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Old 10-11-2006, 01:02 PM
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Default 220v polarity: Does it matter what hot goes where?

I always thought that the two hots of 220 were basically interchangable. The power comes through on one hot, returns on the other, then everything switches. A friend just mentioned that the two are not actually equal but 180* off, that polarity matters and one leg is generally considered 'hotter' than the other. He said that this doesn't matter in 99% of applications, but some very sensitive equipment can have problems if they are flipped. He said he learned this while working at a steel mill where they had problems with something and the electrician found that the wires had been flipped.
Does this make sense? I'm out of my area of knowledge here. Any help would be greatly appreciated as I am going to connect the wires on my welder circuit this weekend.
Did I mention by the way I have a mm210 now? I just love typing that.
--Bob
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  #2  
Old 10-11-2006, 01:18 PM
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I've never heard that either but it might be correct. But, I never took anything like that into account hooking up 220 stuff and I've never had any problem. I just hooked my new MM210 up with no problems. If I were you I'd go out and hook the 2 hot sides up and the neutral in the middle and not worry about "flipping" them. I wonder how you even know if they are flipped?
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  #3  
Old 10-11-2006, 01:41 PM
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What you have is 240 VAC and there is no polarity (to simplify). Both the black or hot legs are 120 VAC. If hooking up a welder you will be hooking up a green equipment ground unless the welder requires 120 VAC to operate something.

Get someone savvy on electrical to do the wiring. Without being unkind your friend may not be the right choice.

The life you save might be yours..
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Old 10-11-2006, 01:43 PM
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Quote:
Originally Posted by Jim-TX
. I wonder how you even know if they are flipped?


When the welder worked backards..
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  #5  
Old 10-11-2006, 02:16 PM
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On a single phase transformer derived system as seen in homes there is no issue.

In an industrial setting where the 220 has been derived from the three phase without a transformer there may be a voltage or noise interferance issue depending on the type of 3 phase input.

An example of why connection choices are important this is a generator but the same basic conditions apply.

I have a customer running a 25 kva generator to power a camp. They purchased the plant from a large rental house without any outside assistance.
This generator has a selector switch to set it up as 277/480 star 120/208 (139/240) star or 120/240 zig zag at 14.4 kva

Since the camp is basic they selected the zig zag connection for true single phase . In fact this is not a true single phase connection but rather is a distorted three phase abortion . The result is that they blow a lot of electric heaters in some of the tents and use a lot of light bulbs again always in the same locations.

When they called me I checked the output it was 120 /135 240 (should be 120/120 240)with the 135 leg being responsible for the heater and bulb failures.

This is not what you normally expect from a zigzag or open delta configuration but results from the way the regulator sensing needs to be done to allow the quick change switch to work. non the less it is not the worst imballance that I have seen from this connection.
Terry
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Last edited by terry lingle; 10-11-2006 at 02:19 PM. Reason: keying error and clairity
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  #6  
Old 10-11-2006, 02:29 PM
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Thanks guys. Moe - yes, I agree, this person was recommending getting a real electrician to check the wiring, he used this as an example of how it could be wrong.
Terry, thanks! That totally makes sense, and is probably exactly what he was referring to.

Thanks Everyone! Don't worry, I'm definitely going to have someone check my wiring before I turn it on, thanks for the concern!
--Bob
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  #7  
Old 10-11-2006, 03:19 PM
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Quote:
Originally Posted by Bob_S2
I always thought that the two hots of 220 were basically interchangable.
You would be correct.

Quote:
Originally Posted by Bob_S2
A friend just mentioned that the two are not actually equal but 180* off, that polarity matters and one leg is generally considered 'hotter' than the other.
Bullsh!t as far as residential goes, now in a plant with 3 phase supply things are much different.

Quote:
Originally Posted by moe1942
Get someone savvy on electrical to do the wiring. Without being unkind your friend may not be the right choice.
Good advice...
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  #8  
Old 10-11-2006, 03:33 PM
stseely stseely is offline
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In some places I have seen what is called a high leg but this usually is only a problem in a building that has 3 phase in it and older ones at that where stuff has been added and changed over the years. One way you can tell is check each leg to ground. Normally it doesn't matter which leg goes where on single phase stuff.
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Last edited by stseely; 10-11-2006 at 04:25 PM.
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  #9  
Old 10-11-2006, 04:19 PM
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Quote:
Originally Posted by terry lingle
On a single phase transformer derived system as seen in homes there is no issue.

In an industrial setting where the 220 has been derived from the three phase without a transformer there may be a voltage or noise interferance issue depending on the type of 3 phase input.

An example of why connection choices are important this is a generator but the same basic conditions apply.

I have a customer running a 25 kva generator to power a camp. They purchased the plant from a large rental house without any outside assistance.
This generator has a selector switch to set it up as 277/480 star 120/208 (139/240) star or 120/240 zig zag at 14.4 kva

Since the camp is basic they selected the zig zag connection for true single phase . In fact this is not a true single phase connection but rather is a distorted three phase abortion . The result is that they blow a lot of electric heaters in some of the tents and use a lot of light bulbs again always in the same locations.

When they called me I checked the output it was 120 /135 240 (should be 120/120 240)with the 135 leg being responsible for the heater and bulb failures.

This is not what you normally expect from a zigzag or open delta configuration but results from the way the regulator sensing needs to be done to allow the quick change switch to work. non the less it is not the worst imballance that I have seen from this connection.
Terry



I guess I'm lazy. I only answer the question asked..
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"If we ever forget that we're one nation under God, then we will
be a nation gone under". ~Ronald Reagan

We should have picked our own cotton...

I love my women hot and my beer ice cold..
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  #10  
Old 10-11-2006, 05:15 PM
harcosparky harcosparky is offline
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In all my years of hooking up to 220V I have never had an issue with which side is which.

However around here ( Maryland ) it gets fun. It used to be you needed only 3 wires to hook up a 220V appliance. Colors were Black/White/Green. Black/White went to the breaker and the two hot legs of the plug, ground went to box ground.

Now they are getting picky and STUPID and require 4 leads. Black / Red / White / Green. Black and Red to hot legs, White to " neutral " and Green to ground. Funny thing is at the box they tie the neutral (white ) lead to box ground. In effect you now have two conductors running to ground.

Come to think of it I am not sure neutral is even effective. I should get my o'scope out and probe the to lines. Could be there are two phases there.

Flipping leads in my experience only comes into play in 3 phase power, when you flip two phases motors runs BACKWARDS.

On a job one time I had to rewire a 3-phase printing press with no schematic and barely a blueprint. Manufacturer was Gutenberg. Well after the job was done the printing manager came to inspect the press and gave me the 'great job' pat on the back. Two days later I get the call, " this damn press is running backwards ". I said " you mean the press I rewired and YOU checked out? "

I went back and on the 3 phase motor swapped two phase and all was well.

Funny how in my past life I respected electricty and did all in my power to prevent sparking and arcing and here I sit today shorting high current power to ground and melting metal!
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