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# electrical phases

#1
08-26-05, 02:31 PM
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Join Date: Apr 2005
Location: ny
Posts: 190
electrical phases

can u please explain to me what a phase A and a phase B meanin regards to electricity?

#2
08-26-05, 02:51 PM
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Join Date: Sep 2002
Location: welland ontario
Posts: 6,171
Although in a house they are not truely different phases.
When power comes into your house there are three wires, two hot and one neutral.
From hot to hot you get 240 volts. From either hot to neutral you get 120 volts. The two hots are often refered to as two phases although they are not really. This would be where the terms phase A and phase B are coming from.

#3
08-26-05, 03:15 PM
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Join Date: Sep 2000
Location: United States
Posts: 18,497
We could probably rant on about this for nearly forever, but if we had some kind of idea as to why you asked the question, we could focus our ranting a bit. So why did you ask the question?

#4
08-26-05, 03:17 PM
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Join Date: Aug 2005
Location: USA
Posts: 995
Ignoring the issues with them in the transformer on your block, let's just deal with the two phases entering your house.

At whatever time A phase is PUSHING 120V, the B phase is PULLING 120V. This is in respect to the neutral (white) wire. So between A and B there is always a 240V difference (-120V to zero + zero to +120V).

They still cycle at 60Hz. And each will always be within 120V of the neutral.

#5
08-26-05, 07:42 PM
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Join Date: Apr 2005
Location: ny
Posts: 190
Originally Posted by John Nelson
We could probably rant on about this for nearly forever, but if we had some kind of idea as to why you asked the question, we could focus our ranting a bit. So why did you ask the question?
i was told by someone from your staff that it is normal for lights to dim for a second when a major appliance, (boiler for example), cycles on and off. could the wiring and the phases be altered to prevent this? (i dont know if this makes sense but someone had told me u could switch the phase or something like that).

#6
08-27-05, 10:53 AM
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Join Date: Feb 2004
Location: usa
Posts: 247
Motor driven appliances (i.e.-AC compressors, water well pumps, etc.) draw an inordinately large amount of current (roughly 4 to 6 times normal) at start up for the brief few cycles it takes them to come up to speed. This large current draw causes the voltage to dip (sort of like the water pressure dropping in your house when someone flushes the toilet). Since the lights see the same voltage source, they dim as well.

What someone may have been referring to as to "changing" phases was to put a heavy 120 volt appliance (like a microwave) on the other leg (as mentioned above) or onto another breaker that only has receptacles on it instead of lights. Of course, you might just be transferring the problem from one to the other, or even tripping the other circuit if you overload it.

If the dimming is coming from a 240 volt load, like the AC unit, swapping legs doesn't help a thing.

On the other hand, if your dimming lights are caused by your neighbor starting a saw next door, they may have been referring to getting the Utility to switch you to a different phase or transformer on the power line. A completely different subject all together.

If the dimming is brief, and merely noticeable (as opposed to making all your clocks reset), it is normal. Most people just get used to it.

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