3-wire electrical circuit serving both 110 and 220V loads?

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I know I'm going to get the "DAGS" for this question, but I just came from there, and I didn't see this specific information. Lots of debate about how much current the neutral wire carries, though :-)
With a 3-wire circuit, is it okay to wire both 120V and 240V outlets on that same circuit? I know it's *possible* by using the two hots for 240 and either hot and neutral for 120, but is it recommended? Maybe this is the main reason for this type of circuit, so I didn't see reference to it on Google. Much of the discussion I read was on balancing the load, which could imply using the 3-wire circuit with all 120V outlets.
I've got a dust collector coming soon, and the extra 6-7 continuous amps is going to result in some frequent breaker trips when I start a saw or planer. Looked at my electrical service, and found there's an unused double-pole 30A breaker, feeding an unused dryer outlet (house had a gas dryer when I moved in). That should give me the 2 hots for a 3-wire circuit.
My thought on this type of circuit is to wire one outlet at 240V (re-wire DC motor to 220V), and the rest wired at 120V. With that 30A breaker and 10 AWG wire, should have no problem handling the loads from a DC and air cleaner running, and startup surge from another tool.
An electrician would be a good idea, too. Hopefully I can find someone willing to consult with me; I can do the wiring myself. But I'm hoping to get some idea if this is the way I want to go so I can cost materials. Won't get an electrician or the inspector on the phone until next week.
TIA
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BTW, I read a lot trying to decide on a 1.5 - 2 HP dust collector. Finally put whatever specs and prices etc. into a spreadsheet to compare them side by side. (Ended up going with a Penn State unit).
If anyone else is shopping dust collector, and would find this info useful, I'll be happy to email the sheet. (Hey, I'm a relative newbie, but I'm trying to contribute *something*).
It's kinda interesting how they line up by price. The Harbor Freight unit is way below anything in price, even adding on $67 for a felt bag upgrade. There's a Grizzly and Jet at the high end. But most of them fall right into a $330-350 range (including shipping and upgrade of bag if it comes with a 30 micron bag).
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If the breaker is double pole and the cable is 4 wire you can do it. Don't share the neutral and ground! They even make a combo 120/240v duplex receptacle for this.
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This type of setup is common on a stove, right? 240 for the stove heating elements, and 110 for the lights/clock?
Personally, I'm thinking of this for my tablesaw/router (router built into tablesaw wing). It would be handy to just have one cordt to trip over, rather than two. But I don't know if you can get 4 wire extension cables. Haven't switched my saw over to 240 yet, so that hasn't been a problem. And since I'm wiring my garage up with 20 amp circuits, I might not bother.
Clint

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Clint wrote:

have a very recent copy to check current, maybe someone else will) allow the bare wire to be used as neutral in this specific instance (under some other specific circumstances as well, basically being a single dwelling). Actually, in a range, the burners typically only draw 110 as well until they're turned to "hi"...
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wrote:

Actually, he wants 3-wire with ground (e.g. 10/3 WG). If he asks for 4-wire cable at Home Depot, all he will get is blank looks. Or thermostat wire.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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Hello Keith, I am just passing by and noticed your post , I am an electrician. I do not know what level of experiance you have so let me start with some warnings, if you know understand what you are doing ignore the next few lines ;)

some transformer-rectifier setup you didn't mention this just wont work, AT ALL.You will end up in flames.Realize you are saying rewire a ?Voltage Direct Current motor to 220Voltage Alternating Current...these are different beast.

220VAC and use one hot and a neutral to get 120VAC and use that for the timer, controls, or whatever. Really there is no reason to have put in a neutral wire in if you weren't needing to use it for that very purpose, ok you can use it for a 3way or 4way circuit, but that only holds for small wire...think about it. If you don't already know the transformer outside your house sends 3 wires. Grounding is done near your house. This is what you get fromthe transformer: L1-N-L2 L1 and L2 are the ends of the winding and N is the center tap that they ground. It is therefore in the middle between the two legs so between L1-N there is 120VAC and between N-L2 there is 120V and between L1-L2 there is 220VAC DIFFERENCE cause it is really all about potential difference between points ;) Also you might be fooled into thinking Neutral is ground it is not . DO NOT use it as such it can be a lethal mistake. The only real problem that I see is this: You have 30 amp double pole breaker. You want to use 220 for *whatever* You want to use 120 for *whatever else* what if the 120v appliance or tool messes up and starts pulling a higher load until it burns up ... The breaker would stop it huh....Nope not always, not if it didn't exceeded the 30 amp breakers capacity. Which is why you should run a seperate line for your 120v loads and your 220v loads that are not related.It is just a lot safer , also ground everything especially on a motor load, it will help reduce staitc and noise on the line as well. In this case I would run a new line, but I am an electrician ;) Oh and by the way always check your local electrical codes they are always diffent and can sometimes surprise you! Hth, M.E.Farmer
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I think DC = Dust Collector...
Maybe AC = Air Cleaner?
Clint

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snipped-for-privacy@hotmail.com wrote:

In a woodworking context, "DC" means "dust collector", not "direct current". He's talking about changing the jumpers on a dual-voltage 120/240 VAC motor on his dust collector from the current setting of 120VAC operation to 240VAC. [snip]

So what? The breaker is there to protect the circuit wiring, not the cord-and-plug connected device. Exactly the same risk exists with a 20A breaker.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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Thanks, M.E. Sorry for the confusion. As someone else noted, I meant Dust Collector, not direct current. Made an assumption about the audience of that post. The dust collector has a 110/220V motor, pre-wired for 110. I believe it's a straightforward task to re-wire for 220. I don't believe it comes with a 220V plug, though; I'd need to add that.

I was just thinking about this on the way to pick up pizza. With 10AWG wire, I'd have a circuit capable of 30A, but the cords from all the tools are 110V/15A. (leaving out the Dust Collector for a moment). Those will plug into 20A receptacles, so say I wire 20A receptacles on the 120V outlets. Now suppose the planer or whatever manages to load down and wants to pull something like 28 amps. The circuit breaker says "no problem". Could run that way all day and it wouldn't trip. But what about that receptacle that's rated for 20A?? Are the connections or conductors inside it going to overheat? Might be some risk there, not to mention the tool itself like you say. I should go look at my reference book; NEC probably doesn't allow a 20A rated receptacle on a circuit protected to 30A.
Then there's 120V/30A receptacles. Haven't looked, but probably cost more than the 20's. The 110V/15A plugs won't fit in them, so I'd have to change plugs on the tools. Even then I have to consider how likely it is that I'll have situations where the tool wants to pull more than its rated current.
The 3-wire circuit starts to look like more trouble than it's worth. It sounded like a slick idea at first, because I have the breaker, and I'd be able to run 3 wires + ground in my conduit. (Finished walls, and I'm not going to tear them up, so I'll be running EMT and individual wires inside).
I am going to ask a local electrician, but starting to think about the original plan of a simple 2-wire 20A circuit. Then go with 220V, on a separate circuit, on the dust collector if it's needed.
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Yep, that's a problem. Hence toller's statement that your 120V circuit isn't rated for 30A. Technically, that's not exactly correct: there's no reason that you can't have a 120V 30A circuit, as long as the wire is 10ga copper or larger. But you're probably not going to find any receptacles that you can plug a 15A or 20A tool into, that are rated for 30A.

Not if you don't feed current through it to some downstream load, *and* the load you have plugged into that outlet never overloads. But if either of those conditions occurs, yes, there is indeed a risk of overheating and fire.

Right, it doesn't.

Right on both counts.

Not relevant -- you have the same issue with a 20A rated receptacle on a 20A circuit.

Have you considered that when you rewire the dust collector for 240V, it will pull only half as many amps? This may enable you to run the DC on a 20A circuit, which you could wire with 12/3 and then add standard 15A or 20A 120V receptacles. (Code *does* permit 15A-rated receptacles on 20A circuits.)
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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There is no argument at all. In a two-wire 115V circuit, the current carried by the neutral is exactly the same as that carried by the hot wire. In a three-wire 230V circuit, there is a property called phase angle or J-factor. This results in current flowing in both directions at the same time (this is not exactly a true statement but a convenient way to consider current flow in a three-wire circuit). As the current on one phase increases from zero to some positive value, the current in the other phase is decreasing from its positive value to zero. The vector sum of the currents on the neutral is exactly the same as the total of both phases. Because of the phase angle, however, that does not mean twice the current. It never exceeds the total current of each phase individually.
Confusing? Yes. One has to study alternating current and understand plane geometry and simple trigonometry to comprehend it.

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Sorry...neglected to answer your original question. I believe the NEC frowns on connecting across one phase of a 230V circuit to get 115V. That WOULD double the current on the neutral and result in a potential overload.

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Utter nonsense. It would do nothing of the sort.
In a circuit wired as he described, with (for example) a 10A load at 240V and a 15A load at 120V, the current in the neutral conductor is 15A. (The 240V load places *no* current on the neutral.)
Now add a 17A load at 120V on the opposite leg.
One hot leg is drawing 10 + 15 = 25A. The other is drawing 10 + 17 = 27A. And the current in the neutral is 17 *minus* 15 = 2A.
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two-phase circuit for a clue that I know what I'm talking about.
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I read that. And you don't.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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On Thu, 23 Dec 2004 23:18:38 GMT, "Chuck Hoffman"

Right church, wrong pew.
All the stuff you wrote about phase angle and AC circuits, etc. seems to be okay, however, you liberally sprinkled in terms that in the real world of electrical wiring just aren't true. Let's take a look.

True. You should have stopped there.

I don't recall J-factor in the NEC. That could be me, though.

And since it's not, you should have left it alone.

Here's the crux of the problem. In electrical systems (the stuff the power company provides) current is delivered in three phases. In the average house, however (and that is what the discussion is about), power is delivered from only one phase of the three phase system.
It is run through a center tap transformer which yields two "hot" legs that are 180 out of phase and a common return leg. All three of these wires, however, are delivered from a single phase.
There is no two phase power. It's either three phase or single phase. No electrician calls either hot leg a "phase."

Yeah, you could say that...if you wanted to answer the question of "what time is it?" by telling us how to build a clock.

Phase angle, as a concept, is perfectly suited for all sorts of theoretical discussions but is unnecessarily complex in single phase wiring. The only "angle" to consider is the 180 that each leg of the single phase power feed to house is out of to each other.

Correct (except for that "phase" thing), but you sure took the long way around to get to it, and lost half the pack doing it.

Please. Opposite legs have opposite values (because they are 180 out of phase) and the current is additive. It's simple arithmetic (albeit incorporating negative numbers).

Given that stoves and dryers sometimes do that very thing, I believe you're wrong...unless you could cite the section in which they "frown" upon it.

This is the part that Doug describes as utter nonsense. And he's right. The ONLY current on the neutral would result from the connection across one LEG of the 230V circuit (to get 115V). There is NO current in the "neutral" of a 230V circuit because there is no "neutral" in a 230V circuit.
- - LRod
Master Woodbutcher and seasoned termite
Shamelessly whoring my website since 1999
http://www.woodbutcher.net
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For the critics, I've been in electronics for 40-some years and have been a technical school instructor. And I worked for an electric utility. I do, in fact, know what I'm talking about.
My late father-in-law was a union electrician and he didn't fully understand AC. He could follow the code, pull wires, bend conduit, install boxes, switches and outlets, etc., but he had no clue about the physical properties of electricity.
Now let's talk about your comments. First you say:
"Phase angle, as a concept, is perfectly suited for all sorts of theoretical discussions...."
It is far more than theory, I'm afraid. It's an important consideration when working with multi-phase circuits. Then you say:
"Opposite legs have opposite values (because they are 180 out of phase)"
That seems diametrically opposed to your first disdainful comment. It is, however, precisely the point I was making.
You also said:
"I don't recall J-factor in the NEC.
I've known people who could recite the NEC chapter and verse but had no clue about the physical properties of electricity or AC circuit analysis. Do you really understand alternating current and its physical properties ? Or are you like my father-in-law?

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I don't know much about the NEC, but I do understand polyphase circuits (4 years of EE in college). I don't think I've ever heard the term J-factor. What is it?
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On 24 Dec 2004 12:41:29 -0500, snipped-for-privacy@panix.com (Roy Smith) calmly ranted:

Rest breaks where union electricians would go toke up?
---------------------------------------------------------------------- * Scattered Showers My Ass! * Insightful Advertising Copy * --Noah * http://www.diversify.com ----------------------------------------------------------------------
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