Wire size to sub panel


  #1  
Old 10-11-07, 09:01 AM
P
Member
Thread Starter
Join Date: Jul 2006
Posts: 23
Upvotes: 0
Received 0 Upvotes on 0 Posts
Wire size to sub panel

In my barn, I have constructed a shop area ( 24' x 15') and plan on heating it with a 240v portable heater. (The Cadet "Hot One;" 4000w). I also need general purpose 120v outlets and lighting, etc.

I have a Cutler-Hammer sub panel that I plan on installing in the shop area and running 8ga THHN from the entrance panel to the sub panel.

Two questions:
1) Does the neutral wire to the sub panel also need to be 8ga, or can it be smaller, such as 10ga. (Probably the max. total load on the lighting and/or outlets will be 15a. ... protected by 15a breakers.)

2) Does the NEC code require 4 wires from the entrance panel to the sub panel i.e., 2 hot wires, 1 neutral wire and 1 ground wire?

Any and all comment greatly appreciated.

Ps: I do have a building permit and plan on having the system inspected when I finish the job. What I want to do now is to have everything right, so I don't have to replace/add wiring.

/roger
 
  #2  
Old 10-11-07, 09:15 AM
R
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes on 0 Posts
How far between the panels?

Do you have any other metallic paths between the buildings (such as water pipes, telephone line, computer line, etc.)?



As an aside, I recommend 20 amp circuits for your receptacles. You will appreciate the extra power they can provide. I also recommend that you keep lighting and receptacles on separate circuits. You don't want to be in the dark WHEN you trip a breaker.
 
  #3  
Old 10-11-07, 09:57 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
Originally Posted by PalouseDIY
Does the neutral wire to the sub panel also need to be 8ga, or can it be smaller, such as 10ga.
I would not recommend a reduced neutral for your situation. Use #8, 8, 8, 10 for hot, hot, neutral, ground respectively in a 3/4" (or preferably larger) conduit with a 50A or smaller breaker.

Does the NEC code require 4 wires
Four wires are required if there are any metallic paths other than the electrical feeder between the buildings. Running the fourth ground wire is a safer installation and allows you to add phone or cable tv to the outbuilding, so I always recommend a four wire feeder.

Any and all comment greatly appreciated.
The four wire feeder will require isolated grounds and neutrals in the subpanel (remove the bond screw, buy a ground bar kit); a three wire feeder requires bonded neutral and ground in the subpanel. In either case, you'll need to drive a ground rod at the subpanel and connect it to the subpanel ground bus with an acorn clamp and #8 copper wire.
 
  #4  
Old 10-11-07, 10:29 AM
P
Member
Thread Starter
Join Date: Jul 2006
Posts: 23
Upvotes: 0
Received 0 Upvotes on 0 Posts
OK ... thanks for the info.
Since the building is a metal structure, I assume that means 4 wires: 8-8-8-10.

The wire run from the main entrance panel is about 75'. (The barn has it's own separate power source, feeding from the power company's transformer, with a separate meter and entrance panel)

Looks like this is the setup:

> 4-wires from panel to panel.
> Neutral connected at both panels
> Ground wire connected at both panels
> Ground rod driven into the ground and connected at sub panel. (10' rod?)
> Ground and neutral NOT bonded at the sub panel.
> I have plenty of 1" sched. 40 to run wires in.

Do I have this correct?

Thank soooooo much.

Sometimes us "semi-pro" DIY'ers get over our head, especially when it comes to wiring. I can get things to "run," but getting it to comply with an ever-changing NEC code is another matter. I think this project is "do-able," for me.

Ps: Earlier I had a 12KW standby generator installed, including transfer switch and separate "essential" circuits. I hired a pro to do this job and even he had a couple of small corrections to make to satisfy the inspector.
 
  #5  
Old 10-11-07, 11:57 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
The answers you have may be somewhat incorrect. Bob and I both assumed the shop was in its own separate outbuilding fed from the house, but I'm not sure that assumption was correct based on your mention of the barn. Just so I'm clear on the layout here:

* the shop is located inside the barn

* the barn has a dedicated electrical service from the power company with a meter and a main panel

* the barn and the shop are separate from the house

If these are true, then the indoor feeder from the barn main panel to the shop subpanel must be 4 wires, and the ground rod should not be installed at the subpanel. The subpanel would not require a main breaker and could be the MLO (main lug only) type. The subpanel should have separate, unbonded ground and neutral bars.
 
  #6  
Old 10-11-07, 07:09 PM
P
Member
Thread Starter
Join Date: Jul 2006
Posts: 23
Upvotes: 0
Received 0 Upvotes on 0 Posts
Ben:
You have it right: The shop is inside the barn, which is a typical steel building. The barn is not connected in any way to the house. The barn is fed separately, just as the house is, (There are two meters on the utility pole. One feed the house, the other the barn.) There is a copper grounding rod at the main service panel.

So, I think I have it. The only correction to my prior post is delete the grounding rod at the sub and don't bond the ground at the sub.
 
  #7  
Old 10-12-07, 06:53 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 9,785
Upvotes: 0
Received 45 Upvotes on 43 Posts
You got it .
 
  #8  
Old 10-12-07, 10:40 AM
P
Member
Thread Starter
Join Date: Jul 2006
Posts: 23
Upvotes: 0
Received 0 Upvotes on 0 Posts
If I can engage you for "one more question:"

It so happens that I have just the right amount of black #6 thhn, left over from a prior project, that will serve as one of the hot conductors. Obviously, I need two more wire runs, plus a ground.

Question: I can get bulk rolls of #6 thhn on ebay for much cheaper than buying locally. (partial rolls, I assume, left over from other jobs.) The rolls available are all of one color, usually either black or blue. Can I wire my project with all one color and "recode" the wires, using colored electrical tape to indicate the proper wire color?

What about the ground wire? Can I use the same thhn or must I use bare wire? (If the answer to that is YES, how do I code it?)

(Hopefully, this will be the last question.)

Thank you for your time!

/roger
 
  #9  
Old 10-12-07, 10:54 AM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 38 Upvotes on 30 Posts
In strict accordance with the National Electrical Code any conductor number 6 or smaller must have the proper insulation color its entire length.

Many local inspectors will waive this requirement and allow for re-identifying black insulation but it is NOT code-compliant. You need to ask your local inspector.

The equipment grounding conductor may be bare or it may be green insulated. Again, your local inspector may allow you to re-identify to green but such is not compliant with the NEC.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: