SUB PANEL Grounding

JStofik

I am installing a sub panel to a new barn.
Circuit Breaker at Feed box to Sub Panel is 20 amp - 2 pole
Pull distance is ~320 feet with 4 wires of THNN #10 through PVC.
(Black, Red, Neutral, Ground)
All outlets will be GFCI protected.
I will be powering:
a horse water tank heater (winter)
3 or 4 box window fans (summer)
3 outside lanterns (60 watts or less each) (all seasons)
4 inside lights (100 watts each) (all seasons)
1 general use outdoor GFCI (intermitent/temporary use)

I am using a Square D sub panel 70 amp box (which I had to add a Neutral bar that is screwed onto the box housing)

My question is: ??
I was intending to install one 10 ft. ground rod which is wired back to
the box for a separate gound bar screwed to the box.

Should the ground wire be connected to a new separate ground bar -(which will be screwed onto the box)?

In effect the ground bar and neutral bar will both be screwed into the box housing, but each bar will have separated neutral and separated grounds.

Should I NOT BE HAVING A SEPARATE GROUND ROD and only use the MAIN green feed that was pulled through (because this is a SUB Panel)
I live in NJ for code determination and 2011 if still a question.

Thanks

Speedy Petey

My comments in RED:
You can eliminate MUCH of this work by simply running a 20A multi-wire circuit to the barn and be done with it.

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stickboy1375

There is no minimum size FEEDER requirement, only a DISCONNECT requirement.....

225.39 Rating of Disconnect


And I agree, just run a 20 amp multiwire branch circuit and be happy.

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Speedy Petey

I thnk we've been though this before.

Most inspectors I know interpret this as meaning the minimum size feeder as well based on calculated load., and I agree.
So if a breaker is the disconnecting means then the circuit or feeder would have to correspond.

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stickboy1375

It says what it says, I dont know what else to tell you... Anything else is just misinterpretation... I can easily provide 20 amp OCD at the house and install a 30 amp disconnect at the remote structure... If they wanted a minimum size feeder then the code would say that... 225.39 is clearly the rating of the disconnect ONLY.... FEEDER size is not mentioned once.

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stickboy1375

WHY? The OCP is at the structure... you are only supplying a disconnect at the remote structure...

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Speedy Petey

Then IMO that code section makes even less sense than all the other crap in the NEC.
If they'll allow a 20A feeder then WHY demand a 30A or 60A disconnect? That's kind of my point.

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stickboy1375

No idea... never really had that scenario before...but why require the disconnect to be suitable for service equipment? that makes no sense as well.... They know its a sub-panel, there fore no bonding of the neutral is required!

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JStofik

ok, I know what and how to use for feed wire to the new sub panel.

My question was centered on the sub panel wiring of ground and neutral.

I am going to double rod the ground wire back to the box where it is connected to the bar that is screwed into the back of the box.

I need to know if the neutral coming in the sub panel and the circuit neutrals are to be isolated from the grounded box.

In main panels ground connector and neutral are used pretty interchangeable. But in a sub panel, do the neutral and ground need to be separate from each other.

I read the NEC rules and I gotta say, it looks like a lawyer wrote it with all the number cross references, instead of easy to follow diagrams.

Thanks for your help.

stickboy1375

Neutrals and grounds must be separated, ground rods at separate structure... eh, its not that hard to read, just gets some use reading too...

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JStofik

Thanks for your help. You made it clear as day.
I guess I got (confusion!) for reading the code.

stickboy1375

dont worry, it happens to all of us...

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Stubbie

Probably left over from the days of 3 wire feeders .... then again if it is a feeder then it lands on some enclosure that has OCPD installed. Most of those have main bonding provisons which really is about all that makes a disconnect "service rated". To a degree a service rating will generally keep you away from disconnects that don't have neutral provisions ( like a/c disconnects) and are not service rated. Though there are a few A/C disconnects that say they can be made service rated by adding a ground bar accessory. Bear in mind also that there are still rural areas that are not on code cycles that require 4 wire feeders and 3 wire are still allowed which will require a main bonding means.
I would like to think that the ROP process will eventually get around to creating language that is up to date.

As for the disconnect rating .. it's based on the number of circuits being used to supply the actual building loads. I am assuming that they want a disconnect that is capable of disconnecting any possible short circuit. I also think they (the NEC guys) expect an electrician to not design small feeders to large amperage building disconnects when there is no practical reason to do so and termination lugs are to freakin big for small wire awg.. So for service rating that pretty much starts you at 30 amps especically when your thinking about using a non-ocpd disconnect at the remote building.

Now another grey area is only one feeder or branch circuit can be ran to the remote building or structure .... there are exceptions but generally the only one that applies to residential is "other characteristics" where it says "different uses" such as to operate exterior lighting. I've heard many arguments on that small sub-section as to its meaning when it applies to residential.

brgmgb

Even at 20 amps, how can you get by with #8 wire? That seems too small to me.