High leg question

[sd4524]

I have a question about wiring a 3 phase air conditioner if it has a high leg. Is the air conditioner usually marked where the high leg gets landed? Does it matter which terminal you connect to? Obviously I have never had to do this? Thanks for any help? Any good place online to read up about this?

 

[hillbilly1]

Electrically, it does not matter, nor does the air conditioner care, as long as the rotation is correct.

 

[chris kennedy]

Electrically, it does not matter, nor does the air conditioner care, as long as the rotation is correct.

Not true for all equipment, some are marked where the high leg lands as there may be 120V control wiring present.

 

[hillbilly1]

Not true for all equipment, some are marked where the high leg lands as there may be 120V control wiring present.

Yes, but the OP specifically asked about a three phase air conditioner. Never seen one of those requiring a neutral.

 

[Smart $]

I have a question about wiring a 3 phase air conditioner if it has a high leg. Is the air conditioner usually marked where the high leg gets landed? Does it matter which terminal you connect to? Obviously I have never had to do this? Thanks for any help? Any good place online to read up about this?

See below, noting the part I highlighted...

430.97 Busbars and Conductors.

?

(B) Phase Arrangement. The phase arrangement on
3-phase horizontal common power and vertical buses shall
be A, B, C from front to back, top to bottom, or left to right,
as viewed from the front of the motor control center. The B
phase shall be that phase having the higher voltage to
ground on 3-phase, 4-wire, delta-connected systems. Other
busbar arrangements shall be permitted for additions to existing
installations and shall be marked.
Exception: Rear-mounted units connected to a vertical bus
that is common to front-mounted units shall be permitted to
have a C, B, A phase arrangement where properly
identified.

 

[hillbilly1]

See below, noting the part I highlighted...

​

The unit still does not care where the high leg is terminated, that was the question, the manufacture will not have a label where the high leg goes.

 

[Mgraw]

I have a question about wiring a 3 phase air conditioner if it has a high leg. Is the air conditioner usually marked where the high leg gets landed? Does it matter which terminal you connect to? Obviously I have never had to do this? Thanks for any help? Any good place online to read up about this?

Look at the unit many have two pole contactors, some have single phase motors. If it has a two pole contactor put the high leg on the phase not on the contactor. If it has a three pole contactor put the high leg on the phase that doesn,t have anything single phase connected to it.

 

[infinity]

Look at the unit many have two pole contactors, some have single phase motors. If it has a two pole contactor put the high leg on the phase not on the contactor. If it has a three pole contactor put the high leg on the phase that doesn,t have anything single phase connected to it.

Why would this matter?

 

[Mgraw]

By code it doesn't. I guess you have not had to work on one.

 

[stevebea]

Look at the unit many have two pole contactors, some have single phase motors. If it has a two pole contactor put the high leg on the phase not on the contactor. If it has a three pole contactor put the high leg on the phase that doesn,t have anything single phase connected to it.

Which brings up another question. Most 3 phase condensers only use a 2 pole contactor instead of a 3 pole. Why?

 

[Mgraw]

Two pole is cheaper than a three pole.

 

[jeremysterling]

Not true for all equipment, some are marked where the high leg lands as there may be 120V control wiring present.

Not that A/C manufacturers have ever done anything to help the field electrician, but to require 120V control out of a 240V delta can be very aggravating.

 

[Chamuit]

At my company and at the last one I worked for, we always put the high leg on L2. This is consistent with the arrangement in the panel. One thing we noticed though, although the unit is labeled L1 L2 L3 and we arrange the conductors in CW rotation the internals of the unit would be CCW.

 

[hillbilly1]

Not that A/C manufacturers have ever done anything to help the field electrician, but to require 120V control out of a 240V delta can be very aggravating.

Not really, most would use a step down transformer if needed, they already do it for the 24 volt controls. I did hook up a 3 phase tanning bed that they imported from Germany, they even sent over an engineer with it. It required 240 volts, and all we had was 208, so I used two buck boost transformers. i asked the engineer if it needed a neutral, and he said no, but I seen an odd looking wire terminated on the ground bar, traced it back, and it was for some 120 controls! Good thing I checked! I was able to put the unboosted leg on that phase.

 

[Smart $]

The unit still does not care where the high leg is terminated, that was the question, the manufacture will not have a label where the high leg goes.

Apparently you did not grasp the full scope of the question.

I have a question about wiring a 3 phase air conditioner if it has a high leg. Is the air conditioner usually marked where the high leg gets landed? Does it matter which terminal you connect to? Obviously I have never had to do this? Thanks for any help? Any good place online to read up about this?

The unit just don't care... unless it gets all emotional on the matter , which I believe is not the case

...and while the manufacturer does not label where the high leg goes, the NEC requires where it goes.

 

[jeremysterling]

... but I seen an odd looking wire terminated on the ground bar, traced it back, and it was for some 120 controls!

That is exactly what is aggravating, the illegal neutral bond.

 

[Mgraw]

...and while the manufacturer does not label where the high leg goes, the NEC requires where it goes.

The section you cited earlier is for motor control centers and buss bars. Is a contactor a buss bar? If you have a two pole contactor and a third wire which is B phase?

 

[Smart $]

The section you cited earlier is for motor control centers and buss bars.

My bad... in skimming through the Code, I thought it was under Part VII Motor Controllers... skimming right past "Part VIII Motor Control Centers". :roll:

I wish the Code would use easier-to-distinguish "Part" header formatting.

 

[BILLY101]

Which brings up another question. Most 3 phase condensers only use a 2 pole contactor instead of a 3 pole. Why?

To save money.

BILLY

 

[LarryFine]

Is the air conditioner usually marked where the high leg gets landed? Does it matter which terminal you connect to?

No and no (unless a grounded terminal is required, as mentioned already.)

Most 3 phase condensers only use a 2 pole contactor instead of a 3 pole. Why?

Because, unlike disconnecting and overcurrent requirements, control doesn't require opening all lines.

For example, many single-phase A/C units have single-pole contactors with a 'pass-through' for L2.

Or, what Billy said. Thus: