3-wire or 4-wire with ground rods?

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Old 06-16-15, 11:19 AM
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3-wire or 4-wire with ground rods?

Hi. I'm working on a garage / apartment / man-cave that is separate from my main house. I'm going to run a 2 gauge aluminum service cable to a sub panel, which will serve 5 20-amp circuits.

After reading around, I'm confused whether I need to run 2-2-2, or 2-2-2-4 wire from the main panel to the sub panel (at about 100 ft distance, btw). I read that a separate structure requires, by code, two ground rods at 6 feet apart or more... no problem here, I can put those in... but do I still need to run a ground wire from the main panel if I am going to be grounding the sub panel through the two ground rods? If so, why?

Also to note, I will be running a coax and cat 6 cable to the same building, in the same trench... which I read could be an issue when not using a ground wire in your electric run...

Thanks in advance for any help to clear my confusion here.
 
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Old 06-16-15, 11:28 AM
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The feeder to the outbuilding needs to be 4 wires. You will also need the ground rods.
 
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Old 06-16-15, 11:32 AM
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I recommend at least 12" separation between the power and data lines (can be a vertical separation in the trench). I also strongly recommend a conduit for the data lines -- they will be obsolete long before your outbuilding is. Make it easy to replace for an extra $30 in materials up front.
 
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Old 06-16-15, 11:32 AM
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Ground rod is the GEC for draining atmospheric charges. The fourth wire on a subpanel feed is for the EGC. The GEC is not a substitute for the EGC. The EGC is to provide a low resistance path for clearing shorts to ground.
I'm going to run a 2 gauge aluminum service cable to a sub panel, which will serve 5 20-amp circuits.
Why #2? Do you have loads to justify that? It doesn't sound like that with just 5 20 amp circuits. If those are general purpose 120 volt circuits #6 on a 60 amp breaker may be enough. Any electric heat, plasma cutters, large air compressors?
 
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Old 06-16-15, 11:54 AM
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The low voltage cables should be rated for use outside and or underground.
 
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Old 06-16-15, 01:46 PM
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I think that #2 aluminum or #4 copper is about right for 60 amps 100 feet away. (Don't have the wire resistance table handy at this moment.)
 
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Old 06-16-15, 03:00 PM
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#6 copper is fine for 100 ft assuming the actual load is less than 60 amps. No indication any of the 20 amp circuits will be fully loaded. If they are then my size estimate may change.
 
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Old 06-17-15, 08:53 AM
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Hey guys -- thanks for the quick replies! As my first post on this site, I'm impressed.

It sounds like I have a little research to do on the difference between EGC and CEG. It's good to know that I need to connect both the ground rods and the ground to the main panel simultaneously. Also, good advice on separating the data lines from the power and running them through conduit -- will do.

To answer your questions Ray,

Why #2?

Actually, a local electrician did recommend 6-4 copper, but it looks like 100 feet of it is about $230... but 2-2-2-4 aluminum would run me roughly $160, and have less voltage drop / more headroom... plus the aluminum is direct burial wire, while the 6/4 copper I looked at was not. I'm not sure if I'm missing something though -- is there an advantage to the 6-4 copper?

Do you have loads to justify that?...Any electric heat, plasma cutters, large air compressors?

In the winter, on cold days, I might have 2 electric heaters, which can draw 10 amps each, in summer an air conditioner, plus a mini fridge, home theatre, stereo, microwave, lights, computers... Also, tools will be run off these circuits at times as well... so I figured it would be best to size the wire for worst-case-scenario 80 or 100 amp draw... I don't know, just erring on the side of caution I guess.
 
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Old 06-17-15, 08:59 AM
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it looks like 100 feet of it [copper] is about $230... but 2-2-2-4 aluminum would run me roughly $160
Then go with it. Electric heat is reason enough even not considering cost. If it is not dual rated* though it can not be used inside so you will need to transition outside.

*Look at the individual conductors. Are they marked as a type of wire that can be used inside? If so then it is dual rated.
 
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Old 06-17-15, 10:03 AM
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Ah, I'm glad you brought this up... it was going to be my next question in fact. So, ya, half (or more) of my run is along the ceiling of the basement / rafters of the 1st floor, before it even enters the ground... so some direct burial wire cannot be run in an open space (such as rafter cavities)?

I was looking at this:

Southwire 2-2-2-4 Aluminum Quad Dyke Wire (By-the-Foot)-55417399 - The Home Depot

Do you think individual connectors, instead, would be the way to go here?
 
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Old 06-17-15, 11:06 AM
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Since you have to go both inside and outside, you either need to use a complete panel-to-panel conduit or switch cable types at the exterior of the building. Quad Dyke or any other kind of URD (underground residential distribution) cable cannot go inside a building due to fire/smoke rating. You could use aluminum SER cable in the house, put a junction box on the exterior wall and switch to the URD there if you want to not use conduit. However the cost of the splice box and insulated connectors will probably be a wash versus just doing the conduit.

I would probably go the conduit route using 1-1/2" PVC from the main panel to the garage panel. The conduit should be at least 18" deep where underground, and there should be LB fittings where accessible corners are needed. Through the conduit, I would pull XHHW aluminum conductors.

Another options is MHF (mobile home feeder) cable which can be direct buried without conduit, or run indoors, but must be in conduit inside.

On copper vs. aluminum, almost all services and panel feeders are installed with aluminum wire due to the significant cost difference when buying larger wire sizes. Copper is somewhat more resistant to corrosion than aluminum, but when installed correctly there is little difference between the two. Aluminum wires need to be larger than copper for equivalent amperage.
 
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Old 06-17-15, 07:47 PM
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I would probably go the conduit route using 1-1/2" PVC from the main panel to the garage panel. The conduit should be at least 18" deep where underground, and there should be LB fittings where accessible corners are needed. Through the conduit, I would pull XHHW aluminum conductors.

I like this idea. I would prefer one continuous run, without having to splice to a different wire type... the simpler the circuit, the better I think.

So, is this the wire you are referring to? --

Southwire 500 ft. 2-Gauge Stranded XHHW Wire - Black-11272207 - The Home Depot

Then just cut 4 runs of 100 ft. wire and keep em' labeled - hot, hot, neutral, ground, right?
 
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Old 06-18-15, 06:44 AM
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The ground only needs to be #6 if you're buying by the foot and want to save a little bit, but yes that's the idea. The neutral must be wrapped with white tape for about a foot on both ends and the ground with green tape to be a code compliant marking.

Use no ox paste on the aluminum wire terminations. After the wires are pulled through the pipe, use duct seal putty to sealed up the conduit at the entrance/exit of each building to prevent condensation in the panels.
 
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Old 06-18-15, 08:22 AM
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Excellent -- thanks so much for the help.

One more question -- when running the ground from my two ground rods to the sub panel, does this wire just get buried in a trench, or does it require some conduit as well?

Also, when reaching the panel, it will connect to the same point that the ground wire to the main panel ties into, correct? In other words, these connections are bridged at the sub panel?
 
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Old 06-18-15, 08:54 AM
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The GEC (bare #6 copper) can just be buried in a trench. A sleeve of PVC conduit is optional, but not metal conduit. The rods should be driven at least 6' apart.

At the panel the EGC (ground from the house panel) and GEC (wire to the ground rods) both connect to the ground bus bar which is screwed into the metal box. You sometimes have to buy the ground bar separately as panels only come with a neutral bar. In a subpanel, the ground and neutral bars remain separate.
 
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Old 06-19-15, 06:22 AM
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What about where the GEC leaves the trench -- does it need it's own conduit running up to the sub panel, or does the wire just get left exposed here as well? In other words, you would be able to see the bare copper running into the house, along side the PVC conduit that houses the feeder conductors?
 
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Old 06-19-15, 07:08 AM
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I often zip tie it to the back of the panel feeder conduit until it gets underground. It can have its own 1/2" PVC if you rather. Most panels have a small roughly 1/4" knock out hole right next to the big main conduit knockout that is for the bare GEC to run through adjacent to the feeder conduit.
 
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Old 06-19-15, 07:56 AM
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Actually, a local electrician did recommend 6-4 copper, but it looks like 100 feet of it is about $230
I think what you meant was 6-3 w/Grd, but if this is what you mean........

Shop 125-ft 6-3 Indoor Non-Metallic Wire (By-the-Roll) at Lowes.com

........you can't use it anyway because it is for dry locations only and that excludes outdoors and underground in conduit.

This is the 6-3 w/Grd you'd need.

Shop 6-3 UF Wire (By-the-Foot) at Lowes.com
 
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Old 06-20-15, 10:21 AM
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Thanks for all the help -- much appreciated. I think I'm all set at this point... (knock on wood). I'll be back if I run into any major snags on the job.
 
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