RV Electricity: Reader asks meaning of AC power

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Reader question:
Forgive my electrical ignorance, but doesn’t “AC” mean “alternating current”? Does that mean that the hot and neutral wires are constantly reversing?

Mike’s response:
The “hot” wire is also known as the “line,” and it is indeed energized with 120 volts AC (in the USA). The neutral wire is “bonded” to the earth ground rod(s) at the incoming service panel (from the power company), so it should always have a voltage potential very close to earth (maybe 1 or 2 volts).

The AC thing is Alternating Current, so 120 times a second the voltage between hot and neutral wire changes from a positive swing to a negative swing. Since a full cycle is both a positive and negative swing, that’s 60 cycles per second, also known as Hertz or Hz.

All modern electrical appliances (and your RV) should have the incoming neutral wire isolated from the chassis. The chassis is connected (bonded) to the incoming ground wire in the power cord, officially known as the EGC (Equipment Grounding Conductor), and also known as the safety ground.

Getting the hot and neutral wires swapped in your extension cord is known as a reversed “polarity,” even though that’s a poor choice of words which causes much confusion. That’s because the AC voltage itself reverses polarity 120 times a second (see above).

However, the incoming wires are known as “poles” and an electrician’s definition of “polarity,” where the hot (black) and neutral (white) wires are reversed, is different from an engineer’s definition of “polarity,” where the voltage on those wires reverses itself 60 times a second from the AC current itself.

 

 

P.S. I’ve asked my friends at Southwire to come up with a decent prize for a contest this week, and they’ve supplied me with half a dozen of their really nice 3-meter kits. Read how you can win one of these great prizes.

And please be sure to read my message to Gary Bunzer, the RV Doctor: A promise between Gary the guitar guy and Mike the keyboard guy.

 

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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9 Comments
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Bob Godfrey
5 months ago

Education is a wonderful thing! Even for an old guy…….thanks Mike.

Mike Sokol
5 months ago
Reply to  Bob Godfrey

You’re welcome. There’s no reason to be bored when there’s so much to learn.

Bob Palin
5 months ago

While accurate I think this answer is too complicated for the questioner, a simple answer would have been “The hot and neutral wires do not reverse, the hot wire changes from +120v to -120v and back to +120v 60 times a second”

John R Crawford
5 months ago
Reply to  Bob Palin

Thank you Bob I agree. This was Mike’s answer to a simple question “The “hot” wire is also known as the “line,” and it is indeed energized with 120 volts AC (in the USA). The neutral wire is “bonded” to the earth ground rod(s) at the incoming service panel (from the power company), so it should always have a voltage potential very close to earth (maybe 1 or 2 volts).” Talk about confusing.

Michael Theis
5 months ago
Reply to  Bob Palin

Thanks Bob. Mike’s ‘in-depth’ responses to questions can hide the actual answer. I appreciate your clarification.

Mike Sokol
5 months ago
Reply to  Bob Palin

But that’s what I said, I think. Remember this is a teaching column so I try to give the simple answer as well as an explanation of where the answer comes from. Plus I’ll go into a little more depth for those of you who need it.

Ray Leissner
5 months ago

Thanks for the description on AC. Could you go a step farther and verify or correct my rather simplistic understanding below on how this produces power at the motor/light at the end of the wire? And then compare this process to that of DC?

Its my understanding that in AC, electrons are pushing on each other and actually moving back and forth along the conducting wire 60 times a second. This movement can create a magnetic field, if you are trying to drive a motor, or light up a bulb when the conducting wire is thinned to that of a filament that glows in a vacuum. DC simply drives the electrons in one direction, and since electrons are always looking for a place to quit moving, it takes more power to push electrons farther. This is why AC can produce power at much greater distances. Thanks.

John T
5 months ago
Reply to  Ray Leissner

No. AC can deliver power to much greater distances because the AC voltage can be increased by a transformer, whereas DC cannot. DC would have to travel at 120V all the way from the powerplant to your house. AC is transformed to 250,000 volts or more for much of the journey. Power = voltage x current (amps), so at 2000 times the voltage, the line only has to move 1/2000 of the current. Power loss in the line = current squared x resistance, so the power loss in the line is 1/4,000,000 of what it would be in a DC line at 120 volt.

That is why Edison had to build power plants every mile throughout a city with his DC scheme, whereas Tesla’s AC scheme could deliver the power from hundreds of miles away. That is why we have AC and not DC.

Mike Sokol
5 months ago
Reply to  Ray Leissner

Ray, sorry good try but that’s not how it works. Let me think about the best way to teach this topic because it’s fascinating.