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Old 01-29-2010, 06:11 PM   #1
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Help please - Upgrading subpanel!


Ok, I currently have a workshop in which I have a 40 Amp sub panel.
This contains two 20A breakers.

This subpanel is fed by a 40Amp tandem breaker on the main panel.
The wiring is #6 AWG and says gas and oil resistant.

I want to put in at least 50Amps, if not 60Amps to the subpanel in the workshop but not if it means replacing the wiring.

The workshop subpanel is a good 100-150Ft from the main panel.

I have done some research and plan to replace the 40A tandem on the main panel with a 50A or 60A (depending on the responses I receive) and then upgrading the breakers in the subpanel.

Also I read that a 50A 240v breaker can feed TWO 50A 120v breakers?
Is this true? I thought I can only use the Amperage specified by the tandem breaker (240v) even though I split it to 120v in the subpanel.

So in summary I think Im safe to upgrade the main panels tandem 40A to a 60A tandem and then replace the two 20A singles in the subpanel with two 30A singles.
I am aware that I would have to rewire internal wiring form the upgraded 30's to handle the 30A load.
This is ok, I just cant afford to replace 100-150Ft of #6 with #4.

If pictures are needed I can supply!

Thanks for your help.


Last edited by wtfh4xx; 01-29-2010 at 06:19 PM.
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Old 01-29-2010, 06:15 PM   #2
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Help please - Upgrading subpanel!


With 100-150' distance the #6 wire was used to offset voltage drop for the distance

Even at 150' the voltage drop is OK for a 50a breaker
Are these newer panels - depends upon if the rating for a 50a is correct

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Old 01-29-2010, 06:40 PM   #3
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Help please - Upgrading subpanel!


I agree with Dave about the voltage drop issue. Go with the 50 amp circuit breaker. The new panel will likely be rated for 125 amps and that's fine. Think of it as having a 1 gallon container but only filling it up half way.

Good luck.
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Old 01-29-2010, 07:20 PM   #4
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Help please - Upgrading subpanel!


Thanks guys!
I figured the reason it was only a 40A at the subpanel was the V drop.
And thats exactly why Im raising it, I see it as half full and am gonna put it 3/4 full :P

So now just to make sure I got this right.
I should replace the 40A tandem in my main panel with a 50A tandem.
Can I then upgrade ONE of the two 20A to a 30A to fulfill the load, or do they have to be done equally? IE 25A & 25A.

If I understand right then I can leave all the wiring on the 20A alone, and rewire the new 30A circuit with 10/3 romex to support the increased load capacity.
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Old 01-29-2010, 07:50 PM   #5
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Help please - Upgrading subpanel!


You would then have 50a 240v

What you can use at any ONE time is restricted by that number (50a 240v)
What you can connect is not restricted
You could have multiple tools & devices connected to breakers
As long as the total load does not exceed 50a on either hot feed the main breaker will not trip

You could have 10 20a singles if they fit in the panel, or 5 20a & 5 30a
Note: 30a requires #10 wire
And normal household outlets are only 20a

A pic is always a good idea so we can see what panel you have
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Old 01-29-2010, 07:56 PM   #6
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Help please - Upgrading subpanel!


Thanks Scuba, your really helping me out here.

I dont know if I made it clear, but the 240V line from the main panel SPLITS at the subpanel into two 120V circuits.

Does this mean that I can have TWO 50A 120V circuits in my subpanel since I have 50A 240V coming in after I get that 50A tandem breaker?

Ill be back in a second with pics of the main panel and the subpanel.
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Old 01-29-2010, 08:15 PM   #7
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Help please - Upgrading subpanel!


A 240v electric panel always has 2 hots
Even a main panel in a house has that

50a 240 = (2) 50a 120v hot bus - to which you connect breakers
Ideally you want to balance the load
A 240v breaker connects to both 120v bus bars to give you 240v
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Old 01-29-2010, 08:18 PM   #8
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Help please - Upgrading subpanel!


Notice how the two black #6 wires each go to a breaker, this si where it becomes 120V if im right.









The 40A tandem that goes to the workshop is the grey one above.
Subpanel is the first two pics, main panel is the second two.

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Old 01-29-2010, 08:34 PM   #9
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Help please - Upgrading subpanel!


Ok, pic tells more...and requires more questions
I assume 100-150' away this is a detached workshop, not attached to the house

How many wires are coming from the main panel ? 3 or 4 ?
Seems like only 3
Lower right that copper wire should exit the building & go to ground rods I think

I'd pull the panel & put a new 100a main breaker panel in
Then feed it with a 50a breaker in the main panel
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Old 01-29-2010, 08:45 PM   #10
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Help please - Upgrading subpanel!


I don't see a neutral conductor at all. Do you?
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Old 01-29-2010, 09:00 PM   #11
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Help please - Upgrading subpanel!


No, I was hoping its hidden due to the camera angle
Maybe they used a black wire for the neutral ?
Looks like one twists to the right under the breakers over to that bar on the right
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Old 01-29-2010, 10:35 PM   #12
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Help please - Upgrading subpanel!


What are the two breakers in the subpanel now powering - 120 volt outlets? Or something else. This may help to figure out what is going on here.
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Old 01-30-2010, 03:03 AM   #13
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Help please - Upgrading subpanel!


Hi guys,
There are two main black wires feeding into the subpanel from the 240V 40A Tandem breaker at the main.
The lower right copper does indeed go down to a ground rod.
The black wires are positive, white wires are the neutral, as the ground is the conduit.
The 14G wires feed 120v outlets, the two 12/3 romex feed 20A GFCI outlets.
The workshop 120v outlets are on the same breaker as one of the GFCI outlets.
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Old 01-30-2010, 08:29 AM   #14
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Help please - Upgrading subpanel!


Only 2 wires ? Is this only a 240v circuit right now ?
What is it powering ? A pump /motor ?
You need 3 wires for a 120v circuit
And for a sub panel you need 4 wires these days

With only 2 wires you would need to pull 2 more to put in a sub
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Old 01-30-2010, 02:24 PM   #15
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Help please - Upgrading subpanel!


Yup, its 240V from the main but because it goes from the tandem 40A to two 20A as power leads, it then becomes 120V because its split.
Two black #6 wires coming from a tandem 40A on the main which go to two 20A breakers powering 120V circuits.
Shoplights and general small power tools are used in the normal 120V outlets.
Bigger tools and welding etc are used off the GFCI outlets.

The three wires for 120V circuit is power, neutral, and ground.
The small black wires are the 120v circuits power, the small white ones are the neutral, and the conduit is the ground.

Why would I need to pull two more wires?
Its been working like this for over the last ten years since the shop was built.
I just upgraded with the GFCI's which are on the Romex.


Last edited by wtfh4xx; 01-30-2010 at 03:39 PM.
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