HVAC

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jmd445

Senior Member
I don't know if this is the right forum but........ is there a standard for wiring a 3 phase rooftop HVAC unit for rotation? For example I'm working with a Carrier 48TJE three phase unit. Do I wire it Phase A,B,C to L1, L2, L3 CW rotation or do I wire it C,B,A to L1,L2,L3 for CCW rotation. I can't find any thing in the mfg's literature and I've asked several electricians / conractors and no one has an answer.

The only solution they offer is to check the blower motor and see what direction it is turning (should the blower be 3-phase). Note: reciprocating compressor will operate either way, scroll compressors will self destruct of operated in reverse.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: HVAC

You are correct pistons compressors do not care although once they run in one direction for awhile should not be run the other way or accelerated wear happens.

The only sure way to verify the rotation on a scroll compressor is with a set of refrigeration gauges connected to it.

The last one I wired was a 40 Ton unit and I made all the connections but left it off with large notes left for the refrigeration mechanic to check rotation. These guys are capable of swapping two leads if needed.

This unit had a tag that said the rotation had been checked at the factory and it matched the fan rotation. Even with that I would not fire it up myself, a new scroll compressor would be costly. Besides that it is really not my job to do a start up on the unit.

[ September 05, 2005, 12:54 PM: Message edited by: iwire ]
 

mdshunk

Senior Member
Location
Right here.
Re: HVAC

If you ever do start up a large scroll compressor that has the rotation reversed, you'll never forget it. It screams like an banshee. As mentioned though, a reciprocating compressor's rotation doesn't matter, but you do want the condenser fans spinning the right direction.
 

ccjersey

Member
Re: HVAC

Most oil pumps which are part of the compressor have the capability to be reversed and still provide forward flow. I am most familiar with smaller reciprocating compressors.

Jim
 

iwire

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Staff member
Location
Massachusetts
Re: HVAC

Originally posted by jmd445:
Unfortunately no one has answered the question ........... are the units normally wired CW or CCW ?

Jim
Jim there is no 'standard' answer.

I would never trust being told if I hook it up L1, L2, L3 clockwise that it is correct.

All scroll units I have installed said in the instruction package to verify rotation with gauges.

For me there is to much money on the line to gamble on someone else connection being correct.


You could call the manufacturer and see what they say.
 

ccjersey

Member
Re: HVAC

Yes that is what you do, but the whole thing is sealed and you have to attach gauges to check pressures.
 

BruceH

Senior Member
Re: HVAC

A 3 phase roof-top with a sealed non accessible refrigeration system, that's odd. I would do as described, bump the contactor for the 3 phase condensor fans or indoor blower motor and note rotation, then after that bump the contactor for the compressor and take a listen (if it's a scroll). I find it hard to believe there are no access ports on this unit for checking pressures, that is part of a normal start-up procedure for a a/c service tech. Also, momentarily operating a scroll compressor in reverse will not result in self-destruction (do ya really think it's never been done before a million times on start-ups?)

[ September 06, 2005, 11:30 AM: Message edited by: BruceH ]
 

james wuebker

Senior Member
Location
Iowa
Re: HVAC

jmd445
Most of all the units I've done are wired up cw and you can use a phase tester to do that but that doesn't mean it's the correct way. The factory has pre-wired everything so that if you wire up the main correct everything will run correct. So again like george said wire it up and bump the contactor for a fan and if it's turning the right way all other items on this system should be correct. But if you try it and the fan is turning the wrong way then all you need to do is switch any 2 wires around on your main feed for the unit.
Jim
 

ccjersey

Member
Re: HVAC

Don't think I or anyone said there were no ports on it, it is just that most electricians I know don't keep a gauge set in their back pocket, unless they do refrigeration as well.

Jim
 

james wuebker

Senior Member
Location
Iowa
Re: HVAC

jmd445
Most of all the units I've done are wired up cw and you can use a phase tester to do that but that doesn't mean it's the correct way. The factory has pre-wired everything so that if you wire up the main correct everything will run correct. So again like george said wire it up and bump the contactor for a fan and if it's turning the right way all other items on this system should be correct. But if you try it and the fan is turning the wrong way then all you need to do is switch any 2 wires around on your main feed for the unit.
Jim
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: HVAC

Originally posted by james wuebker:
The factory has pre-wired everything so that if you wire up the main correct everything will run correct.
In my mind that is a large assumption that the factory never makes a mistake.
 

macmikeman

Senior Member
Re: HVAC

n my mind that is a large assumption that the factory never makes a mistake.
Amen to Brother Bob for that. Yesterday I hookup up a 7-1/2 ton compressor 3 phase. The factory installs pigtails from the line side of the contactor down to a wire cabinet for us to connect our incoming 3 phase conductors from the disco to. Sombody at the factory forgot to even slightly tighten up the lugs on the line side of the contactor. I figured it out real easy when one of the pigtails slid right out of the machine as I only pulled it slightly. If I hadn't of noticed, or done my usual tighten check of factory lugs, could have had a loose hot wire as soon as vibrations started.
 
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