New Shop Wiring

14 years ago

Well, I'm finally getting around to adding some additional circuits to the detached garage in my new home. The previous owners built the garage in 2003 and had it wired at the time with a large sub panel (100A) and ran some lighting circuits, 120V outlets, and one 120/240 welding outlet. Previous work was done by an lic'd electrician. I'm adding a few more circuits and have some questions about what they did and what I'm about to do.

1) Their welding circuit consists of 12-3 wire running to the sub panel. At the subpanel each hot (red and black) received an individual 20AMP single pole breaker. This would allow one hot leg or the other (or both) to be off or on. Also if one leg trips, the other wouldn't. I can't figure out why you would want to do this. No recepticle is installed at the outlet box, all wire ends are capped with wire nuts and taped. (They built this garage and then never used it for anything)

2) I'm adding two 4000W electric element heaters. They each draw 17A and each requires their own dedicated circuit with a 20AMP breaker and a NEMA 6-20 outlet. This is a hot-hot-ground configuration. I've run 10-3 wire incase I ever desire to upgrade to a 5000W (22A) heaters, the additional wire capacity will let me swap to a 30A outlet along with a 30A breaker without having to redo the work behind the wall. Is there anything wrong with using the larger capacity wire than necessary on these two circuits?

3A) The use of hot-hot-grnd on 240V circuits baffles me a little. Since I'm using 8-3 on a 40A breaker for my compressor (5HP motor, plate amperage is 22A) why exactly don't I use the neutral wire at all? Compressor plug is hot-hot-grnd, can't remember what NEMA type it is.

3B) Basically same question for the two electric heating units. The NEMA 6-20 plugs are hot-hot-grnd. All of the advice I've seen here and other places says to cap the neutral in the outlet box and disconnect & cap the neutral at the sub panel in a scenario like this.

I always thought the neutral wire is what carried my "spent" electricity back to the source - so it was somewhat integral in getting a circuit to function (a closed loop if you will). On these 240V setups is the bare ground wire actually carrying the spent electricity?

Thanks,

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Comments (9)

petey_racer
14 years ago
NO, the ground is not serving any purpose other than a backup way to open a circuit breaker in case of a fault.
It does not, and should never, serve as a current carrying circuit conductor.

A 120v circuit is a line-to-neutral circuit.
A 240v circuit is a line-to-line circuit.

NO neutral is used or needed in ANY 240v load.
the_analyst
Original Author
14 years ago
Yes, thanks, unfortunately that is the part I understand which is why I asked "...why exactly don't I use the neutral wire at all?"

The best answer that I've been able to find on my own is that the polarity of each 120V branch in a 240V circuit is different - one is negative, one positive. On a 120V circuits you don't have the cancellation of charges, so the neutral is used to carry the current flow back to source then to ground. I hope my summation of the more technical explanation I found didn't dillute with what i just typed.

I already understood that the neutral wasn't used on PURE 240V circuits / equipment.

K
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The majority of the switches and receptacles are not installed yet. Just enough for light and a few receptacles. Before jumping on the "electrician" here, I wonder whether the electrician did his or her version of a "rough-in", with the expectation of returning to install switches, receptacles and other fixtures before the circuits were put into service. If so, there may be other issues involved in moving from rough to final. What was the agreement as to the scope of work the electrician would perform? Who put in those few switches and receptacles that are already installed? Was the can you moved already energized (i.e., "hooked up" to a circuit with usable power)? If the electrician installed working switches, receptacles and lights with hots and neutrals connected but with grounds left unconnected and/or unbonded, then I'd definitely call him/her out on that. It's just plain wrong. If anything, most of the grounding work should be done at rough-in and the fixtures installed later. (And, naturally, you can't connect an EG to a fixture that's not there yet.) I plan to install the receptacles and switches myself so I know that will be done right. No offense intended here, but the nature of your original post makes me feel a little nervous about that statement. Are all the equipment grounds continuously bonded with only pigtails for fixtures left unconnected?
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What you are missing is that if there is no ground wire in the existing cable, there will be no ground for the circuit and any extension of that circuit. The new cable's ground wire can therefore be ignored because if it were connected it would not be doing anything. Connecting it would only mislead someone who was working on the circuit later. The purpose of the ground wire is to trip the breaker if the housing of the exhaust fan came in contact with the wiring and would shock someone who touched it. It is unlikely that someone would be touching the housing unless there is a light in the fan housing. I'm not sure what the electrical code says about extending a non-conforming circuit to a new fan but I doubt it is allowed. Of course, it is also likely that only a licensed electrician is allowed to do this work. In any case, if the installation caused a fire it is unlikely you would be insured if there was evidence the fan was not installed by a previous owner. Also, if the new junction box is metal you must use cable clamps where the cables enter through the knock-out holes. The box not only provides isolation of the connection but secures it so it can't be pulled apart. The cable must also be stapled to the framing at certain intervals; it can't be lying loose in the attic. 2 wire ungrounded non-metallic sheathed cable is rare in my area. Post knob & tube, home wiring was metal armored (AKA "BX") and the metal provided a ground to the metal boxes although it was not always reliable for long circuits. In the 60's an aluminum strip was added inside the cable to improve the continuity of the ground. Non-metallic (plastic) sheathed cable (AKA "Romex") became popular in the 50's and in 62 the electrical code made a bare ground wire mandatory for all branch circuits. For your wiring to have only 2 wires and no ground it would probably have been installed in the 50's by someone who didn't want to use BX cable. But this is not all bad because the insulation on BX wiring tended to crumble when exposed to air and is more dangerous than any other kind of wiring wiring. For any further help you should post photos of the original cable and the main panel with the cover off. Tripping the main breaker is wise but NEVER assume a branch circuit is dead without using a voltage detector. If a breaker can be turned off, it can be turned on again by someone else. Always put a note on the panel. A 120v wire can stop your heart instantly. A voltage detector is the smartest $10 you will ever spend. Here is a link that might be useful: voltage detector
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joed
14 years ago
The neutral does not carry current back to ground. It carries it back to the transformer and ultimately the power plant. The ground plays no part in power delivery. It is a safety issue or lightning protection only.
brickeyee
14 years ago
"The best answer that I've been able to find on my own is that the polarity of each 120V branch in a 240V circuit is different - one is negative, one positive."

Negative and positive do not have any real meaning when the voltage is changing 120 times a second.

The system used is a center tapped transformer winding.
Across the whole winding is 240 V.
The center of the winding is grounded and 'declared' as 0 volts.
From either end of the winding to the center is 120 V.

The net effect is that the two halves are 180 degrees out of phase with each other (NOT the same as 2-phase power though, that is 90 degrees).

When one end is at a maximum positive voltage, the other is at the maximum negative voltage. They then reverse 120 times a second to produce 60 Hertz.
the_analyst
Original Author
14 years ago
Thanks guys, now that we have the least significant part of my original post answered, can someone provide an answer or clues regarding question #1 and #2... 2 is less critical as I have a feeling there is absolutely nothing wrong with using a larger gauge wire on a circuit as long as the proper breaker size is paired to the proper recepticle / equipment rating.

K
normel
14 years ago
1. I doubt this is a welding circuit, sounds more like a multiwire branch circuit that was never used.
2. Go ahead and run 10/2 if you wish. Using 10/3 is a waste of money.
the_analyst
Original Author
14 years ago
Guess I'll give the company that did the wiring install a call and see if they have any docs / plans that detailed that welding circuit.

The breaker is labelled "welder" circuit by the installer, because they're using company logo'd labels at the breaker and the hand written notation on the label matches the employee's signature on the installed electrical company sheet taped to the inside of the subpanel.

K
billhart
14 years ago
1a. A 20A circuit won't support a typical welder. Maybe some specialized type of smaller welder.

1b. A 240 V circuit really should use a 2-pole breaker, not individual breakers. In some cases it is required by code. If not always required, it is still always a good idea.

2. There is nothing wrong with oversized wire except the cost and difficulty in working with it, so long as the breaker and receptacle can accommodate it. If it doesn't fit in the termination, then you would have to splice a short piece of the "normal" gauge wire on.

3A/B I think you basically understand it. In some cases an appliance (kitchen range) may use both 120 and 240 so they do send some current back on the neutral. If it is only 240 then the neutral is not used.
hrajotte
14 years ago
I believe your 4000W heaters (16.7A) will REQUIRE a 30A circuit with 10 gauge. Electric heat is a "continuous load" and its circuit can only be loaded to 80% of capacity. Since 80% of 20A is 16A, and the load is 16.7A, you probably have to upsize even though it is only 0.7A more. This would also allow you to use the 5000W heaters (21A). You could load the 30A circuit up to 24A.

New Shop Wiring

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petey_racer

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petey_racer

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