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Adding a 220v sub-panel
Asked Nov 25, 2007, 07:35 AM
I am in the process of building and will soon be wiring a detached 20' x 12' shed in my yard. The run from the service at the house to the sub-panel in the shed will be a total of 47 feet(including the vertical portions of the run), so, I'm using 50 feet as a basis. I intend to have the smallest electric water heater that I can find. 5 to 10 gallons if possible, and a welding machine that pulls 20 amps. I have decided to run 8/3 with a ground buried in PVC fed by a 30 amp circuit breaker. I have done considerable reading on the subject, and I have been confused by some things that I read. All of the switch boxes that I use will be plastic. So, according to what I've read, I don't need a continuous grounding wire. However,some things that I have read imply that I will need a dedicated outlet for the water heater with a continuous ground. ( The water heater will not be continuous use. Perhaps one to two days per month. The rest of the time, the breaker or switch feeding it will be off). Also, I have read that sub-panels don't require a main breaker (or disconnect) upstream of the branch circuits. If I read correctly, the feed will run as follows : the double pole 30 amp breaker (for 220) at the service panel, 8/3 with a ground to the sub-panel bus bars, and the neutral bar , and then on to the branch circuit breakers. (I would prefer to have a main disconnect at the shed). The first receptacle of the two branches containing receptacles will be GFCI protected. The neutral and ground bars of the sub-panel should not be bonded so, where will the continuous ground flow to? Two final questions, does the sub-panel need its own ground from the panel frame to an electrode, or does the ground wire connection to the service panel suffice, and what size PVC do I need to run the 8/3 with a ground ? Thanks
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Senior Electrical & Lighting Expert
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Nov 25, 2007, 10:40 AM
My answers are in bold.
So, according to what I've read, I don't need a continuous grounding wire.
Where did you see that? A subpanel should have a separate equipment ground wire run with the feeder. An 8-3 cable with ground will work fine. Three wire feeders are allowed, but not practical and seldom done, this allowance will be deleted from the next Code in 2008.
Also, I have read that sub-panels don't require a main breaker (or disconnect) upstream of the branch circuits. If I read correctly, the feed will run as follows : the double pole 30 amp breaker (for 220) at the service panel, 8/3 with a ground to the sub-panel bus bars, and the neutral bar , and then on to the branch circuit breakers. The neutral and ground bars of the sub-panel should not be bonded so, where will the continuous ground flow to?
The neutral will go to the neutral bar, the one with the insulating feet. The equipment ground needs to connect to a separate ground bar, needs to be added as an accessory, available where you get panels. Bolt the ground bar directly to the panel using machine screws, you will see threaded holes in the back of the panel for this purpose. Read the panel label, it will tell you the part number for the ground bar. Only bare and green ground wires connect to this bar. Only white neutrals connect to the neutral bar. Do not install the green screw thru the neutral bar into the panel.
(I would prefer to have a main disconnect at the shed).
The panel in the shed does not need a main breaker, the breaker in the main panel is sufficient, however you may have a main in the shed panel if desired
Does the sub-panel need its own ground from the panel frame to an electrode,
Yes, any subpanel in another building needs a ground rod, connected with min #6 copper wire to the equipment grounding bar in the subpanel.
Or does the ground wire connection to the service panel suffice,
No, see above.
What size PVC do I need to run the 8/3 with a ground ?
What kind of cable? If using NM-B Romex, then it cannot be used outdoors at all. You may use UF cable and direct bury it 24" deep. I would not pull any kind of multi-conductor cable thru conduit.
A conduit can be 18" deep.
Once a conduit system is installed using all the proper fittings, elbows, etc, use individual conductors pulled thru. If you change to cable in the home, a junction box will be needed to splice the cable to the pulled wires. Here you can use THHN/THWN or XHHW insulation on copper wire.
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Nov 25, 2007, 02:44 PM
I have 200 feet of individual #8 cable from a previous job. I intended to use it. I'm not sure of the type but, I had an electrician do a similar installation for me about 5 years ago at my previous home, and this is the type of wire he used, and it was run underground, in PVC conduit. I was going to use three separate cables, and add a bare ground. So really,, I need to know what size the PVC should be, and how big the ground wire needs to be for this service...
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Nov 25, 2007, 03:03 PM
Originally Posted by acajun4u
-To find the type, read the markings on the wire. If THHN/THWN then you can use 3/4 inch PVC.
-A #10 Copper wire will work for your EGC
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Nov 26, 2007, 05:39 AM
It never hurts to go up in size for your conduit. It will make it easier to pull. And I would bury an empty conduit right along side it in case you want to add a phone line, CAT5 cable, coax, etc. at a later date. I would go so far as to leave a couple of strings in the conduits.
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