Coiling 40 ft of 2/0 conductors in a Jbox. Any code violations?

[Kalyan]

I have a situation at a PV installation where the point to point fault current had to be limited to 10000 A. Hence as per the fault current calculation for a 2/0 wire with a 14,318A fault current available at point 1(line tap), i had to make the conductor length at least 40ft to be able to achieve the less than 10000A fault current at point 2 (PV system disconnect). The wire size has to stay at 2/0 as per utility. With just 5 ft physical distance between Point 1 and Point 2, the only way to accommodate 40ft is to coil the excess conductors in a jbox (Which is almost next to impossible for a 2/0 wire size). Are there any restrictions as per NEC or any other safety/ good practices that would stop me from coiling the conductors? Please suggest any alternative methods for making this work.

See the attached rough sketch to help give you a better understanding.

 

[petersonra]

I don't think it is a safety issue but you might end up adding some inductance to the system.

that would be a pretty large j box too.

is there some clean way you can make the conduit run(s) longer so less of it has to be hidden in a box? maybe move the disconnect 20 feet away?

ETA: could you use DB cable and hide the excess underground?

 

[Kalyan]

I don't think it is a safety issue but you might end up adding some inductance to the system.

that would be a pretty large j box too.

is there some clean way you can make the conduit run(s) longer so less of it has to be hidden in a box? maybe move the disconnect 20 feet away?

ETA: could you use DB cable and hide the excess underground?

I think moving the disconnect 20 ft away would be the best solution but I was trying to avoid doing that. I wanted to see if there were any better ways to coil 40 ft of wire. I will probably move it and run conduit on the wall. Thanks for clarifying on the safety part. I also like the DB cable solution.

 

[Fulthrotl]

I think moving the disconnect 20 ft away would be the best solution but I was trying to avoid doing that. I wanted to see if there were any better ways to coil 40 ft of wire. I will probably move it and run conduit on the wall. Thanks for clarifying on the safety part. I also like the DB cable solution.

it's going to act like a choke to some extent, depending on how many
turns are in the box.

i was just wondering if tickets were available to the coiling ceremony.
packing a marshmallow in a piggy bank has entertainment value....

 

[Jraef]

I posted a similar problem (on a larger scale) here a few years ago; someone had to go from 47kA AFC to below 42kA on a 1200A service because the MCCs were only rated 42kA and the transformer came in larger than originally designed (because of lead times). The contractor friend who was in this pickle ended up deciding that his least expensive option was to trench around the building and make a giant loop in the ground, adding the resistance of about 150ft of extra conductor. A Current Limiting Reactor (his only other alternative other than replacing the transformer) was way too expensive.

 

[Daniel malack]

Are we talking DC? AC would be a problem.

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[Sahib]

I think moving the disconnect 20 ft away would be the best solution but I was trying to avoid doing that. I wanted to see if there were any better ways to coil 40 ft of wire. I will probably move it and run conduit on the wall. Thanks for clarifying on the safety part. I also like the DB cable solution.

Why can't you try interposing a 1:1 transformer to reduce fault level if it is more economical?

 

[petersonra]

Why can't you try interposing a 1:1 transformer to reduce fault level if it is more economical?

40 ft of 2/0 conductor is less than $100. You won't get much of a transformer for $100.

 

[Tony S]

How about an inline reactor?

Not cheap I know but it will look slightly better than a pit with 40Ft of cable jambed in it.

 

[jim dungar]

Keep all 3-phases relatively together, do not coil each conductor or phase individually.

 

[electrofelon]

Was the fault current value calculated from the transformer specs or from a value obtained by the utility? IF the latter, you could try going off the actual equipment and see if that gets you there.

Another option would be to change some equipment. You said "PV disconnect" and "10,000A" max in the OP which implies maybe you have a non fused safety switch. Going with a fused switch would solve the problem. Somewhat of a hassle but not a huge expense.

 

[Sahib]

40 ft of 2/0 conductor is less than $100. You won't get much of a transformer for $100.

If OP had a spare transformer at hand..........

 

[kwired]

the only way to accommodate 40ft is to coil the excess conductors in a jbox (Which is almost next to impossible for a 2/0 wire size).

Impossible would be to try to fit it into a 12x12x4 box, 48x48x12 becomes much easier, but definitely more cost.

 

[petersonra]

If OP had a spare transformer at hand..........

and spare OCPD and whatever else would be needed to add a transformer.

 

[jaggedben]

...
Another option would be to change some equipment. You said "PV disconnect" and "10,000A" max in the OP which implies maybe you have a non fused safety switch. Going with a fused switch would solve the problem. Somewhat of a hassle but not a huge expense.

Based on his diagram it had better be a fused disconnect. But I was going to make the same general comment. I know disconnects can be expensive but I was wondering if the problem was the disconnecting interrupting rating and if it would make more sense to just upgrade it.

 

[Daniel malack]

I would drop the coiling idea #1, basic electricity (COIL) what happens to the circuit when you coil 40' of wire?

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[kwired]

I would drop the coiling idea #1, basic electricity (COIL) what happens to the circuit when you coil 40' of wire?

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You raise the overall impedance, just like the OP intends to do. Overall length changes the resistive component, how you coil it is what will make a significant difference in inductive or capacitive components.

 

[Jraef]

Impossible would be to try to fit it into a 12x12x4 box, 48x48x12 becomes much easier, but definitely more cost.

Also, mind the box fill issues. 2/0 is not small and each wrap of that coil will count as two conductors. 40' of 3 phase cable coiled in one box, the box volume is likely larger than you might imagine. I'm not going to do the math but I'm thinking that 48x48x12 might Be the MINIMUM size, if that even works.

 

[ggunn]

You raise the overall impedance, just like the OP intends to do. Overall length changes the resistive component, how you coil it is what will make a significant difference in inductive or capacitive components.

It seems to me that the double thickness of insulator material between conducting metal along with the very small voltage differential between adjacent points will make the capacitance effect very small.

 

[GoldDigger]

It seems to me that the double thickness of insulator material between conducting metal along with the very small voltage differential between adjacent points will make the capacitance effect very small.

True for a loop of a single phase.
It gets more complicated when you coil all three phase wires.
But at worst capacitance comparable to a coiled four conductor cable. And lowest value of inductance close to zero, as for a coiled cable with balanced current flow.
I guess you should plan to coil the neutral (if any) too.

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