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Old 03-21-2007, 10:18 AM   #1
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Subpanel feeder questions



Last edited by Silhanek; 05-17-2007 at 10:04 PM.
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Old 03-21-2007, 10:27 AM   #2
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1. Type NM cable (Romex) is not approved for use in wet locations and underground conduit is defined as a wet location. You can use UF cable for the entire run or transition from NM in the attic to either individual THHN/THWN wires or UF cable.
2. Schedule 40 PVC can be exposed, but it is not approved for physical protection. You need schedule 80 for that.
3. You definitely have to install a ground rod if the structure is detached. You have the option of using a 3-wire or 4-wire feeder if there are no other metallic paths between the buildings. I would use the 4-wire feeder and keep the ground and neutral separate in the subpanel.


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Old 03-21-2007, 12:45 PM   #3
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1. Most, if not all, individual conductors available nowaday carry a dual THHN/THWN rating.
2. Use Sch 40 PVC for the attic and underground portion, sch 80 for the exposed, it will be cheaper. 1 inch will work, but I would use 1.25 inch to make pulling easier.
3. You must have a ground rod regardless of using 3-wire or 4-wire. Using 4-wire does not restrict you from adding a phone or cable line later. The ground can be #10 when using #6 ungrounded conductors.
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Old 03-21-2007, 03:12 PM   #4
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2. Conduit. Am I able to use the ridgid PVC schedule 40 conduit from Home Depot for what I want to do? Labeled on the exterior of it reads "For outdoor and indoor use." But, I've also read online that anywhere conduit is exposed, it can't be PVC. If I can't use PVC, my next step up would be EMT conduit with water tight connectors. Would I be ok doing that?
I dont like emt underground though it may be listed for direct contact with earth. My experience with it underground is that it rusts out in a few years. I am also glad you are choosing conduit, it is a much superior installation than direct bury cable.

As house helper said use sch.40 pvc everywhere the conduit isnt exposed to physical damage. In most cases like yours this area of physical damage will only be where the conduit exits the ground just before you enter the detached building and where you exit the house to go underground. In these areas use sch.80 pvc. Last time I looked HD didnt carry sch. 80. That is a mystery to me as they are missing a key component to all pvc installations that have areas of physical damage. Try an electrical supply for it, you will only need a few short sections. If you cant find it then use EMT in those areas only. transition fittings are readily available. Connect the emt to the pvc right at ground level. Probably a 10 foot length will be all that you need. Use premade pvc 90 sweeps where you transition from underground horizontal to above ground vertical and pvc Lb fittings to enter and exit structures. You can look at these in the pvc fitting section at HD. I think you can figure out how they work.

3. Grounding bar. My garage is detached with no water/sewer lines or anything else connecting it to the main structure. I'm pretty sure I need to drive a grounding rod into the ground at the detached structure. If I do this, I will need a cable from that to the subpanel grouding bar and need to keep that seperate from the nuetral bar? Do I run a seperate ground cable from the grounding bar in the subpanel to the main panel then?
Yes... you need to install an accessory ground bar if one didnt come in the sub-panel... most dont. The bus that came in the sub-panel will be your neutral bar, notice it is on insulated standoffs from the metal of the panel. The ground bar will install in premade holes in the back of the panel. Get a ground bar for your make of panel that fits these holes or hole. These ground bar kits are sold at HD.

You are confusing the grounding electrode system with the equipment ground system. The grounding system is the ground rod. The ground wire ran with the feeder back to the main panel is the equipment ground. The two systems have entirely different purposes. The grounding (earthed) system is for high voltage events like lightning and provides property protection. The equipment ground system is for human safety and facilitates the tripping of breakers during fault conditions.

Essentially what your going to do when you run 4 wires (H-H-N-Grd) is install the ground bar in the sub-panel and you will not install the bonding means that connects the neutral bar to the metal of the panel because this will bond it to the ground bar. You dont want this when running 4 wires to the sub-panel. Sometimes this is a green screw but depending on the panel it varies. Dont use it.
At the ground bar your going to connect a #6 solid copper grounding electrode conductor (wire) to the ground bar and route it out a knockout to the ground rod. Some knockouts you will see a much smaller disc than the others this is for the grounding electrode conductor. Use a listed ground rod clamp to fasten it to the rod. All this can be purchased at HD. Protect the wire where it is exposed in a sleeve of pvc till it goes into the trench you dug to get over to the ground rod.

Your then going to connect the #10 awg copper equipment ground wire you ran with the feeder to the sub-panel ground bar. On the other end at your main panel both the neutral and the ground wires are bonded if the panel houses the main disconnect so it doesnt matter which bar if there are more than one you connect the feeder equipment ground.

If you see two bars... a neutral and ground bar with only whites on the one with insulated standoffs (neutral bar) and bares and greens on the other bar and no main breaker in the panel then connect the feeder equipment ground to the bar with the ground wires not the white neutral wires. This setup indicates that a main disconnect is located somewhere besides the main panel.

You most likely are going to need 2 ground rods unless the inspector plans to test ground resistance. Ask him...if he isnt then drive 2 ground rods to satisfy code. Rent a rotary hammer drill with ground rod chuck to get the rods driven. Sledge hammers are a pain with t-post drivers coming in as a second choice to hammer drills.

If you are going to have several bends in the conduit be sure to install conduit bodies with access covers (except underground) every so often between bends to use as pull points. The rule is no more than 360 degrees of bend before a pull point needs to be installed.

If you choose 3 wires to the sub check with the local codes department as this may not be allowed. In 2008 the NEC is expected to require all feeders to contain 4 wires so I would side with househelper and run four wires.

Post back if you go with 3 wires as in this case you do not seperate the neutral and ground in the sub-panel.

Also if your going to have 220 volt loads like welders or air compressors most 60 amp panels only provide two spaces for breakers with 4 120 volt circuits possible if you use tandem single pole breakers. A 220 volt load requires a double pole breaker which will fill a 60 amp panel. I would suggest getting a 100 amp panel that has 4 spaces or 6 spaces then just feed it 60 amps with the #6 awg feeder. This will give considerable more flexibilty for branch circuits out of the sub-panel.


Last edited by Stubbie; 03-21-2007 at 03:38 PM.
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Old 03-22-2007, 06:30 PM   #5
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Originally Posted by Silhanek View Post
2. Conduit. Am I able to use the ridgid PVC schedule 40 conduit from Home Depot for what I want to do? Labeled on the exterior of it reads "For outdoor and indoor use."
Make sure the conduit that you use is the grey/DZYR type mentioned in the NEC. They don't sell it a most of the BORG's in my area and inspectors (mine at least) do not like to see ASTM pipe used for electrical installations.
Well, now, there's what's right and what's right and never the twain shall meet.
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