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

[Keith Carlson]

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.

[Keith Carlson]

Saw quite a bit of this debate on various NGs. Some felt it's dangerous because someone will screw up the neutrals at a later time, or a future homeowner might replace the double pole breaker with two, and put them both on the same leg of the service. Others thought, anyone qualified to work on the wiring or panel should not be confused by this type of circuit.

This might be my downfall. It's not going to be practical to use 30A outlets so the whole circuit can be considered 30A rated.

It's looking more like that's what I'll do... back to the original plan and see how it goes. My thinking, though, was that now on a 15A circuit, I'll have the breaker trip sometimes on starting a machine. Table saw - about 10% of the time. Miter saw - maybe 15% of the starts. Planer - probably 1/3 of the starts trip the breaker. So I was thinking 20A circuit. But now add the dust collector, drawing 6A continuous (12 on startup). Now I've got about 14A "headroom" for other motors to start.

Yeah, I knew that. I'm running the wiring in EMT conduit though, so I don't have to tear up finished walls. I thought I read somewhere that you can't run more than one circuit in the same conduit run. If that's true, two circuits means more cost. I suppose another option is replace the 30A breaker with a 20A double-pole, make it a 20A, 120/240V circuit. Could still use the 20A receptacles then.

Anyway, talked to my neighbor who referred me to another neighbor who's an electrician. I may be able to get some inexpensive guidance from him.

[Keith Carlson]

Yeah, this is what I was talking about: 2 hot wires, 1 neutral, 1 grounding wire. In my HP Richter book, they called it a "3-wire" or "split-wire" circuit. "3-wire" sounded less confusing to me. Don't know how common that term is in sparky-land.

Hmm. I do have trouble tripping the breaker on the current (present) 15A circuit. Could be that the breaker is old and trips easier? From what you're saying, starting up a TS or planer shouldn't draw a high current long enough to trip the breaker.

[Doug Miller]

I read that. And you don't.

-- 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]

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)

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]

Yes, that could very easily be the case. Breakers are cheap; replace it and see what happens.

-- 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.

[LRod]

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

[Chuck Hoffman]

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 "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?

[Roy Smith]

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?

[LRod]

I, also have been in electronics for more than 40 years. I have a pretty good idea of what I'm talking about, too. I've known all sorts of people over those years; some with the golden finger who didn't even need schematics to fix complex equipment but who couldn't pour piss out of a boot with the instructions on the heel.

I've known people that couldn't pass a Morse code exam if it was sent one character every two minutes but who could teach brilliantly.

And now I know someone with more background and experience perhaps than I have who can't explain a simple concept involving a single phase, two pole, center-tapped electrical circuit without complicating it beyond measure, and then when trying the simple answer gets it completely WRONG.

Excuse me, but when you're discussing phase angles, AC circuits, impedance, etc., THEORY is EXACTLY what you're talking about. Or, to be even more precise, anything involving the movement of electrons is electrical theory. Want me to prove my point? Let's discuss current flow; electron or conventional current?

But the issue at hadn is a special circumstance; it is the set of properties that becomes simple, additive arithmetic when the phase angle is 180°. It uniquely occurs in the center tapped, two pole circuit that is residential house wiring. Discussion of other phase angles overly complicates the simple additive (albeit with negative numbers) process required to understand and calculate resultant current in a two pole, shared neutral circuit.

Yes, I'm afraid I do. But with all your experience and expertise, explain this:

That was your response to a question about running a 120V load on one half of a 240V circuit. I said then:

And I stand by that question; how do you explain this?

You should have left well enough alone.

- - LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

[Chuck Hoffman]

This has become an obvious standoff and it will serve no purpose to continue it.

[Larry Jaques]

On 24 Dec 2004 12:41:29 -0500, [e-mail] (Roy Smith) calmly ranted:

Rest breaks where union electricians would go toke up?

----------------------------------------------------------------------

• Scattered Showers My Ass! * Insightful Advertising Copy

• --Noah *

[Duane Bozarth]

I think one of the problems in this thread is the confusion of using "phase" to refer to both true phases as in 3-phase versus the 180-deg phase difference between the two hot legs of a single-phase AC circuit.

I don't know when it became prevelant or popular (if it actually is or whether it's still fairly isolated) to refer to the two hots as two "phases" but I spent a significant amount of time trying to break my hired hand of the misconception over the last year. He was trained as an aircraft mechanic and was/is pretty darn good w/ hydraulics, mechanics, most simple wiring, controls, etc., but apparently was taught this as gospel regarding single phase AC power...

[LRod]

Yes. It even confuses tech school teachers with 40+ years in electronics who work for the power company...if you can believe that.

I think it's isolated. I remember years ago when I was "educated" about it. All my prior knowledge of AC theory didn't count for squat while my friend who educated me was pounding it into my head. Oddly, one of the things he kept saying was, "the power company won't like hearing you call it 'two phase.'"

- - LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

[toller]

As I understand it, everything is pretty much the same whether you have true

240v (one 240v hot and a 0v neutral) or "2 phase" 240v (two 120v hots 180 degrees out of phase). Everything will work identically. (With the understanding you would need a 120v "neutral" in the first instance to get 120v to H-N)

However 3 phase electricity is very different. Or so I have heard.

I think that is why 240v is not called "2 phase"; it can be thought of as 1 phase, but is completely different than 3 phase.

[mefjr75]

Respectfully there is no standoff you are wrong. there is only a single PHASE in household power supplies unless you have a huge shop and have 3 phase power(rare). What you haven't noticed was the fact that it is a SINGLE phase that is SPLIT by a transformer with the center tapped to ground for reference. Not 2 phases , just a split-phase.........DO NOT CONFUSE! I don't really understand the difficulty if you have 40 years of electronics you should have played with tranformers a little by now. Oh wait a minute this isn't electronics........ Leave it to ELECTRICIANS. I also know electronics and love to build things but when I started in the electrical field I discovered most of electronics is diffrent than electrical even the symbols( capacitors (electronics) look like open-contacts ( electrical) and it took many years to sort it all out (oh yes I just knew that electronics would help me....bahh). Just to qualify myself I have wired many 3 phase transformers and motors and know what the difference in a delta, wye or delta-wye transformer, and can wire them correctly too. M.E.Farmer Family full of electricians even my mother.

[LRod]

In the U.S. I don't believe there is any such thing. Household supply is obtained from a single phase of a three phase system. For all intents and purposes, when the power grid appears in your neighborhood, forget about two of those phases. You will never be involved with them. Only single phase is getting into your house.

The single phase current is transformed from a fairly high voltage off the feeders to 240V from the output of a center tapped transformer up on the pole. With regard to that 240V, there is no neutral. The 240V comes from the two secondaries of the transformer. And there is no + or -. This is AC, not DC. It is 240V across the two poles.

Our well known 120V supply comes, as you know, from one hot leg and the center tap of the transformer. Either hot leg will work.

Here is the most important thing: ALL of this; the 240V, and each 120V is developed from the same single phase of the three phase system. It is single phase power. There is no "two phase" power.

There is no "two phase" power. The two legs or poles are 180° out of phase, as you say, because they come off the opposite legs of the secondary of the transformer. They are NOT two phases of the three phase system. It is single phase. All of the current in your house comes from the same, SINGLE phase of the three phase system.

Yes and no, but I will leave that discussion to those with experience with it. I have none.

It IS single phase. There's no "thinking" about it.

Sigh. Yes, you have stated a tautology.

- - LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

[Greg]

In real "two phase" the phases are 90 degrees out and it is a 4 wire service. Topology is like the 4 poles of a compass.

[Swingman]

54 degrees?

[Doug Winterburn]

It is 240VAC, but it is similar to differential signaling (each wire wioth equal but opposite instaneous voltage & current) as in twisted pair ethernet vs. AC with a ground reference as in coax TV cable (the ground stays at a zero volt reference and the voltage on the center conductor varies symetrically positive and negative in relation to the ground).

- Doug