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Q: Subpanel with 120V from 240V ( Answered,   3 Comments )
Subject: Subpanel with 120V from 240V
Category: Family and Home
Asked by: yorksprings-ga
List Price: $10.00
Posted: 28 Feb 2006 13:23 PST
Expires: 30 Mar 2006 13:23 PST
Question ID: 702026
In the electric panel (200A) is double breaker 2x40 A. It was intended for welding.
Now, I wish to use this "outlet" ; extend it to the attic, install a
subpanel and use it for up to 10 lines( 15-20 A).
How should I proceed ?
Subject: Re: Subpanel with 120V from 240V
Answered By: weisstho-ga on 01 Mar 2006 18:33 PST
Dear Yorksprings-ga,

Our usual disclaimer about our providing ?general information? which
is not intended to substitute for informed professional advice must be
emphasized here. May I suggest that any work that you do be inspected
by a licensed electrician; indeed, perhaps your plan should be
reviewed in advance by an electrician.

Now, let?s see what we have here:


You have two 40 amp legs of 120 volts each from both bus bars in your
main panel, with a functional and normal 40A breaker. Each side of the
breaker of course is a separate 40 amp 120 volt circuit, and since
they are from separate busses, when combined they create 240 volts. I
assume that the two switches on the breaker are ?ganged? meaning that
they are hooked together, probably by a small aluminum or plastic bar
or connection such that if one trips, it will take the other one with

Keep this 40A breaker in place and then run a wire from the main panel
to the new subpanel located in an appropriate place (with adequate
access and, importantly, adequate clearance from the front of the
panel to the wall in front ? allowing a person to pull away if

Install the various boxes for receptacles and fixtures. Wire them up
to the subpanel.

You will probably want to install a 2X40A breaker as a ?submain? in
the subpanel so that the busses can be isolated.

Connect the wires to the 15A or 20A breakers in the subpanel.

GREAT HELP TOOL has just the site that you are looking for:  How
To Install A Subpanel.
Thank you!!


Of course for 15A circuits a 14-2 wire is used, while 20A circuits use
12-2 wire. Here?s Lowe?s pricing and descriptions:

The line running from the main panel to the subpanel will probably
take a #8 wire. Now, whether this wire should be 8-2 w/ ground, or 8-3
w/ground is a technical question. The 8-2, with the two positive legs
being serviced by the black and white, the neutral by the ground is
what appears to be the most widely recommended.  See Home Depot?s site
on 40 amp circuits.


This is a pretty technical area. This site is more for planning load
factors for an entire home:

But here is a more useful calculator for an individual circuit:

A very basic (and simplistic) tool for circuit calculations in your attic is this:
P = I times E

P= Power in Watts
I= Amperage
E= Volts

P = 20 Amps times 110 Volts
P = 2200 Watts 
A 20 Amp circuit for a 110 Volt Line will support 2200 Watts


I suppose the trick is going to be running the #8 wire from the main
panel to the location of the subpanel in attic. Of course, if it is
run outdoors, a special wire for that purpose must be used. Perhaps
conduit may be appropriate and, indeed, there may be a state or local
electrical code restriction governing this.

Here is a PDF that is designed for electrical inspectors. Its
usefulness here is as a checklist of issues that you will face.

As usual, Wikipedia has good stuff:

Permits?  Inspections?  Here is some general info:

AND NOW, I am going to share with you, Yorksprings, this Top Secret
information about the best electrical tool EVER made: Don?t leave the
basement without it:  . The
8 inch H/L Diagonal Cut Plier D2288 by Klein Tools.  Or, as the pros
say:  ?The D-228-8.?  Accept no substitutes. And keep it to yourself!!

If you require anything more, please hit the CLARIFICATION button and
I?ll get right back to you.


Search Strategy:

Residential electrical installations
Electrical load calculations
Klein D228-8
Electrical subpanels

Clarification of Answer by weisstho-ga on 06 Mar 2006 07:38 PST
I have been considering the issue of the wire feeding from the main to
the subpanel. Although the "8/2 w/ground" should be acceptable, the
BETTER alternative would be to use "8/3 w/ground" and configured as: 
Black and Red for each of the 120v legs, the White from the
Neutral/Ground bar to the Neutral/Ground bar, and the Ground wire as a
separate ground which would be connected to the ground lug in the
subpanel and then to a separate grounding point near the main panel.

It seems that inspectors like grounds and the more of them, the
better. A separate grounding of the subpanel would make the inspector
happy, may be a requirement of the local code anyway, and slightly
enhances safety in any case.
Subject: Re: Subpanel with 120V from 240V
From: whsingleton-ga on 14 Mar 2006 07:54 PST
The cable to the subpanel should be 8-3 w/ ground and NOT 8-2 w/
ground.  At the subpanel the ground and neutral should be isolated
from each other.  You CAN NOT run a common ground/neutral wire to a
subpanel.  This violates NEC.
Subject: Re: Subpanel with 120V from 240V
From: weisstho-ga on 15 Mar 2006 07:49 PST
The commenter is absolutely correct, hence my clarification. 

Thanks whshingleton!
Subject: Re: Subpanel with 120V from 240V
From: nitro77-ga on 18 Mar 2006 13:13 PST
Tell me if I'm off base but it seems to me we are missing a 100 amp
breaker at the subpanel.

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