220V to two 110V circuits

[pmichener]

Is it possible to take an existing 220V circuit and convert it to two 110V circuits. This is a four wire 10 gauge circuit for air conditioner that is no longer used, but is ideally suited to run to my outbuilding shop. Wires are black, white, red, and ground. I know that both the white and black should be hot, but can you use the red as a common neutral on two separate 110V circuits? Or am I completely off base???

[tfischer]

You can do it, if you add a small subpanel off of the existing wiring to feed the new circuits.

-Tim

[sacherjj]

And make sure that you keep ground and neutral isolated in the sub-panel. They should only join together at the main panel. Normally you need to break off a tab joining them in the sub-panel.

[just4funsies]

This can get complicated. Make sure you do, in fact, run a neutral from your subpanel back to your service panel. Many have tried to split a 220 into a pair of 110's with only a ground. Tried and DIED, that is... The neutral MUST be run, and MUST be tied into the service ground ONLY at the service panel.

Remember, too, that you do not want to be feeding your 110's through the breakers on the 220 circuit. Take out the double breaker, and put in two proper 110 breakers. It can become troublesome if you imbalance your load between the two legs.

And lastly, remember that the wiring to the original 220 circuit is sized for 220, not 110, and may now be too small if you run a large 110 load on one of your new circuits. At equal motor loads, 110 pulls twice the amps as 220, and it is AMPS that heat wires and burn down houses.

Sorry if this makes the process sound difficult, but simple hurts more, and better to be safe than .

[JimZ]

IF you were an electrician.

This would not be terribly difficult. Unwise, but not difficult. But electricity is dangerous, so call a professional. That's my advice. Even after working on 34.5 KVAC, I still respect 110 VAC, and 220 is that much worse.

After 30 VAC, it's all lethal.

But hypothetically, IF you were an electrician, you'd know that for existing circuits, generally

RED = 110 VAC
BLK = 110 VAC
WHT = Neutral (not really "used" in 220, but still present)
GRN = Ground.

And hypothetically, how could you test this ? Why carefully, of course.

1. Electricians live by lockouts. Or they die by not using them. This means that if an electrician disconnects power, he takes pains to make sure power is off, AND to make sure some helpful soul doesn't "reset the popped breaker" while he's working. Anyone working on electrical circuits should heed this. LOCKOUT or TAPE OFF breakers to protect yourself.

2. NO electrican prefers to work on live circuits. An smart electrician would NEVER work on a circuit live if you can drop power. 15 seconds of convenience is not worth his life.

3. NO electrician should ever work completely alone if he can help it. Having someone around to call 911 is a really, really good idea.

So, using our color guide above, our hypothetical electrican would trace the wiring back to the panel or subpanel. Then he'd drop power and open the panel. He'd carefully trace the wires, and see the white lead bonded to the neutral bus with a bunch of other white wires. And he'd see the green lead bonded to (generally) a different bus -- the ground bus - with some other green wires. And he'd see the red and black wires going to a ganged 220 V breaker. "Aha!" he'd say, "someone wired it right for 220 !"


Or, if for some reason this was not possible (hard to imagine but possible), our intrepid LICENSED ELECTRICIAN would prove out the wiring another way. More tedious, but do-able.

First, he'd drop power to the circuit. Then he'd take his wiggle stick, AC voltmeter, or VOM and check each female connection in the receptacle at 0 volts AC and 0 Volt DC (without opening the main panel, leave nothing to chance) with respect to every other connection in the receptacle. As an electrican, he'd know enough to NOT TOUCH ANY METAL ON THE TEST PRODS.

Once he checked each pin dead to every other pin, he'd repeat the test with each pin to ground (the metal case). That's in case all 4 pins are at the same 110 V potential.

Now that the receptable is CHECKED DEAD (not *assumed* to be), he could open the face plate and examine the wires. He'd make a diagram of which wire went to which pin, reversing it so it read correctly from the front.

Then he'd check continuity white to green. He should get about very low to 0 ohms, indicating netural and ground are tied. Checking continuity white to either red or black should read an open if the breaker is correctly opened.

He'd also note the size of the wires, particularly the white lead. More on the later.

Carefully reassembling the receptacle, he'd turn power back on. Then he'd TURN THE METER OFF THE OHMS SCALE AND BACK TO A RANGE THAT HANDLES 220 VAC. Unless he likes the smell of burning test equipment.

With power back on, and avoiding the metal parts of the test prods, he'd check eack pin to each other and to case ground. He should see

RED to BLACK = 220 VAC
RED to WHITE = 110 VAC
BLACK to WHITE = 110 VAC
RED to GREEN = 110 VAC
BLACK to GREEN = 110 VAC
RED to CASE = 110 VAC
BLACK to CASE = 110 VAC
WHITE to GREEN = 0 VAC
WHITE to CASE = 0 VAC
GREEN to CASE = 0 VAC

And at last, our intrepid 'trician knows the score. Except for one thing.

When he examined the wires inside the creptacle box, did he find the white wire fatter than the others ? If he wired two 110 VAC circuits,

RED to WHITE with GREEN as ground and
BLACK to WHITE with GREEN as ground

He's got the voltage right, but here's the big fly in the ointment: THE WHITE LEAD IS CARRYING THE COMBINED CURRENT OF BOTH CIRCUITS. NOT good for serious machinery. For anything more than a transistor radio, he'd have to run a second neutral lead to handle the current. If our electrician were not familiar with the ampacity of wiring, he should not attempt to judge this.

AND HE'D BETTER MAKE DARNED SURE HE GOT THE WHITE NEUTRAL LEADS CORRECT ON THE 110 SOCKET. Reversing polarity is dangerous.

Of course, the real question is why our electrician **doesn't just PULL TWO NEW RUNS of 110 wiring (12/2 NM with ground, for example)**. Is the convenience worth the potential cost ? Overloading the neutral is very UNgood.

Our hypothetical electrican might run a clock radio on one leg of this mucked-up circuit, but he'd never hook a table saw to one and a planer to the other and try to run both at the same time.

And of course, this is only what I'd imagine a hypothetical electrician might do. Electricity is dangerous, and we live in a litigious society. So I would never, never recommend this to anyone who's not a licensed electrician. And I'm not, either, I'm just a crummy engineer. So I wouldn't dream of trying it myself.

Y'all be careful out there.

[tfischer]

He indicated he has a 4-wire circuit, so the neutral is there.

As for amps @ 120 vs. 240V, amps is amps, as far as wire gauge is concerned. A 30A circuit @ 120V needs the same wire gauge (typically 10 gauge) as a 240V circuit @ 30A. It's true the watts are doubled at 240V, but the 2 hots involved are from opposite phases adn will balance out in a true 240V load (the neutral is only there to carry an unbalanced load, e.g. one of the hots having more current pulled than the other).

-Tim

[leehljp]

This can get complicated. Make sure you do, in fact, run a neutral from your subpanel back to your service panel. Many have tried to split a 220 into a pair of 110's with only a ground. Tried and DIED, that is... The neutral MUST be run, and MUST be tied into the service ground ONLY at the service panel.

Remember, too, that you do not want to be feeding your 110's through the breakers on the 220 circuit. Take out the double breaker, and put in two proper 110 breakers. It can become troublesome if you imbalance your load between the two legs.
. . .
Sorry if this makes the process sound difficult, but simple hurts more, and better to be safe than .

I knew better but took the advice of a Japanese electrician who said that I could use the ground without a neutral. - I tried but I didn't die from the electricity. The house main clicked off, leaving LOML in darkness during an afternoon hen party - the TV went off as they were watching it ! I almost got killed by her!

Imbalanced loads on double breakers ARE a problem. DAMHIKT also!

In the end, I think something was lost in the translation from that electrician.

[pmichener]

Thanks for all the replies. I'm going the subpanel route, seems easiest and safest.

Phil

[JimD]

For anybody in doubt, I would go with JimZ and not Tim. The neutral carires the same amount of current as the red and black hot wires. One neutral is not enough for two circuits. Grounds are not the same as neutrals and only carry current when the circuit is shorted (the ground carrying current is better than you carrying current but is not something that you should cause to happen deliberately).

You can set up one 20A 110V circuit without a new wire but to have two, you need another neutral.

Jim

[tfischer]

For anybody in doubt, I would go with JimZ and not Tim. The neutral carires the same amount of current as the red and black hot wires. One neutral is not enough for two circuits. Grounds are not the same as neutrals and only carry current when the circuit is shorted (the ground carrying current is better than you carrying current but is not something that you should cause to happen deliberately).

You can set up one 20A 110V circuit without a new wire but to have two, you need another neutral.

Jim

Jim --

In a 240V 4-wire circuit, the neutral only carries the difference current between the two hots, which will never exceed the current of either of the hots. This is ONLY true if the two hots come from opposite legs of the service -- but this is guaranteed with a two-pole breaker.

I'll bow out of this thread now since it seems my knowledge is in doubt (even though I've wired up several subpanels and done a ton of electrical work which has been inspected and passed code...)

-Tim

[just4funsies]

As for amps @ 120 vs. 240V, amps is amps, as far as wire gauge is concerned. A 30A circuit @ 120V needs the same wire gauge (typically 10 gauge) as a 240V circuit @ 30A.

That was not my point at all. What I was trying to say is that to do the same amount of work at 110, it takes double the current (amps) as it would at 220. If you were going to put a 2hp pump on a 110 circuit, it would pull twice as much current as the same pump wired for 220. If the wire's already there, you cut your working capacity in half when you go from 220 to 110. (I've seen this done when somebody wanted to get an extra circuit without running extra wire.) And wire size is about amps. That is why big things run on 220... because the wires required to run at 110 would often be as big as your arm. If the wire size didn't matter, there would be no need for 220 (or even 110) power anywhere. If you're going to run a circuit at twice it's rated amps, I'll bring the marshmallows...