Why should I use a two-pole contactor on a single-phase 240V motor?

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htroberts

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I was asked recently why I was using a two-pole contactor to control a 1? 240V motor (versus a marginally cheaper single-pole contactor).

Not using a two-pole contactor feels wrong to me, and from a safety perspective I don't want to not switch any ungrounded conductors--that's the same to me as switching a neutral in a hot-to-neutral circuit.

240-15 says that OCPDs have to open all ungrounded conductors, but I can't think of a code cite that says that a contactor has to.

I think that switching one side of a 240V circuit is former common practice in HVAC systems, but I also think that most everyone stopped doing that several years ago...
 
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dicklaxt

Senior Member
I think 430.84 of NEC 2008 lets you go with opening 1 leg ifffffffff you also have a disconnect if not then the controller must open both legs,,,,,,,,,,,,I don't know what 2011 says.

dick
 
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htroberts

Member
dicklaxt said:
I think 430.84 of NEC 2008 lets you go with opening 1 leg iff you also have a disconnect if not then the controller must open both legs,,,,,,,,,,,,I don't know what 2011 says.
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I think you're correct. It just feels like bad practice to me, and I don't like it. 2011 code says the same thing.
 
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htroberts

Member
busman said:
I've never seen a residential A/C compressor with anything but a single pole contactor.
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It seems like the older ones (mfd in 1970/80's) usually had two-pole contactors, but sometime after that manufacturers started using single-pole, presumably to save cost.

An instructor in the HVAC program at one of the local community colleges told me that common practice had recently gone back to switching all non-grounded conductors.
 
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dicklaxt

Senior Member
From the design side of the road,we always spec'ed or designed around 2 pole,however the contractor could challenge and go with any ruling by the inspectors or owners.

dick
 
Jraef

Jraef

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You have to remember that the Code is not a design guide, it is a collection of MINIMUM standards that must be met. Some things, like this, meet that MINIMUM standard but are not "best practices". The code DOESN'T say that you can't, but that doesn't mean that you SHOULD. However an OEM looking to shave every dime in cost because they make thousands of these per year, can argue that there is nothing preventing it and be technically correct. But when the same concept is applied at a retrofit one-off application level, it's just done out of cheapness. The difference in cost for a 1 pole vs a 2 pole DP contactor is so minimal it's not even funny.

BUT... that said, NONE of the DP contactors are actually UL Listed, did you know that? They are all UL "Recognized Components" signified by the backward UR symbol. That means that they can ONLY be used in a LISTED AND TESTED package with something else. So from that standpoint, if a packaged HVAC unit is built with a single pole contactor, TECHNICALLY you are not allowed to replace it with a 2 pole, unless that SPECIFIC manufacturer has the same unit UL listed with a 2 pole contactor and even then you would be required to use the EXACT SAME 2 pole contactor. Does anyone pay attention to that? Not much, but technically, that's the way it stands.
 
iceworm

iceworm

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petersonra said:
It is a common practice.
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I agree with Bob. Break all of the hot wires. It is often part of the owners' specs. It is for all of my customers.

dicklaxt said:
From the design side of the road,we always spec'ed or designed around 2 pole,however the contractor could challenge and go with any ruling by the inspectors or owners..
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That surprises me a bit. I would think your specs are the owners specs. And the answer to the contractor would be, "Ahhhhh .... How about "follow the spec" - that's what you bid on.

ice
 
Rick Christopherson

Rick Christopherson

Senior Member
It's a control circuit, not a service disconnect. So breaking only one conductor is acceptable. Anyone that would service the motor without opening the service disconnect is, well, thinning the heard. :D
 
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dicklaxt

Senior Member
I guess I should have been more explicit,,,,,,,,,,,,,,,,,,In the absense of an owner spec/preference,we designed around 2 pole,once it was issued for construction we didn't have much say in things other than answering questions for clarification of intent or to redesign if owner requested it .The final in place design was often changed by AHJ/Owner/Contractor but our skirts were clean as we had documents signed off by the owner or his rep. The contractor by General Specifications was required to do as built markups of certain design documents,These markups were approved or disapproved ( supposed to be )by the owner before they were sold.

If we had a concern during the design phase with the owners requirements, they the owner, were required to respond to an issue in writing which again kept our skirts clean from a legal view point. The plant owners oft times underwrote their own insurance so it was there game to change the rules if they saw fit.

dick
 
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htroberts

Member
Jraef said:
You have to remember that the Code is not a design guide, it is a collection of MINIMUM standards that must be met. Some things, like this, meet that MINIMUM standard but are not "best practices". The code DOESN'T say that you can't, but that doesn't mean that you SHOULD.
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Sorry, I should have specified that I'm asking the question in a design context. I'm looking for good *design* practice, not necessarily minimum code requirement.

Jraef said:
BUT... that said, NONE of the DP contactors are actually UL Listed, did you know that? They are all UL "Recognized Components" signified by the backward UR symbol. That means that they can ONLY be used in a LISTED AND TESTED package with something else. So from that standpoint, if a packaged HVAC unit is built with a single pole contactor, TECHNICALLY you are not allowed to replace it with a 2 pole, unless that SPECIFIC manufacturer has the same unit UL listed with a 2 pole contactor and even then you would be required to use the EXACT SAME 2 pole contactor. Does anyone pay attention to that? Not much, but technically, that's the way it stands.
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I think that "definite purpose" means, by definition, that it's intended to be used in a particular, definite, application--that the part will be in an assembly that ends up being tested/certified/listed as an assembly.
 
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