Electrician questions

You need three (3) wires AND Ground to get 240 and 120 at the garage and you will surely need 120. Assuming you are in the USA of course.

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John G

I have a service panel on the side of the house that feeds a sub panel inside. Over the years a few breakers got added to supply an air conditioner and electric clothes dryer (in the outdoor panel directly under the meter).

I was planning on running some power out to the garage - about 80 feet of cable. I figured to just tap into the outside box (the inside panel is ancient and has plug fuses);.

Run 240 out to the garage via a regular 12 AWG "2 wires plus ground" and put a 20 amp 240 volt GFI breaker in the outside panel on the house to a small sub panel in the garage for lights and a workbench.

The cable will run in a trench buried in the ground and only be protected with conduit where it goes into the ground and comes out. There should be no cause to dig in the area and the GFI should protect against shock should someone dig.

Intent is to bury plain old vinyl insulated cable. I don't see any that is rated for direct ground burial in the hardware stores - is this important and why?

Any reasons this won't work and be safe?

I do see "outdoor cable" in the store, but from reading the specs it appears that it's only claim to fame is the UV inhibitors in the vinyl

- other than the label, it is indistinguishable from the indoor stuff

- cuts and strips just like the indoor cable no tougher outside covering. The package says it can be buried... and it costs roughly double.

GFIs can and do fail to trip. That's why they have the test buttons on them.

Well, you're probably just a troll, but: Plenty. Also quite a few reasons that it's penny pinching and pound foolish....Right off the bat, you're short one wire, so "you doing the work yourself" is not looking like a good plan, and anyone qualified to do the work won't be doing it "your way." Likewise, anyone willing to do it "your way" is not qualified to do the work.

Indoor cable insulation is not waterproof. Any exterior cable (including those in conduit, which is assumed to be wet, because it usually is) must be waterproof (there will be a W in the "type") Within a relatively short time (might be a few years, but a proper installation lasts many decades) the insulation will fail.

Because you are a penny pincher, you'll then have to dig the whole trench up again. If you use conduit correctly, you only have to dig once. Conduit is CHEAP. Trenches are EXPENSIVE. If you were planning to dig the trench yourself, it's quite expensive in terms of labor - a shallow trench is unsafe, and violates electrical code. 18-24" minimum to the top of the line or conduit. A good job goes below frost line to keep the line from being stressed each year with ground movements, but that is not required by code, just by economics (long-term version). Warm up the pick and shovel muscles, you've got a lot of dirt to move.

The correct (waterproof) wire to run in conduit and conduit are usually LESS expensive (together) than correct direct burial wire - and the conduit solution is also much less subject to rodent or rock damage. Direct burial cable is a failure waiting, none too patiently, to happen. Any sensible person will oversize the conduit and add extra conduit when digging a trench, as both are trivial compared to having to dig it up again when you decide to run something else (ie, network) out there, or larger service, or some utility you don't even have yet.

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Cats, coffee, chocolate...vices to live by

Do yourself a favor, do it right!

First off, you should rent a trench digger and check with your local gov for the depth you need it at. Some places require a min of 36" others 42. etc..

Get 4 of #8 cable Black,white, red and grean/bare wire for ground. You can get DUR how ever, its not needed because you should be using ABS pipe coupled together as you pull the cable through it. Use what size you need that fits.. 1 1/4 sounds about rite. Get some plumbers pipe glue, plastic couplings and 1 1/4" rigid elbows, straights that will come up out of the ground into your boxes on both ends. You don't want just cable coming out of the ground or plastic pipe. It has to be protected with a strong wall of something. This is to keep pets, kids, wild animals from chewing on it, and maybe the weed whacker! The plumber shops have the proper couplings to join plastic and metal hardware.

P.S. The ABS keeps water out, bugs out and allows you to pull new wire through if you ever have a mother nature strike! Jamie

default used his keyboard to write :

If you big devices only have 3 wires then they do not confirm with today's practice which requires that the ground wire does not carry any current except fault current and the neutral has wire of its own.

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John G

default brought next idea :

There is a space required between power and data wiring but I do not know the details for the USA.

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John G

Also the stove and dryer, probably AC too, are on separate circuits for each of them. There are no other appliances downstream sharing 120V "ground" wires.

Why? my electric range, AC, clothes dryer are all three wire devices. In the case of the electric range the neutral is carrying current when the 120 volt outlet on the stove is used, likewise the oven and range top lights.

'Thanks. That makes sense. Downside of conduit is that unless it is flexible it won't fit the trench a gas powered wire laying trencher makes, and if it is flexible it won't offer much protection. Ideally I need flexible to curve around some tree roots.

It was my plan to drop some coax, cat 5, etc. in the trench too.

I lived in one house where someone buried a three wire extension cord out to an outbuilding and had a 240 volt welder running from it (not me).

I just dug a line for water 12" (below the frost line - to building codes) took me three days with a mattock, shovel and axe to hack through tree roots. This time I figured to rent a trencher and do it in a few minutes. (they claim 5 feet a minute worst case)

PVC "rigid" conduit makes large bends with no effort at all when you put a few sticks of it together. What seems "stiff" with one stick becomes floppy when you get to three or so (30 feet.) You can also get actual sweeps if you need to take serious corners, but don't use too many without also using a pull box or you won't be able to pull cable. Either drive the trencher fairly straight (stretch a string and paint a line to follow), or come back and clean up the trench a bit with your mattock after it. If you are "bending" around tree roots in a larger circle (as to avoid damaging the all the roots of a tree you like) rather than in small jinks (as to bend rather than cutting one root) it should be fine, if you use your conduit, dry-fitted, to mark out the line to dig, and then follow it. If your trencher won't dig through tree roots, it's not worth renting, or you should rent a different tool (or tool and operator

- have somebody come dig the trench with an excavator and you fill it back in by hand.)

Also - use PVC conduit (gray), not plumbing pipe. Since you're barely willing to consider conduit, you probably won't consider schedule 80 (heavywall, not generally seen in chain stores, much stronger) but you really should. You have to find someplace (generally an electrical supply house) that sells it, and it costs a little bit more than schedule 40 (what the chain stores sell.)

12" minimum, face to face (ie, if you have a 2" conduit for power and another 2" conduit for network, cable, etc the nearest two faces need to be at least 12" apart, not the two conduits running 12" on center (only 10" face to face.) Some local codes may require more separation.

If you have the digging done, vertical separation with the power below the communications gives one more warning (besides the "Buried Electric Line Below" warning tape you WILL put in the trench as you are filling it back in, so it gets dug up first) before the next person to dig gets into the electric line. Otherwise you can go side to side in a shallower trench. Given the typical roll size on that tape, might as well put in a couple of them at various depths - the top one should be no more than

6-8" under the surface of the ground, so it's easy to find well before the wire is in danger.

With conduit you might get away with interior Cat5 up to the point you want to replace it with the next thing, or a fresh piece of cat5. If you direct-bury interior cat5 (and/or coax) it will fail, and you won't be able to replace it, nor will you have a hole available to pull (say) fiber through if fiber around the home becomes normal at some later date.

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Cats, coffee, chocolate...vices to live by

Yes, deeper is better, The house I'm in has a 60 ft run of PVC conduit about

12" below ground to a shed.. I have dug two times to put a tower in and a post. I hit the darn conduit both times! Recently I put in several fence posts, I hit the cable tv line going to the house. But my bad luck is sometimes good luck, I was beginning a dig to install a cleanout on a sewer pipe and hit --- a cleanout!--- Saved me a weekend of hard labor.

Mikek

I thought that might be the case. I checked out some new ranges and they do have four wires.

You've already heard the right way to do it: electrical (not plumbing) plastic conduit, 4 wires, buried at LEAST 18", etc.

I think it best to plan the entire installation on paper, before lifting tool 1 or spending penny 1 on it.

Consider the design: almost all the work is in the trenching. Saving money on parts is just plain not worth it. #12 wire feeding a workbench

• lights over an 80' distance is a not a good choice. At _least_ one size bigger is better. Consider what you are burying in the trench: one feeder circuit to the garage subpanel. Suppose you want to be able to turn on a light, mounted on the outside of the garage, from the house? Why not build that capability in while the trench is there? Add a 3 wire + ground of #12 to enable switching of a garage light from the house or garage.

Do you have plans for adding a generator in the future, to power the house during power outages? If so, you might want to house the generator in the garage, and you might want to consider burying cable for that at this time.

As to GFI: if your intention is to split the 240 into 2 120V branches in the garage, don't use a 240 GFI breaker. Instead, use 2 GFCI receptacles, each installed as the first receptacle on the branch and wired to protect all of the "downstream" outlets on the branch. It is _much_ cheaper that way. If you later decide to use a 240 volt branch from the subpanel, you can add the 240V GFI breaker at that time.

_Grounding_

You are feeding a separate building on your premises. You need to ground the sub-panel properly, per the electrical code. The neutral bus MUST NOT be connected to the ground bus in the sub panel. The ground wire in the feeder cable from the house MUST BE connected to the ground bus in the sub panel. The only place where neutral and ground are allowed and required to be connected together in your service is at the main service panel. You need to understand what (and where in your subpanel) the "bonding jumper" is. It MUST be removed.

Ed

Which raises another question. The GFI breakers I looked at have a white wire coming out of the breaker (prepped - coiled and stripped white "pigtail" sold already in place) where would that go on the sub panel in the garage ground or neutral?

It would go on the neutral. On the main panel the neutral and ground are usually the same but on your sub-panel they are (should be) isolated. Tom

She will be able to switch the garage light on and off from the house

- via a relay and communication cable to be buried with the feeder. That is already in Plan A. Because I can already see it - she will want lights inside the garage then want to know they are off or on and be able to control them from the house too.

This, I feel, is likely, but I don't want to put ideas in her head.

That is called "mission creep" by the military (and it is one of my faults too). Keep it simple. This is for my wife's house and she's footing the bill. A generator isn't in the future, and her use of a workbench would be limited.

The rule of thumb is 3% voltage drop. With 14 AWG that would be about

17 amps, so 12 AWG is already one size larger.

I was thinking the wire underground needs GFI as much as any circuit outdoors. No?

I'll probably just put in 120 to keep the cost down after reading the responses here.... The 240 would be if she wants to insulate the garage or a part of it and add air conditioning or heating. The building really isn't worth putting that into it in my opinion, but my opinion doesn't count.

That is interesting. This is the deep south and electricians don't necessarily have opposed thumbs... There was no ground rod at the meter, I installed one some years ago trying to get the hum out of her stereo - that problem turned out to be no ground on the power transformer at the pole. I called up the power company and they added a ground and the buzzing went away.

BUT to get back to what you are saying, so I understand it. (assume a plain old vanilla 120 circuit - cost will determine if it is 240) The neutral will run from the panel on the house to the sub panel in the garage, but must be isolated from the ground at the sub panel - while the default is to have them connected as they come from the store?

Which raises another question. The GFI breakers I looked at have a white wire coming out of the breaker (prepped - coiled and stripped white "pigtail" sold already in place) where would that go on the sub panel in the garage ground or neutral?

Not quite. #14 wire is 3.14 ohms per 1000 feet. The computation:

17 * 3.14/1000 * 160 = 8.5408 volts; 8.5408/120 = .0711 or a bit over 7%

Using #12 wire at 1.98 ohms per 1000 feet:

17* 1.98/1000 * 160 = 5.3856 volts; 5.3856/120 = .0448 or 4.48%.

My guess is that you used 80 feet in the calculation instead of 160. You have to use 160 because there are two wires in the 80 foot run.

No - you can't come into contact with the buried wires. GFI is not to protect wiring - it is to protect personnel from conducting current to ground.

Yes.

Likely.

The white wire always goes to the neutral bus, regardless of whether the GFI is installed in the main panel or a sub panel. GFI never needs to be connected to the ground bus, nor does a GFI in the panel or GFCI receptacle need a connection to ground to function properly. At a GFCI receptacle installed in a grounded system, the ground wire must be connected to the ground screw on the receptacle. That serves the same function as the ground wire connection to a regular receptacle - to ground the 3rd prong of a plug (and therefore ground whatever device is plugged in). It has nothing to do with the ground fault operation.

Ed

That is exactly what I did. Thanks for pointing out the error.

My ARRL handbook puts 14 AWG at 2.575 ohms per thousand

and 12 at 1.619 ohms/1000.

8.7 amps with 14 14 amps with 12 for 3%

Guess why I am familiar with that particular error. :-(

There are several different wire tables floating around, so it's not unusual to find different values for the same wire. For house wiring, I figure it's best to use the table in the NEC (National Electrical Code). The table in the ARRL book may be more accurate - I don't know.

Ed

The ARRL table is for winding transformers.

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You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.