How do I know what volts my shop wiring is?

[Jake]

I bought a new compressor pump and reading the directions it says I shuldn't run it on 208 volts. How do I know what my garage is rated at? The motor is a 5 horse, single phase, 230 volt and 32 amps. Thanks

[John McGraw]

I bought a new compressor pump and reading the directions it says I shuldn't run it on 208 volts. How do I know what my garage is rated at? The motor is a 5 horse, single phase, 230 volt and 32 amps. Thanks

Unless you have 3 phase power in your garage, the voltage is 230-240 volts.
Single phase power is used in residential and light commercial areas and 3 phase power is usually only used in heavy commercial. In single phase power, when a 115 volt circut is at + 115 volt potential, the adjacent phase is at - 115 volt potential giving a 230 Volt potential phase to phase since the phases are exactly 180 degrees apart. In 3 phase power, you have 3 adjacent circuts instead of 2 and the phases are 120 degrees apart. Two adjacent 115 vlot circuts only total up to 208 volts because the peaks do not fall exactly across form one another because of the 120 degree phase angle. There are a lot of single phase motors that can run off either 208 or 230 volt, but the motor usually has to be de-rated to run on 208 volt.


Regards, John McGraw

[CDJr]

Great info, John! Jake, I think you may have to run a dedicated circuit to your garage just for the compressor if theres not one already there thatll handle that large of a load. Most homes wont have a circuit run to the garage thatll handle that unless it was added on. Personally, Id run 8 ga wire or maybe even 6 ga if thats the running amps. Or do you already have a compressor there and you just changed out the pump?

[Jake]

Thanks John, I now know I have 230. CDJR, I am just replacing the pump. I ran under ground cable to my garage years ago, not sure of the size but each wire is about 3/8 of an inch. Jake

[Phil V]

I have a question thats bothering me. If the directions for the new pump say not to run that pump at 208 volts, then is makes absolutely no sense. A pump is a mechanical device that is run by a belt from an electric motor. That pump doesn't have a clue and could care less what the voltage is going into the electric motor.

I can understand where a European 208 volt source may have a problem with one of our 220V AC electric motors. So why would that mechanical pump co. add that disclaimer to their pump instructions ?

[John McGraw]

I have a question thats bothering me. If the directions for the new pump say not to run that pump at 208 volts, then is makes absolutely no sense. A pump is a mechanical device that is run by a belt from an electric motor. That pump doesn't have a clue and could care less what the voltage is going into the electric motor.

I can understand where a European 208 volt source may have a problem with one of our 220V AC electric motors. So why would that mechanical pump co. add that disclaimer to their pump instructions ?



Phil,

It is really a function of the motor and not the pump. If the motor is not a dual voltage rated motor, it will burn up the motor if you run it on 208 volt.
In order to perform the same work on 208 volt as 230 volt, the motor has to run at higher amperage. If the motor does not have a sufficient "over design" built into it, then the motor will jus burn up from being overloaded. Remember, on an AC motor the speed is constant and does not change as the voltage changes, so if you lower the voltage the motor will still run at the same speed, which means the compressor will still comsume the same horsepower. A horsepower is 746 watts and watts is Volts time Amps. If the voltage drops, the Amps must go up to maintain the same horsepower.
With cost being a main consideration for a lot of equipment purchases, it only makes sense that some compressor makers will stretch to the edge of the capability of machine design to sell a less expensive compressor. Dual voltage rated motors are more expensive since they have a greater safety factor built in. There is an old saying about buying compressors, "there are 3 factors to take into account in purchasing a compressor, Price, Performance, and longevity. You can have any two of the items, but you cant get all 3!"
You can get a cheap compressor that will have high performance, but it will not last, you can get a cheap compressor that will last forever, but it won't have very good performance.

Regards, John McGraw

[Jake]

You are correct, it is not the pump. It just says in their instructions that I need at least a 5 horse motor and that is where it is talking about the wiring requirements. Jake

[dchristian]

Be careful, you said you have a wire ran underground to your shop. I assume from your house. I assume you are also running 120 volts for your lighting and other tools. That wire must carry the entire load so make sure include that in your calculations.
The length of the cable must be taken into account too. The resistance in too small a wire can cause it to overheat and start a fire. Also, an electric motor will have a higher inrush current momentarily when it starts. Also make sure you are running a neutral and a ground all the way back to the source (house breaker panel) or you could get ground currents that can be deadly. A current of as low as 10 micro amps through the heart can cause cardiac fibulation. And a current from one hand to the other takes a path through the heart. In short if you are not sure get a REPUTABLE electrician to check it out. The few dollars you spend might save your life and or loss of property. Sorry didn’t mean to preach but I have been in the medical electrical / electronic field for 35 years and I have seen people come in the ER and never go home again