Sub panel

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Wilson01

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I want to put a sub panel in my shed for a few circuits. It’s roughly 150’ run from my panel in the house to my shed. I already have conduit run the whole way. I was given two pieces of wire that I wanna use and I already have a 8”x8” junction box. One wire is 6/3 THW submersible pump wire with a #8 ground. The second wire is 4/3 tray cable THHN/THWN with #8 ground. Sixty feet of the conduit is inside my basement where I will put the junction box before going outside. One question is can these wires be put in conduit and second does it matter whether I start from my panel with the 6/3 or 4/3 ? Obviously I know I can’t over amp the smaller wire so I was thinking of putting a 60 amp double pole breaker in for the circuit. If I’m reading the chart right the maximum amps for #6 THW copper is 65. What should I use for a splice for putting a #6 and #4 wire together? Thanks for any help
Steve
 

wwhitney

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With a subpanel at the shed, the shed will need a grounding electrode system, most likely 2 ground rods, unless it has a recent concrete foundation with a Ufer ground that you can use instead. The ground rods get connected to the panel EGC bar, and grounds and neutrals are separate as it is a subpanel.

As to running the cables in conduit, that's fine as long as the conductors inside are NEC recognized wet location wiring types (which THW and THWN both are). The conduit gets sized based on the largest cross sectional measurement of the cable. So if your 6/3 plus ground THW pump cable is flat like this one:

https://www.wireandcableyourway.com/6-3-w-grnd-heavy-duty-flat-jacketed-submersible-pump-cable

it has a largest measurement of 1.32" across the flats. That would control assuming the tray cable is round. With one wire in the conduit, the conduit can be filled at 53% based on area, so the inner diameter of the conduit would need to 1.32" / (sqrt(.53)) = 1.81". So you'd need a 2" conduit.

Then for the junction box, with conductors of size #4 or larger, the rules get more complicated. If you have 2" conduit and #4 conductors, you'd need your junction box to be at 12" wide (6 x 2") in any dimension parallel to a conduit coming into the side, and at least 2" deep opposite the removable cover (based on the #4s).

Generally you'd be better off pulling individual conductors in conduit, and Al XHHW is often cost effective. (4) #4 Al would fit in 1" Schedule 40 PVC, but if you're going to the trouble of running conduit you might as well run at least 1-1/2".

Cheers, Wayne
 

Wilson01

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I thought if I ran 4 wire and bonded the panel I didn’t need ground rods ? Also the 6/3 submersible pump wire is twisted and not jacketed. And the junction box I have is 8x8x6” deep. Would that work ? Thanks for your help

Steve
 

wwhitney

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1) Nope, every detached building gets grounding electrodes. If it's supplied by a service from the utility, then the main panel gets the the neutral-ground bond and the grounding electrodes are connected to the neutral. If it's supplied by a feeder from another building, then for new construction the feeder needs an EGC, the grounds and neutrals are separate in the subpanel, and the grounding electrodes get connected to the ground bar.

2) What is the diameter of each of the two cables? The minimum inside diameter of conduit for a single cable is the 1.37 * cable diameter. [That's 1/sqrt(0.53)].

3) So it depends on where the #4 cable comes into the junction box. If it's the back of the box opposite the cover, that's fine. If it's the side of the box, then it depends on the conduit size used. If the #4 cable is in 1-1/4" conduit, then it's OK. If it's 1-1/2" conduit, then would the box would need to be at least 9" in that direction. And if it's 2" conduit, it would need to be at least 12" in that direction. 1-1/4" Schedule 40 PVC, if that is what you are using, has an ID of 1.36", so that would be allowed if your #4 TC cable diameter is under 1".

PS #6 Cu THW has an ampacity of 65A, so you can put it on a 60A breaker, or if the calculated load is between under 65A, you can put it on a 70A breaker. If the calculated load is above 65A, #6 Cu is too small.

Cheers, Wayne
 

Wilson01

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The main panel in the house that’s fed from the utility company does have ground rods. I also have a sub panel in my first floor because it’s a modular home. I would be feeding the sub panel in my shed off the main panel box. The 4/3 TC measures 3/4” and the 6/3 pump wire measures roughly 9/16”. The pvc is 1-1/4” schedule 40 and it comes in the side of the junction box in the middle but on the bottom half. So there’s 4-3/4” from the cover to the conduit.
I’m using the sub panel for a circuit of lights and outlets and the a designated outlet for my RV which is a 30 amp single pole breaker. Thanks Wayne
Steve
 
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