Reply by LoneRider

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Posted on MORE Power!!! 120 vs 240 volts

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27 posts in 3877 days

#1 posted 05-17-2008 06:42 AM

Like a fool I will jump in, I know I will offend some, sorry. I don’t know if I am an “electrical whiz”, but I am the son of an electrician, EE, and currently am working on automotive power accessory power systems at work.

Power is power, simple and true, running a motor on 220V will be more efficient, heat dissipation in the wires is cut by 1/2 give or take, but that is only waste, the “work” being done, or power used to turn the motor is the same. So you are by no means using 1/2 the electricity, less yes, but not 1/2, just about 1/2 the current.

As well, regardless of if you are using 120V or 220/240V, (I’ve never lived in a house/apartment that was fed with two legs of a 3 phase, always a single phase transformed with 180deg taps, but that is another discussion :-) ) there is power going through both wires, has to, current going into the motor on one wire by definition has to go out the other.

Which is the main advantage of running everything you can on 220/240V in your home shop.

So you have three wires (excluding ground) coming into your house/shop, the middle on is the common, and then you have two power lines, in 240V they are 180 deg out of phase, and 220V 120 deg out of phase. Ether power line to ground has 120V potential.

Follow me so far?

So when you run a 120V 1200 watt load on one of your power lines, you have 10 amps coming into your house on the that one power line, and 10 amps going out on your common line.

Okay, now, turn on another 120V 1200 watt load on the same power line, you now have 10 more amps on each line, or a total of 20 amps coming in on that first power line, and 20 amps leaving on the common.

Now switch that second 1200watt load to the other power line, so you have 10amps coming in on each power line, because the loads are identical and 180 out of phase, they offset each other and result in no current going through the common line, which is a good thing™.

recap, so everything on a single power line (phase), you have the full load coming in one one of the phase and leaving on the common, but if you have the load split over both phases, you have 1/2 the power coming in on one phase, the other half of the power coming in on the other, and nothing on the common, life is good.

So if you have your table saw, dust collection, air conditioning, band saw and such all 240V, then technically you can run all of the tools without a common wire. Now of course, you can then go to the next level, and split your shop lights evenly between the two phases. The later has the advantage if you blow a single big fuse, only half your lights go out :-)

So being balanced is good and fine, but there is a really good practical point, so say, with the AC, dust collection, unisaw going through 6/4” oak, there is a lot of power draw. If all three of them where running on 120V and I accidentally plugged them into the same phase, all that current going through the common wire would cause a voltage drop on the common, equal to that on the power line BTW. That voltage drop would make the common line electrically closer to the power line with the tools on it, thus by definition further away from the other power line, 180 deg out of phase with the first. Realistically, it should not be that much, but at extremes I have heard of house with weak ground/common wires causing bulbs to burn out. Or of course, with all of the loads on the one phase you could accidentally trip the breaker for the house/shop bases on one of the phases. But if all of the big loads where 240V, split on both phases you will never have to worry about load balancing.

Well, sorry for all of those who actually read through my post, yeah, I am a newbie on this forum, work got me behind in some of my web browsing and have some more catching up to do.


-- Tom - Canadian in N. Texas

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