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

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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...
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On Fri, 24 Dec 2004 13:40:40 -0600, Duane Bozarth

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
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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.
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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
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"LRod" wrote in message

54 degrees?
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On Sat, 25 Dec 2004 01:33:37 +0000, LRod wrote:

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
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Actually, you haven't the first clue what you're talking about. I hope that your time in the tech school wasn't spent teaching how residential electrical service works.
You wrote that adding a 120V load to a 240V three-wire ["Edison"] circuit doubles the current in the neutral wire, and creates a dangerous overload.
It does nothing of the kind.
And I'd love to see your explanation of how it could.

Apparently, neither do you.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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Doug Miller wrote:

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 <only> 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 | | | |<---240v--------->| | | | L1<--120v-N--120v-->L2
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
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remember that watts x volts =amps total your watts x if 240 take it times 240 if 120 take x 120 this will give you a idea if your over loaded your breaker and yes a neutral dose carry some voltage if your circuit are unbalanced the amps will go up on the neutral it's also the return of current. some 240 stuff doesn't take a neutral

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

so the whole circuit can be considered 30A rated.

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.

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.

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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

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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).
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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.

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.
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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.
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The breakers you have may have a **very** short time current curve (you can always get different breakers with longer curves) or the motor may pull excessive startup currrent (tho this isn't my area of expertise). Once the motor is up to speed, the current will stablize -- so you can judge by ear how long the surge will last.
Matthew

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On Thursday, December 23, 2004 10:59:07 AM UTC-8, Keith Carlson wrote:

There is a common misconception about watts and volt-amperes. People misunderstands both
the same. But there is a whole different way of calculating the both. W and VA are both units of measurement for power, but that's where the similarity ends.
Watts do work or generate heat, while volt-amperes simply provide you with information you
need to size wires, fuses, or circuit breakers. Watts add linearly, while volt-amperes doe
not. And to measure W, you need a special wattmeter. You can calculate VA by using a
standard multimeter to measure VRMS and IRMS and finding the product The idea for determining the Real power VS apparent power for ac is uncomplicated, We care less of apparent power, its the utilitys problem! it is true true,or CONSIDERED
true for all practical purposes for circuits having motors,etc. Electronic products list ac line voltage,frequency, and Amp ratings. 100Watts = 100 VA
1 watt Watts (W) is calculated by multiplying Volts (V) times Amps (A), so 1W = 1VA true true,or CONSIDERED true for all practical purposes In reality A pf OF .9 WILL MAKE 1 AMP ABOUT .93 AMPS.
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YOU RESPONDED TO A 10 YEAR OLD POST.
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