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New Shed - Electrical Service - Review please

1K views 7 replies 5 participants last post by  rjniles 
#1 ·
Hello everyone,
Have been researching the requirements for feeding electrical service to a new shed on my property.

The FACTS: sorry for the length and rambling ....

Entire distance for wire run in 3/4" PVC conduit = 130ft.
Shed electrical needs would involve a maximum of four 20 amp breakers for lighting, normal 120VAC electrical tools, and a small 120VAC AC unit.
Haven't finalized whether there will be a sub-panel rated for 60A (smallest I can find) or a main lug breaker load center rated 100A (again the smallest I can find).
Plan to install two ground rods minimum 6ft apart at the shed that will connect to the sub-panel or load center (whichever I end up using).
Plan to use an existing 50A breaker that is no longer being used as a feeder from my main house panel.
This breaker currently feeds a manual disconnect mounted on wall at rear of house for the old pool heat pump. I planned to remove this disconnect box & install a new 4sq PVC junction box. I heard something about code not allowing a disconnect to be junction box.

The wire size is #8 THHN/THWN-2 ....
There are only 2 -#8 wires w/a #10 grd wire... So I already know I need to add another #8 for a neutral.
From the PVC wall junction box - another 90ft to the shed for total wire run of 130ft. Wire the same #8 - 3 conductor ....

It is my understanding that I do not need to carry a ground wire from house panel to shed. Reason being it serves no purpose in the PVC conduit and that code requires shed panel to have its own separate ground (grd rods). Neutral and ground must/will have separate bus bars in shed panel. Correct?

Major concern of mine is wire size. Reading the NEC amp charts I see #8 THHN/THWN-2 is good up to 55A. But most of the web info I've read seems to indicate #6 is required...why?
If I need to pull all new #6, I will but hopefully this won't be necessary.

Anyway these are my main concerns. I'd appreciate any knowledgable advice about these issues.
I've already applied for the necessary permits and hope to do all the work myself as most of the electrical estimates are way over my budget. I have worked as an apprentice for about 3 years in commercial electric industry before moving into the electronics industry where I've maintained, perform PMs, repaired, troubleshot, and installed a wide variety of electronic equipment in hospital environment as a Biomedical technician for 25+ years.

I'm now retired (1year) and living in sunny Florida...

Thanks in advance,
Gary
 
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#6 ·
You misunderstand two code requirements.

Others have already noted that, yes, you DO need a ground wire - plus ground rods at the shed. You are NOT allowed to treat the shed as a separate service. It will have a panel with a feeder circuit.

Ground RODS are for static and lightning - every lightning target needs its' own ground rod.
The ground WIRE is what lets the breaker trip if something goes wrong.

As for wire size ... you can't use the 90 degree column for THHN in this instance. Buried pipe is a 'wet' location, which means you'll have to go by the THWN rating (the 75 degree column). You're further limited by the temperature ratings of your breaker connections, which might mean you need to use the 60 degree column.

Plus, there's "voltage drop" to consider over that distance. So, you'll need to use #6 copper wire.

Four #6 wires are the most you're allowed to have in 3/4 PVC conduit. Considering the length of the run, that will be a difficult pull. Besides- what if you want to upgrade to 100 amps later?

Pipe is cheap. Digging is tough. Use bigger pipe.

The box stores sell "spa panels" or "hot tub disconnects" that will serve just fine as your outside disconnect on the shed. Typically, they have plenty of room for your connections, a 50-amp GFCI breaker, and places to land the ground and neutral wires. Just make sure you're is intended for outdoor use.

Yes, I also advise a breaker in the house panel, at the beginning of the run.

There is no reason you can't use the existing disconnect as your starting point. If you insist on replacing it, a 4" box will be way too small. Get at least a 6x6x4 'can", NEMA 3R or NEMA 4 rated.
 
#4 ·
Thanks for the correction on the ground wire having to be included in the entire run from main breaker to the shed panel.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To answer the question about 240VAC service.... NO I am NOT planning on having any 240VAC circuits in shed.
 
#5 ·
You would run 240v to a subpanel in the shed then split it off to 120. It would be 240 to protect against the voltage drop across 130'. Look up voltage drop charts and they will tell you wire size for the run you have and max current plus voltage drop.
 
#7 ·
KarenStein and others,
Thanks for the technical info and advice I've received so far. It appears that the unfortunate fact is I cannot use #8 wire as planned. So I've done some rethinking of this issue and believe I may have found a answer to the wire problem.
Is there any problem to this install if I just replace the current 50A main feeder breaker with a 35A breaker (if it exists, I haven't checked yet)? That would reduce the available trip current to 35A which I believe would the #8 wire could safely carry. This lower amp capacity doesn't matter to me because I don't ever see any time I would need more than anything approaching 20-25A....
It solves the wire issue although I am still going to have fun yanking the necessary neutral #8 wire in the first leg of the already installed wire run ( that's 40ft to the old disconnect area).
If that doesn't work then I'll just run a completely new 1" PVC with #6 and a 40 or 50A main breaker.... I'm tired of messing around. And just so you all, I didn't dream all this electrical scenario by myself....I had no less than 4 licensed electrical contractors come to my home to provide estimates for this job. It may not surprise you but all 4 companies came up with completely different methods...although all of them didn't bat an eye about using the #8 wires. I don't know how they figured to bypass all the physical laws regarding the wire but it really surprised me when I started looking online for advice and discovering it wouldn't work.
Thanks everyone!
Gary
 
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