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Old 03-09-2008, 11:06 PM   #1
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50amp GFI Trips on Commercial Truck


Please help. 50amp GFI trips when running anything on 120v.

I have a 50amp GFI Square D (not the homeline) breaker in my warehouse panel wired to a 50amp 4 prong outlet. Our warehouse has just been completed so its up to the latest code.

We run a #6 cord from our box truck and plug it into the 4 prong outlet. The truck as a Square D residential 125amp panel. The truck panel has 2 breakers 50amp and 20amp. The 50amp is for a 50amp heater that only has 3 wires (white, black, green). We took green to common (white) from the cord. This part all works fine.

Problem comes in when we plug something into a 120v outlet wired from the 20amp breaker in the truck. Trips right away. I don't have any leakage on the truck side. I took the cord and measured at the plug common to ground and have no leakage.

What is causing our 50amp GFI to trip, I've look at everything. Inside our warehouse panel a show a direct short between the common and the ground so I'm guessing they are bonded outside our building somewhere. When I plug in and measure at the truck, before the breaker trips, it shows a short between common and ground.

Any ideas?


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Old 03-10-2008, 01:04 AM   #2
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You have the neutral of the 120 volt circuit connected to the same bar as the ground and neutral from your feeder cord. The neutrals and grounds must be seperated in the truck panel. Neutrals must land on a individual bar for neutrals only and grounds land on a ground bar separate from the neutral bar. What is happening is that you have created a parallel path for neutral current returning to your warehouse panel and the 50 amp gfci is seeing the change in current on the neutral wire vs the hot wires in the feeder cord. Your ground in the cord is carrying neutral current and the gfci sees this difference of current in the neutral as compared to the hots. Remember your gfci is protecting the entire panel in the truck. Since your heater circuit is 230 volts and no neutral then the gfci won't trip as it only needs to see the hot current in both legs. However when you you operate the 120 volt circuit you have a neutral current flow which is splitting on the ground and neutral at the truck panel. So only half the current that the gfci wants to see is flowing on the neutral wire of the feeder cord. The other half is on the ground of the feeder cord. This is causing your 50 amp gfci in the warehouse to trip. You need to install a separate ground bar in the panel in the truck and do not bond it with the neutral bar...ie... do not install the bonding means in the truck sub-panel. Land your neutrals on the existing bar in the truck panel and your grounds to your added ground bar. I threw together a quick diagram to show you what is happening below. If you disconnect the ground of the feeder cord from the warehouse 4 prong plug to the truck panel you find that your gfci will hold when you operate the 120 volt circuit. this is because you will have eliminated the parallel path for the neutral current and all neutral current will now flow on the neutral of the feeder cord which is what the gfci wants to see. Click image to enlarge.
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Last edited by Stubbie; 03-10-2008 at 01:20 AM.
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