Temporary sub-panel with GFCI

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  #1  
Old 01-05-05, 02:13 PM
Rick Hagen
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Temporary sub-panel with GFCI

I am attempting to hook up power between the main panel (on newly installed transformer) and a building which is 225' away. I installed a 20amp GFCI outlet on the main panel. I have a 250' extention cord (12/2 outdoor romex) run above ground to the building. This is due to the fact that the ground is frozen and can't be buried till next summer. I installed a sub-panel in the building with a 20amp circuit breaker. When I connect power the GFCI trips. I believe this is due to a N-G fault because of having the neutral and groud wires connected (same bus) in the sub-panel. Question is; can I have the 250' power GFCI protected (while it is above ground) and still have a circuit breaker in the building? In order to make this work I think I would need to remove the ground wires from the sub-panel (thereby isolating them from the neutral wires) and tie them together.
I believe once the ground thaws the correct procedure will be to run a non-GFCI line to the building's sub-panel and GFCI protect from there. I do currently have an earth ground installed at the sub-panel.
Sorry if this got lenghty, but would greatly appreciate assistance.
 
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Old 01-05-05, 02:26 PM
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For 20 amps at that distance you'll need much thicker wire.

Try this (approximate) wire size calculator: http://www.csgnetwork.com/wiresizecalc.html
Note where you enter "One Half The Total Circuit Length " that's half the run there and back.
 
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Old 01-05-05, 02:42 PM
Rick Hagen
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Thanks for the reply. Is this the reason for the GFCI tripping? I did remove the GFCI and everything works fine.
 
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Old 01-05-05, 03:13 PM
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To be honest, I don't understand GFCIs. They're more sensitive?

250' 12/2 will only give you 5 amps before voltage drop begins to seriously eat into the more current you pull through. The lost power turns into heat along the cable length. The solution (assuming you want to run more than lights or a small power tool) is to use thicker cable, shorten the cable, or both.

I don't understand "everything's working fine". Exactly how is your subpanel working now that you removed the breaker protecting it?

***

Maybe, since you're just waiting for the ground to thaw, you could complete the ultimate feeder, circuits, breakers now and protect the run above ground with concrete blocks and so forth, then drop it all down when you've got the trench dug?
 
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Old 01-05-05, 03:30 PM
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In less things have changed.
The main panel is the only place the neutral wires and ground wire are joined.
For the sub panel the neutral buss is floating. You need get a second buss for your ground wires.
You may see the two screw holes in the panel for the grounding buss.
 
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Old 01-05-05, 03:32 PM
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The GFCI is tripping because the ground and neutral are connected together at the subpanel. This is wrong anyway, so fix that part now.

However, you won't be able to use that panel for much now because of the voltage drop.
 
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Old 01-05-05, 03:33 PM
Rick Hagen
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What I meant by everything is fine is that I removed the GFCI outlet (not the circuit breaker) installed a regular outlet and all the power works - the circuit breaker doesn't trip. I would like to have the temporary above ground line GFCI protected (especially because the transformer is located on the edge of water, as our electrical supply comes from under the lake. The other factor i neglected to mention is that we are building a cabin in the next year or so and I would like to run power to this building from the cabin and not from the transformer as it will be a shorter distance and I can run a switch wire in the same trench as the power line. The trench becomes a major issue here because I am dealing with a great deal of ledge rock.
 
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Old 01-05-05, 03:37 PM
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Originally Posted by Rick Hagen
Thanks for the reply. Is this the reason for the GFCI tripping? I did remove the GFCI and everything works fine.
It's my understanding from past experience that the long extension cord is probably causing the GFCI to trip.
 
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Old 01-05-05, 03:47 PM
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It's tripping because the ground and neutral are bonded at the subpanel. They can only be bonded at the main panel.

If you remove the bond (you install a second ground bus bar in the subpanel) the GCFI should not trip, unless the cable is damaged.

Either way, 250' with a 12 gauge wire is not very good. You'll experience a large voltage drop, especially with high resistive loads (electric heaters). Runa a thicker wire.
 
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Old 01-05-05, 05:30 PM
Rick Hagen
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I now understand the problem with the GFCI tripping and will seperate the neutrals from the ground wires at the sub-panel. I would also like to run #10 or #8 gauge wire to the building but am wondering how to connect it temporarily to the GFCI at the main panel?
Thanks to each or you who have responded. Your help is greatly appreciated!
 
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Old 01-06-05, 02:54 AM
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"GFCI at the main panel" had me confused - I thought you meant a GFCI breaker not GFCI outlet. So this truly is an extention cord lugged onto the subpanel.

That's right the ground and neutral must be isolated absolutely everywhere except the box your main service is in. Just to be clear, that means you treat the neutral as a conductor - which it is. Your subpanel should have a bus on plastic standoff for this.

#10 or #8 is just a step in the right direction. 20 amps over 225' calls for #4. If #8 then use a 10 amp breaker. I know a lot of people will tell you "use #12 for 20 amps" but this is only true for typical short runs inside a house. As it is I think your cord will smoke before a regular 20 amp breaker trips. And you'll be out of a cord, best case scenario.


Can you move the building?
 
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Old 01-06-05, 10:51 AM
Rick Hagen
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I wish I could make the buiding closer but not possible. Thank you for your help! The power needs to this building are mainly just lights and a couple outlets for lamps. Total of maybe 900 watts. Yes, I am basically connecting the main panel and the subpanel with an extention cord. Maybe it's working because its under 1.5' of snow? If I understand, I should change the breaker to 10amps just to give more protection to the extention cord, correct? Also, should I have a separate earth ground at the subpanel and which buss should this be connected too? The neutral or the ground buss?
 
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Old 01-06-05, 11:10 AM
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Originally Posted by Kobuchi
"GFCI at the main panel" had me confused - I thought you meant a GFCI breaker not GFCI outlet.
#10 or #8 is just a step in the right direction. 20 amps over 225' calls for #4. If #8 then use a 10 amp breaker. I know a lot of people will tell you "use #12 for 20 amps" but this is only true for typical short runs inside a house. As it is I think your cord will smoke before a regular 20 amp breaker trips. And you'll be out of a cord, best case scenario.
Where is he going to get a 10 amp circuit breaker? Does he live in Canada? Are 10 amp circuit breakers available in Canada?
 
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Old 01-06-05, 02:30 PM
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Originally Posted by Kobuchi
I know a lot of people will tell you "use #12 for 20 amps" but this is only true for typical short runs inside a house. As it is I think your cord will smoke before a regular 20 amp breaker trips. And you'll be out of a cord, best case scenario.
This is false. The temperature of the wire depends only on the current flowing through it, not on the length of the wire (as long as the wire is spread out, not coiled up). A longer cord will waste more power as heat than a shorter cord, but it dissipates this power over more wire. The temperature rise at any point on either cord will be the same.
 
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Old 01-06-05, 02:36 PM
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Mike is correct. The longer the extension cord, the easier it is on the cord, but the harder it is on the stuff attached to the far end of the cord.
 
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Old 01-06-05, 03:01 PM
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No, 10 amp breakers are not available in Canada (At least, not to my knowledge).
 
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Old 01-06-05, 03:33 PM
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Square D has them. 1 Pole 10-70A, 2 Pole 10-150A, 3 Pole 10-100A. Actually...I've seen a 10amp breaker..very small.
 
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