I have run 4 wires to the detached garage: 2 hots for 240V, 1 neutral, and 1 ground to bond both structures (thank you Bill Hartmann). It’s all for the in-laws and they’ll never need 240V, but someday, someone might. So, say this future someone has a large table saw, 3hp and able to run on 240V.
How large should the wire gauge be to this outlet? I know you can run 1.5 hp table saws on 12 gauge/20 amp/120V circuits. I’ve heard a saw running on 240V is still pulling the same amperage as one half as powerful on 120V; it’s just pulling the same amperage twice as often through 2 hots. Is this true? If yes, then 12 gauge, 3 strand w/ ground would be adequate, right?
Thanks to all you Sparkies for getting me this far correctly and safely.
Replies
Well, you say that, "I've run...." , so maybe this is closing the gate after the horse is out, but if not, I'd give some thought to the possibility of a subpanel. The cost of #10, #8, or even #6 is minimal compared to having to do the whole thing again due to current shortfall. If it were me I'd do a load calc on what I'd use, and then install the next bigger conductor. More current is better, and remember that, from the subpanel, you're still going to protect each branch circuit with the appropriate breaker.
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”
On 220v it draws half the amperage from each leg. So if your tablesaw draws 18amps 110v then it will draw 9 amps on each leg at 220v. A 220v breaker pulls 110v off 2 different bussbars therefor it splits the amperage between each bar.
I'd go heavier than 12ga wire. What size breaker are you using? I did my garage with a 220v 30a breaker and ran 10ga wire.
If it's going to be a subpanel in the garage ( no main breaker) you need to seperate the ground and the neutral bar in the box. I'd also install a ground rod at the garage to be safe.
Headstong, I'll take on anyone!
It's a requirement of the NEC to installl a grounding electrode at a separate structure that is served by more than one circuit (that is, one that has a subpanel).
Also, whether you keep the equipment grounds separate, or bond them to the neutrals at a subpanel in a separate structure has nothing to do with whether the subpanel has a main breaker or not.
It depends on whether there is any metallic pathway (metal water pipe, phone line, CATV cable, intercom wire, etc) between the building with the service and the one with the subpanel. If there's a metallic pathway between the two buildings, the feeder has to be a 4-wire (hot-hot-neutral-ground), and the grounds and neutrals in the subpanel have to be kept separate electrically (called a floating neutral).
If there's no metallic pathway, you can run a three-wire feeder (no ground) and then you treat the subpanel sort of like a mini-service--you bond (connect) the neutrals and grounds.
In either case, there has to be a grounding electrode at the separate building.
A multiwire circuit (shared neutral) is considered one circuit--so it's a neat way to get two, 20 amp circuits to a garage or shed, without having to install a subpanel. I sometimes put in a cutoff, just for safety and convenience, if the building is a long way from the main panel or service.
Cliff
Cliff, I have been reading your posts and explanations for years now and I must say that you have a unique talent for breaking things down and explaining them in simple terms.
Obviously, you have a complete understanding of how electrical systems work, but not many people have the ability to convey it in writing like you do!
IIRC, the separate structure requirement has an exception if the distance between the buildings is five feet or less. My garage is 61 inches from the house. ;-)
-- J.S.
Not exactly true. The current is not drawn off two bus bars at the same time. It only reacts to the difference in voltage between the bars (220). It flows between them and reverses direction 60 times a second.
All you need to remember is that voltage X Current = power
So if you double the voltage the current will be halved for the same power. The saw will run better also. I agree with the sub panel idea."With every mistake we must surely be learning"
The sub-panel is not in yet, but the THWN to it is 6 gauge over 125'. Sub-panel will be something like 60 amps, but will check my NEC for distance and other potential deratings. Main panel at the house will have a breaker as well. Obviously, I'm no sparky, I have the NEC for a class.
Distance to a potential table saw in the garage from the sub would be 40' at most. Garage was built by previous owners, but someone was smart enough to stub out rebar from the stemwall. One can only assume it has 20' of embeddment to effectively act as an EGC.
Thanks for the clarification on how 220V motors pull power.
There's a handy voltage drop calculator at http://www.electrician.com/vd_calculator.html. Assuming you ran copper wire, if you plug in 240vac, 6 awg, 125 feet and 60 amps you get a voltage drop of 3.1%. VD should be kept to 3% or less, but on the other hand it's doubtful you'll ever pull a full 60 amps out at the garage so 6 awg wire should be okay.
Edited 1/6/2006 1:27 pm ET by Stuart
You need to say how far the wires run. That and the load determines the gauge.
So how big are the 4 wires you have already run? You certainly cannot run any larger wire to your saw outlet.
3 hp @ 240v will draw 17 amps - slightly over the limit for a 20A line which should not be loaded higher than 16 amps on a continuous basis - could be argued as to the duty cycle of the saw; i.e., what is continuous duty?
3 hp @ 240v - I would use #10 wire and a 25A or 30A breaker - run all day.
Jim
Never underestimate the value of a sharp pencil or good light.
Wires to the as yet uninstalled sub are 6 gauge. Thanks for the drawing potential of a 3hp motor. I'll probably run 10 gauge to the saw.