductors w/ braided ground) cable be used to power this subpanel?


Question 2: What other types of cable can be used for a subpanel? I have (or can get) some heavier gauge SE (alum, 2 ga., 3 insulated conductors with ground) as well NM (6 ga. 3 conductor w/ ground).

Thanks in advance for all feedback,
-Mick
 
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  #2  
Old 06-09-07, 02:39 AM
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Well, I just did this so I'll tell you my story and you can gleen from it what you want.

I wanted to install Central AC in my home. I had existing 4/2 (with ground) AL cable going from the main panel to the laundry room that was used for the furnace before the home was converted to natural gas. It was suggested to me to use this cable to power a subpanel since running new wire would mean tearing apart my basement, so I started buying parts and reading what research I could find.

The first big obstacle: no neutral. The neutral is the white wire that carries 110 current back to the panel. You need 4 wires to properly supply a feeder (sub panel), and I only had the 2 hots and a ground. Since the neutral and ground are together at the main panel, but must be separate at the feeder, I used the ground for the neutral which now left me with no ground.

At this point I was told I would need to pound 2 8' (yes, that's feet) ground rods to give the feeder a ground. I also had to buy a supplemental grounding kit for my subpanel and 6 awg bare wire to run to the ground rods (which must be more than 6' apart otherwise they're not much more effective than one).

Well, I got my permit from L&I and got to work. I had to splice the wire in a metal J box (which was not fun) to get to a wall where the appropriate code clearance could be observed, and of course since the box was metal, it needed to be grounded as well. Sounds like you'll have to splice your AL wire as well. Make sure you get a reducer big enough to handle the gauge and get your anti-oxidant paste as well. You'll need it for every aluminum connection. If at all possible, DO NOT BUY #6 COPPER AND SPLICE IT TO THE AL UNLESS THE SPLICE IS RATED CU/AL. I would seriously recommend keeping the run AL to the subpanel anyway, but that's your call.

The trouble with what I did, and what you want to do, is that it isn't wrong, but it isn't right either. Since the neutral carries current, it needs to be insulated. It is insulated by the cable, but once you run it into the box it will be bare and you'll need to tape the c**p out of it (get white tape).

Mine passed inspection, but I've talked to electricians who wonder how it did. What's worse, is they've asked other inspectors in my town who said they would've failed it. Interestingly enough, the inspector made me ground the panel to a copper water pipe so the main panel would know if one of the hots got disconnected and energized the subpanel box.

I'm not worried about it because it works just fine. I only use the neutral for the 110 outlet required by the NEC to be within 25' of the condenser. The breaker to it is off unless it is needed. The 20a 220 circuit for my AC condenser is happy as can be.

You best first step is to talk to your local inspector. The ones here are available for code questions from 8-8:30am. Try to talk to the inspector who works your area, and ask him what he thinks of your plan.

Getting the permit and getting it inspected are really important, IMO.
 
  #3  
Old 06-09-07, 05:58 AM
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> can this SE (2 insulated conductors w/ braided ground) cable be used to
> power this subpanel?

No. A subpanel feeder in the same building as the main panel requires 3 insulated conductors plus a ground.

> What other types of cable can be used for a subpanel?

A 50A subpanel is typically wired with #6/3g NM-B (which may actually be used up to 60A if you need the extra 10A). The #2-2-2-4 aluminum cable is good for up to a 100A subpanel, and is probably cheaper than the #6 copper.
 
  #4  
Old 06-09-07, 06:09 AM
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stringer,

> It was suggested to me to use this cable to power a subpanel
> At this point I was told I would need to pound 2 8' ground rods
> the inspector made me ground the panel to a copper water pipe

You got very bad advice.

> The trouble with what I did, and what you want to do, is that
> it isn't wrong, but it isn't right

Sorry to say, but there are major things wrong with your installation. The inspector should have failed the installation. The "bootleg" three wire feeder to the subpanel is a clear code violation as are the additional ground rods as is the taped up neutral as is the water pipe ground.

> DO NOT BUY #6 COPPER AND SPLICE IT TO THE AL

This, however, is good advice.
 
  #5  
Old 06-09-07, 07:10 AM
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Location: New Bern, NC
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There is nothing at all wrong with pounding two, or even 20 additional ground rods in and arround the same building, and bonding them to the equipment ground. What is wrong is that this CANNOT replace, or be substituted for the other grounding requirements.

The other problem is that they serve no purpose but to waste your time and money.
 
  #6  
Old 06-09-07, 08:11 AM
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Join Date: Feb 2007
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While I certainly did get bad advice, hindsight is always 20/20 and I don't plan on adding anymore circuits to this panel. What I needed to do was use the existing 4/2 wire because it was already running to the main panel. In truth, my feeder is just a glorified way to reduce from 4/2 AL to 10/2 copper to feed my ac condenser. It was a pain in the a**, but it was worth it to have the AL and Copper wire as far from each other as possible.

Yeah, the ground rods were a waste of my time and money, but I won't be the first DIY to sing that song, and the additional supplemental grounding doesn't hurt my feelings at all. I'm certainly still grounded to the water pipe system and the original supplemental ground rod as well.

I would certainly suggest that the OP run new wire, just trying to shed light using my experience.
 
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