Hey all you electrical engineers and electricians. I need some help with an air compressor I just received. It is between ten to fifteen years old, but it is in really good shape. It has an electric motor that can be wired for 115V or 230V. It is currently wired for 230V, and that is the way I would like to keep it. The problem I have is that it has a three wire system...two hot wires and a ground wire. According to the current building codes, all 220V lines must be a 4 wire system. This means that there will be two hot wires, a neutral wire, and a ground wire. I was going to rewire the motor to be a four wire system, but I am not sure how. What exactly is the neutral wire for? What happens if I do not use it? Here is a diagram of the current setup:

P P T T T T T
1 2 8 3 2 5 4

P1--unground line(one of the hot wires).
P2--taped off(has a wire nut on it).
T8, T3, T2--taped together(using a wire nut).
T5, T4--wire nutted with the other hot wire.

now, according to the label and by looking at the wires, T5 and T8 have been switched so that the motor rotates in a counterclockwise direction.

Can anyone give me an idea on how to convert this from a 3 wire system to a four wire system? If this can't be done, I will probably install the outlet from the breaker box using a 4 wire setup, and will wire the three wires on the compressor to use a four blade plug. What will this do if I do not use the neutral wire?

Thanks for your help

 

Hmm, that's a new one on me. Where are you located? That must be a local thing.

Yea, I'd just wire the outlet to code and then just don't hook up the neutral in the air compressor. There really isn't any way to convert it. You don't need a neutral for anything. All you need it two hots and a ground.

 

[updated:LAST EDITED ON 28-Nov-02 AT 03:05 AM (EST)]The nuetral wire is connected to the ground buss at the breaker box, but should be kept separate, not connected to ground at the compressor. Neutral should be tied to the appropriate connection at the compressor junction box. Your ground wire is tied to the metal frame (junction box) of the compressor, then should be tied to the wall junction box (if metal), and tied to the ground wire going back to the breaker box. Neutral and ground may logically be the same, but there is a reason for this. I am not sure what that reason is, so I won't venture a guess that would probably be wrong!

I highly suggest having a licensed electrician do your wiring, both the house and the compressor. Remember, your safety, and your property is well worth the $$$ having a pro take care of this if you're not an electrician. I am not an electrician by trade, but have installed many compressors in dental offices, both 110V and 220V.

To quote Clint 'Dirty Harry' Eastwood, "A man's got to know his limitations..."

 

[updated:LAST EDITED ON 28-Nov-02 AT 01:40 PM (EST)]Not sure of the reasoning for the 4 wire code or even where the neutral would be utilized in a 230v curcuit. You don't need to rewire anything inside your compressor if it is already hooked up for 230v and this is the voltage that you intend to run. You need two hot wires (black and red on different phases)and a green which is ground and attached to the metal case of the compressor. The green (ground) will also be attached thruout you supply to outlet boxes etc. Green (ground) and white (neutral) are both attached back at the breaker box to the ground bus as rikfish said. While this may seem strange and redundant the reason for this is in a 115v volt curcuit if the hot were ever to short to the case and/or the neutral was somehow compromised so the breaker could not realize the short then anything coming into contact with the compressor (you) would complete the curcuit. The only thing that I can think of for your 4 wire code would be redundant grounds for safety purposes??? Better ask your local electrician on this one. I know on my pressure washer (230v)there is an external lug provided to connect an additional ground to. I dont use it but it would be safer in case the ground in the hookup cable would somehow become compromised. This may be the situation in your 4-wire code. Let us know when you find out for sure.

Randy

 

I can answer your question RIKFISH. Neutral and ground may seem the same but they are not. In standard residential wiring you have 1 hot, 1 neutral, and 1 ground. The Hot carries the entire 115 volts on it. The Neutral completes the circuit. Thats what it is there for. Some engineers thought it would be safer to only have one hot wire instead of two. On ships they have to use two hot wires because there is no earth ground. Hope this helps.

 

[updated:LAST EDITED ON 28-Nov-02 AT 11:59 PM (EST)]Thanks for the reply JR. I understand the need for the nuetral to complete the circuit, just never got into why a separate ground. My guess is using a neutral isolates your motor, and the ground provides the safety aspect.

My neighbor is a licensed electrician, I will get 'THE' definitive answer when he gets back from Florida in a few days.

 

Thanks for the replys.

I called the city building inspectors and asked them what the codes were on wiring circuts for power tools. They had to do some research to find the answer. When they got back with me, they told me only circuts for oven/ranges, refidgerators, and dryers required 4 wire circuts. Anything else can use a 3 wire circut. I have wired the circut and the compressor is up and running.

 

The reason for a sep neutral and ground is that in some applications parts of the device use only half of the 230V, or 115V to neutral. Examples would be timers, controls, and light bulbs in appliances. The ground circuit in the house does not carry current. The ground wire is for life safety and EMI purposes only. The neutral wire carries load current and as such is "live". Technically, contact with the neutral in a circuit could cause an electrical shock, particularly if there is a damaged or improper connection in the circuit.

230V devices that use 115V as part of their operation would require a 4-wire circuit.

230V devices that use only 230V would be able to use a 3-wire circuit. (Like a normal compressor)

Many (but not all) clothes dryers with a three prong plug are a perfect example of an inappropriate use of a three wire circuit. Many houses are/were wired with 3 prong dryer circuits (including mine) in order to save wiring costs. I have discovered that many RV's and RV parks are wired improperly also.


Thanks for using FTE!




[font color=red]As a Moderator I do not "Subscribe" to most topics,
please send email/pm if you need a reply -Thanks![/font]
-