Neutral & Ground Potential Difference

[Randal Vincent]

I've come across a problem I can't solve. It seems I?ve got a potential difference between my ground and grounded conductors. I have a small sub panel feeding my pool (220 30amp. w/insulated ground back to main panel.1 circuit gfi breaker 20 amp for pool pump. 1circuit gfi 20 amp for 2 120 volt pool lights. I noticed when I touch the neutral bar and ground bar with my screw driver I get sparks. It seems to happen with either load running(pool filter or lights). At the sub panel, when I check for voltage across hot to ground I get about a 3 volt difference from hot to neutral. I also measure 3 volts from the neutral bar to the ground bar. I know my sub feeders are good. I know all my connections to all 3 loads are good. I've also checked all ground and neutral connections.


I know the neutral and the ground wires are connected to the same point in the main panel, but I can not understand why I would see a potential difference in them at the sub panel. Any comments, suggestions, or explanations are most welcome.

Thanks,
Randy

 

[realolman]

I think that you would have a voltage drop on the neutral conductor because of the load, that you would not have on the ground.

When you touch them together, the current divides to flow in each, causing the spark. If you have each lighting breaker on a different phase, the neutral currents should tend to cancel each other out. If they're both on the same phase they will add up.

with #10 wire you would probably need about a 15 amp. load to get a 3 volt drop over 200ft.