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

[Keith Carlson]

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

[Keith Carlson]

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).

[Greg]

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.

[mefjr75]

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 ;)

I am not sure what you mean here D.C. is NOT a.c.! Unless there is 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.

O.k. that is pretty reasonable there are many appliances that run on

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

[Clint]

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

[Chuck Hoffman]

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.

[Chuck Hoffman]

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.

[neal.don]

remember that watts x volts =amps total your watts x if 240 take it times

[toller]

It is not recommended. Anything confusing is dangerous, and this is confusing.

However, if you lose the 30a breaker, it is possible. You cannot use a 30a breaker because the 120v circuit will not be rated for 30a. You must get a

20a breaker and use #12 for all of it. (I am assuming #12 is adequate, which might not be true if your run is too long and the current is too high; you have to check that first)

If 20a will cover the DC and whatever you want to also run on it, you can do it. Otherwise you can't.

Overall it is a better idea to run two circuits. Did anyone mention that you will need 12/3 wire for the combination circuit but only 12/2 for the separate circuits? It is probably just as easy to run two 12/2 as one 12/3.

That was funny about not being able to change DC to AC.

[mefjr75]

Hello, sorry that just isn't right I'm sure you meant ; P=I*E Or rearranged : Watts = Amps*Voltage Amps = Watts / Voltage Voltage = Amps / Watts etc.. But even that isn't fully all of the story if you are talking AC power and Inductive loads..... then you deal with Impedance and other fun things (power factor), long story short ... it's complex. Hth, M.E.Farmer

[Clint]

I think DC = Dust Collector...

Maybe AC = Air Cleaner?

Clint

[mefjr75]

Correction ;) Or rearranged : Watts = Amps*Voltage Amps = Watts / Voltage Voltage = Watts / Amps Sorry for the screw up. M.E.Farmer

[Doug Miller]

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)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.

[Doug Miller]

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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[Doug Miller]

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.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.

[Matthew]

Agreed, this is a confusing issue. But there are lot of people with experience on the rec.

Yes, mixing 120V and 240V is possible, and many circuits are designed this way. That is why (almost) all 240V installations have a neutral wire. If you used only 240V loads, there would be no need for that neutral wire.

If you want to mix 240V and 120V on the same circuit, at least 4 wires will be needed. Call them L1, L2, N, and G (for Line1, Line2, Neutral, and Ground). The "3-wire" terminology was confusing. Hence 3-wire with ground is common, if a bit of a mouthful.

My tablesaw and jointer have 3 prongs on their plugs (L1, L2, and G) -- no neutral is needed, since only the 240V motor is used. My bandsaw has 3 prongs (L1, N, and G) If, however, I wanted to add a 120V light bulb at a

240V tool, 4 prongs would be needed (L1 L2 N & G). I would need to add the neutral wire to carry the 120V current. That's a hassle, which is why most people would just run a separate plug. :)

Even if the startup surge would cause the current to exceed the breaker rating, the breaker probably would not open. Circuit breakers have what is called a time-current curve, which tells you how long a circuit breaker can carry excess current before it opens. Most breakers will carry 2x or 3x their rated current (ex: 30 or 45A on a 15A breaker) for ten seconds or so before they open; even short time delay models will do it for a second or so, which covers the startup surge. Google on "circuit breaker time current curve" to check it out for yourself.

Once you go above this level, the breaker will trip very quickly. For example, when a hot wire shorts to the neutral or ground, a current of many time the rated current occurs (10x? 20x?), and the breaker trips within milliseconds.

The general idea is that larger overcurrents trip quickly, but that 2x or 3x currents can run for a long time. This protects the wiring, which is one of the reasons for circuit breakers; a "shorted curcuit" level of overcurrent would damage wires quickly, but lower overcurrents would take a long time to overheat the wiring.

Calling qualified help is never a bad idea, but this sounds like a simple project. And this group will be helpful.

Matthew

[Duane Bozarth]

Yes, and electrical dryers as well. At least NEC to (I don't 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"...

[Roy Smith]

Or a step-down transformer inside the machine. Not that that's not a hassle in its own way.

The best suggestion I ever saw was just put two bulbs in series.

Or, I suppose, find a source for 240V bulbs (they must exist).

[Chuck Hoffman]

No, it isn't. Look at my simplified description of the current in a two-phase circuit for a clue that I know what I'm talking about.

[mefjr75]

Right on Doug, Exactly thats the idea behind LOAD BALANCING! if the are equal there is no current on the neutral. The whole idea of the neutral is a 0 volt refrence We really try to achieve 0 volts potential across the neutral it is technically a ground wire of sorts but is not the GROUND wire . It isn't magic if you look at how they send it too you from the 'pole pig' transformer they send L1 and Neutral (ever seen the ground wires running down the light pole? That's were they ground the neutral at on there side) and then L2. This is a simplification and is only meant to help illustrate the idea. Please look into it if you want to verify any of this. ['pole pig' transformer step down] the v's are windings and the = is the seperator between the windings.

M1-vvvvvvvvvvvvvvvvvvv-M2 13.8kv or some other mains voltage ====================== L1-vvvvvv-N-vvvvvv-L2 240v split phase | | | || | | | L1L2

Excuse my ascii. The legs L1 and L2 are 180 degrees out of phase with each other that means one should be at -120 and the other at +120 . Notice that the Neutral is at the middle...that gives you a ZERO volt refrence to ground. Why is it needed well we need a reference to ground so we can use our appliances and not be subject varying voltage levels from ground. A simple 240v circuit could actually be floating 1000v above earth ground but between legs only have a potential of 240v but since there is no ground reference it could be 1000v above earth ground from just 1 leg, that would hurt a bit ;)

Once again it is all a bit complex. One could study for years and still be confused. And ponder this, what is electricity, is it the just the flow of electrons .... or is it more to it, like maybe the electrons are just buckets that travel the circuit slowly tranfering 'photonic energy', hint they are never consumed in the circuit yet we do get power from the circuit;) M.E.Farmer