240 VAC Compressor wiring help

[hmiller]

Question on installing the electrical service conductor to my 240 vac air compressor.
The compressor will be installed in a detached outbuilding, which has a 100 amp "distribution" panel (sub panel from the homes' main panel.)
It has a floating ground neutral, meaning the ground buss and neutral buss are seperate, the way they should be.

So my question is this: If the 20 amp, 240 vac, 12-2 conductor has two ungrounded conductors (black & white), the ground then is attached to the ground buss ......or the neutral buss?

I think Paul A. will know the answer on this one.

Thanks in advance

[Marc D. Shunk]

Your compressor requires no neutral. The conductors that are black and white in color will terminate on the breaker. The white will be reidentified. The bare equipment grounding conductor will terminate on the ground bar, same as any other equipment grounding conductor.

[hmiller]

That sounds right to me also....but I wasn't sure about the difference when the panels ground and neutral are not bonded together. I guess my hang up is that I am always thinking of the return path (neutral) and wondering why if the ground completes the circuit, that it would not be instead hooked up to the neutral buss?

[Marc D. Shunk]

The equipment grounding conductor is only utilized on a fault condition. It does not carry current under normal circumstances.

[hmiller]

Thanks again Marc. I appreciate your patience as this is probably very basic.
So now I understand how it should be hooked up properly.
Now my only question is how does it function with no neutral?
As I understand our two legs of 120 vac travel from the 240 VAC breaker along the black and the white (relabeled black) conductors to the motor. .....Normally on 120 VAC this is where the neutral completes the circuit back to the neutral buss in the panel. Disconnect the neutral wire and nothing works as it breaks the circuit.
....So on 240 VAC with no neutral, what completes the circuit? The ground?

[Paul W. Abernathy]

Basically you have to understand that a 240V circuit is a balanced load circuit ( for a lack of better term ) meaning lets say you have 5 AMPS on line 1 and 5 AMPS on line 2....you have a total of 0 amps on the neutral and designed that way no neutral is needed but as marc stated the effective ground fault path must be maintained...thus the bare EGC...so 240V motors take all this into account.

Remember the neutral carries the unbalanced loads....which are always the case in 120V circuits...but not in 240V circuits.....

About as brief as I can get.....lord knows we would not want me to elaborate on this and take pop shots over it...hope it explains it

Note: in some appliances like Ranges with some internal equipment that need a 120V circuit to function...ie: lights and control features as such usually then get a grounded NEUTRAL conductor....this leaving the EGC to serve it's intended purpose, kinda why they changed to require 4 wire setups to ranges and such now...but some appliances like you stated do not need the neutral circuit because they are not trying to obtain 120V readings.

[David A. Andersen]

I should let Mark at this as he has it under control!

You only should see a neutral in a panel with 120 VAC circuits as well as 240 VAC circuits (stoves).

You need a neutral to make the circuit run.

If it is a 240 VAC circuit it uses two legs of power L1, L2, not one hot, one neutral. The second leg is basically the neutral. This is not correct but may help your perception of the circuit.

Paul likely has a good graphic to show this.

Dang Paul, how did you get in there ahead of me? LOL

[Paul W. Abernathy]

I am off today as I have an appointment with the doctor today so I took off...

marc probably has a better explanation than I do....I was trying to be brief since i tend to confuse....Go for it Marc...

[hmiller]

Paul
I see the light now...
The principle of balance versus unbalanced helped tremendously.

The wiring for other 240 VAC appliances like an electric range and dryer were also throwing me for a loop. Because they have a neutral on modern applications. But they also have 120 vac lights, clocks, timers, etc.

OK guys...all a big help...much appreciated!

[Paul W. Abernathy]

Yeah......it is easier to look at it this way because we could get REALLY technical in how the path of a 240V circuit really goes but it is not needed so we come up with an easier way to convey it.

Yep...the Neutral is needed in those appiances for the potential of 120V loads in the appliance and the individual EGC is to remove the possible issues of stray voltage onto the metal case due to a inproper connection....which is present on older range's and dryers....this is what prompted the changes to the NEC in the past.

[David A. Andersen]

Glad you "see the light"!

I used to teach Oil Burner Techs A/C. They never saw 240 VAC!

They would all be standing around a Heat Pump in school trying to figure out how they would get around blowing up the world when they had to hook up a 240 VAC unit!

[hmiller]

Originally Posted by dandersen

Glad you "see the light"!

I used to teach Oil Burner Techs A/C. They never saw 240 VAC!

They would all be standing around a Heat Pump in school trying to figure out how they would get around blowing up the world when they had to hook up a 240 VAC unit!

Ya, it makes sense now...
I see the same hookup on 240 vac electric baseboard heaters, heat pumps, etc. all the time..... But I had the misconception that the ground acted as a return path, like a neutral does on 120 vac, ....in the absence of the neutral (on 240 vac)
In a main panel with the neutral and ground busses bonded or common, it hardly made any difference to me.
But where I got confused was when we get into the downstream distribution panel with "floating" ground /neutrals. I was still thinking a circuit needed to be completed to the neutral buss.

But, thanks to you guys I get it now.

[jcundiff]

240 vac is always soooo fun to explain! Just for the fun of it... here's the way I explain it to my associates when they are having trouble grasping it.

-I stand in front of them and take both of their hands in mine.
-I explain that I am the source and they are the load.
-I extend one hand while pulling back the other, push/pull, push/pull, like a choo choo
-Each hand represents 0-120 volts of potential
-As the first hand moves forward, it represents the first arch of the sign wave and voltage in that hand increases while the voltage in the other hand decreases.

Can you visualize that? One is pushing, one is pulling.

In 120v unbalanced load, its all push, push, push,

Try it with your kids, at least they'll get a kick out of it.

[Roy D. Cooke, Sr]

I can just see it now so many home inspectors who thought they had electricity figured out are now completely confused.

Roy Cooke

[jcundiff]

Like I said- just for the fun of it But trust me... it works.