The scenario: My sisters new lake house needs to have its dock mounted power pedestal placed/connected. The brother (me) was asked to do this simple project. The previous home owner had given them his complete set-up for this. There is a pedestal mounted disconnect with 240 30 amp plug-in point at shoreline - 100 feet of SO cord with twist-locks for connecting between the marine grade dock power pedestal and shoreline power. I understand you cant protect this whole thing with GFI protection without nuisance tripping. I simply don't feel comfortable running 100 feet of SO cord zip tied (suggested method) for 100' along a dock. They have a 4 year old daughter who is very inquisitive... I want to say sorry I dont feel comfortable connecting this but, I know the handyman will be next in line to do it.
Any suggestions? Money is not an issue with them. I was thinking (if I do it) that I could possibly protect the SO cord with PVC along the dock without gluing it so the whole thing could be dis-assembled before dock removal in the fall.

Thanks John

Mount some lengths of PVC, not connected together, as a route for the SO cord.
That way you can run the cord through the pipe and most of it will be concealed from the view of the kiddie.
You're not using the PVC as a raceway so much as using it as a support for the cord.

Twin City Sparky:
The east coast way (NJ) is to 'hang' the SO at the side of the dock, with 'slack'. As it is out of site, it should be OK. I've seen PVC cable hooks screwed to the beams, the cable laid-in and done. BTW, not a lake, but bay, river, & lagoon tidal waters.

John

I am not sure why you say this can't be GFCI.

Portable SO cord has a certain amount of capitance leakeage to ground. I've read that 100ft of SO cord alone can leak the 5 milliamps to ground resulting in a false trip... John

I would say if there is that much leakage it is not safe for the application.

With any long run, even with good wire, capacitive coupling between the circuit conductors and the EGC or steel raceway may become an issue.

If it was up to me I would not GFCI protect this feeder to the pedestal. IMHO it is never a good design to GFCI protect a feeder.

I would make sure my ground fault path back to the source was as reliable as I could possibly make it.

No GFI used. I ran PVC along the top outer edge of the dock. replaced all cord ends, checked all connections. Looks nice and safe - sister and hubby are happy! Thanks everyone for the replies.

John

This "coupling" BS is just a way to rationalize water in boxes and other real ground faults. If 100' of wire couples enough to trip GFCI I would have them tripping all over my house.

BS?

Greg it is not BS, it is a real issue.

As a an example when you wire a line isolation monitoring panel you should not even use THHN/THWN at all as its dielectric strength is to low.

Now I fully understand that LIMs are more sensitive to this than a GFCI but the cause and theory are the same.

Now instead of the LIM take a less sensitive class A GFCI and run a couple hundred feet of circuits on the load side of it and you may have a real problem.

If you are interested in some more info read the linked article.

Impedance of Wiring on Hospital Isolated Power Systems used in Operating Rooms

Another example of capacitance coupling can be observed when testing the continuity of long cable runs with a good meter. You will get an ohms reading until the cable (capacitor) charges.

If that was really a problem the 250' of extension cord I had strung out yesterday should have tripped the TWO GFCIs (in series) that were feeding it. We were running a table saw, several drills, an impact wrench and a skill saw with no problem at all. This is very common in construction sites all over the country where they run very long cords from T poles or the garage across the street. Why isn't this "capacitance" and the "motor myth" shutting down construction sites?
I put this in the same class as those who say refrigerators and washing machines are supposed to trip GFCIs. GFCIs trip on ground faults.
I am planning on doiong some pressure cleaning next week and we will be using my 50 foot 30a 240v SO extension cord (5HP motor). I will put a clamp on it and see what the ground current is at the source. If 100' of SO cord is supposed to trip a GFCI I should see 2-3ma of ground current from 50 feet (plus the 35 foot SJOW cord on the cleaner).

BTW your example of a "good" meter only means that it has many megohms of impedance so microamps will excite it.

OK folks
Want to get your bets down?
50' of 6ga SO cord
35' of 10ga SJOW cord
35' of 10ga Romex
5HP motor load on the end
Will I measure on the green wire at the panel?
(A) much less than a milliamp
(B) about 1ma
(C) 2-3ma
(D) 4-5ma
(E) Greater than 5

I read your article. This is what it said about capacitance ... unless I missed something



The OP is talking about SO cord in plastic pipe.

Greg I am a great supporter of GFCIs

I think we should use them more often than required and I do just that, I install GFCIs at the point of use many times even when not required. Like you I feel an appliance that trips a GFCI has an issue that needs to be fixed.

That said I find it discouraging that you refuse to consider a real issue that can cause problems no matter what may have worked fine at your own home.

Some reading that is more on target.

From EC&M


Tony Locker, BSEE, MSEE


From a University of Idaho pdf.



From Square Ds GFCI breaker installation pdf



I am in no way saying it will always happen, what I am saying is it can happen and in my own opinion a good design will place the GFCI as close to the point of use as possible to prevent this 'pre-loading' of the ground fault detecting circuitry.

A GFCI is worthless if it does start nuisance tripping and is removed by the customer.



In steel conduit is a worst case, the simple fact a copper EGC runs in parallel with the circuit conductors is also a problem although of lesser significance than if they where run in steel conduit.

One thing does bother me about all this capacitive coupling. Wouldn't BOTH current carrying conductors couple to ground a similar amount and if so the net current differential across the CT in the GFCI would be close to zero? In fact the current in the ground wire would null too.

I just can't imagine this is a real world problem when I see those T poles with 2 or 3 cube taps in them and hundreds of feet of extension cord plugged in. Certainly they trip now and then but I think that is more a function of the lousy cords and defective tools.

True Greg,, but what it can compound it some of the tools used the electronic device to regualation the speed of the tool sometime it can read very funny current wave fourm some case it can cause the GFCI to trip much more eaiser when the sine wave is gone like chopped sqaure wave or moddifed sine wave it can cause the GFCI's CT to be reading at wrong time can cause to trip

so that one thought it can happend

Merci , Marc

The current will be proportional to the voltage differential. For a 120V circuit, one conductor is at 120V while the other is at 0V with respect to ground. There will be current due to capacitive coupling from the hot conductor, but not from the grounded conductor, as it is at nearly the same voltage as the grounding conductor.

For a 240V circuit, the capacitive coupling would likely be balanced between the two current carrying conductors, as they are both at 120V.

The winner would be the one who picked 2-3ma.
I didn't see anything on my UEI clamp so I ended up using a current probe on my Fluke 8060A

You took the long way but I'm glad you're in agreement. GFI would have been bad for my application.