Question on wiring up 220v motor

[Mike Routen]

Gentlemen,

I'm getting ready to wire up the motor to my hydraulic press pump. Can anyone tell me if it's okay to switch just one leg of the 220 that's going to the motor, or do I need to switch both legs? ( I know that there are all kinds of openings for jokes here about switching legs....)

Thanks!

- Mike

[Jamey Saunders]

Either will work, but there are a couple of things to consider. Let's say that you switch one leg. It's possible that if the motor shorts out, you won't be able to turn off power to it with the switch. You'll have that one un-switched leg shorted out to the ground, which means (if your breaker doesn't kick off) that you'll have 115 volts going through that motor until you can get to the breaker. I'd switch both legs.

Also, make sure that you're using a switch rated above the amp rating of the motor, just for safety, and make sure it's rated for 220v. I tend to lean toward using a mechanical contactor for switching 220v motors, but it's surely not needed. They can get pricey (well, $50 or so, best as I remember) but it'd be worth it to me for the assurance that I have a truly heavy duty switch controlling the motor. Remember, that motor will routinely draw three to four times it's rated amperage during start-up, and all that power has to go through your switch.

What type of switch you use is up to you, but I'd go with a mechanical contactor that switched both legs.

[Mike Routen]

Jamie,

Thanks for the info. Can you tell me what guage of wire I should be using from the switch to the motor? It's about 3' between the two of them and I'm trying to fit everything into 3/8 flex conduit. I was going to run 10 guage but I don't think I have enough room in the conduit.

Thanks again.

- Mike

[Jamey Saunders]

There's a calculator at http://www.elec-toolbox.com/calculators/voltdrop.htm that will help you. It all depends on the length of the run (3 feet) and the amperage of the motor. Plug in those numbers and this page will calculate the minimum wire size for the application. I'd go one size larger. (Which means smaller -- 14 gauge is bigger than 16 gauge. The smaller the number, the bigger the wire.)

[brucep]

We need the full load amps (FLA) off the motor or the motors horsepower to size the wires.

I agree you should switch both legs. Your breaker in your panel will need to be sized to the wire gauge also.

[Jamey Saunders]

Hey, Bruce, you're not that far from me. I'll be in Dawsonville this weekend. The wife wants to go shopping. Are you going to be at the GCKG meeting in August? It's up in Cleveland.

[Mike Routen]

Thanks guys! The motor is a Harbor Freight motor that was originally sold as a 5hp, but is now sold as a 3hp. I'm pretty sure its FLA is 16 but I'll check when I get home. I'm going to use the dryer outlet in the garage wich is a 50 amp circut. I know that I'm nowhere near that.

[brucep]

Jamey,

Alas, while I get the GCKG emails, it is unlikely I will be able to make a meeting as they are generally on the same day I run the Action Pistol match at my gun club.

I JUST got my new propane forge running, hopefully I can attach a pic here.



It's made from an air compressor tank.

[hogcat]

You have to shut off both 110 volt legs of the 220!!!!!! The motor will still be electrically HOT shutting only one leg off!!! No such thing as Jamey says shutting just one leg will work. You must for saftey shut off all power with the switch. So a double pole switch is a must have for 220.

[Mike Routen]

Gary,

Thanks for the heads-up! I went out and picked up a double-pole single throw switch and will be switching both legs just to be safe.

Cheers!

- Mike

[pjelect]

Mike, with a motor drawing 16FLA you will need to use a 20A TWO pole CB in your panel and #12 MTW or THHN wire. Your 20A circuit is, by NEC code, derated to 80% of the branch circuit protection, ie .80X20A=16A. Once up and running, the motor will draw a lot less amps. 3/8 tubing is just too small for this, use 1/2". Also use a DPST Switch(double pole single throw), of the proper rating.
Do NOT skimp(or overkill, for that matter) when it comes to setting up your electricial system for your shop; do it once, do it right, and move on to utilizing your equipment, knowing that the safety aspect of it has been taken care of. PS. NEC code requires you to include a grounding conductor along with your circuit conductors in metalic tubing 6' or longer and a must in any nonmetalic tubing. A good practice is to ALWAYS include a grounding conductor no matter what your conduit system will be. Be safe. Pat McGroder

[Mike Routen]

Pat,

Thanks for all of the info! I am using 12g. solid wire from the DPST 20a. switch to the motor (2') inside of 3/8" flex conduit. From the switch to the outlet (12') I'm running 10g. cable. I'm currently using the 50 amp clothes dryer circuit in my garage, but will eventually be running underground conduit out to a shop when I build it. Then I'll run 3/4" or better conduit to handle a 220 circuit and a 110 circuit along with a grounding conductor.

I've given the press a little test run and everything seems to be working correctly.


There is still a little more welding to do before I paint, then I'll give everyone a peek at my FrankenPress.

Thanks again to everyone for their help!!!

- Mike

[pjelect]

Mike, just remember when you get your new system set up in your new building to use a 20A circuit breaker(2 pole). Your protecting the circuit wires, not the appliance. I see you didn't run a grounding wire with your other conductors. If the 3/8 tubing is metalic and short (less than 6ft) and has the proper fittings on the ends, and either the switch box or the motor is properly grounded, you should be OK for now. But if the tubing is non-metalic you should definitly add an equipment grounding conductor. Pat McGroder