Does the code allow for a 240V circuit to have one hot leg switched and the other not? A driveway ice melt system is setup to do this and the mfg has told the homeowner this is allowed. I have never been allowed to do this and have had to use a 2 pole contactor in this situation. Am I missing something here?
240V Crk with one leg switched
- Thread starter ZCBee
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jumper
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Yes the controller/t-stat can break only one leg, but ya still gotta have a disconnecting means. See 426.50 and .51
So, the 240V ckt can have 120V out to heating elements 100% of time? Code does not require both legs to be open or closed together? Yes, the cb is 2 pole device.
jumper
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Sure, no different than baseboard heat. The element needs 240V to work.
426.51(B) and (C) would allow those controls.
Industrial v home
Industrial v home
OK, thanks for the info. Just as a point of order, in industrial applications we have not built circuits like this for a long time, at least where I work, for personnel safety requirements. I think it is a good idea to kill both hot legs for safety whether it is in a homeowners yard or in a large scale industrial setting. Just my point of view. Thank you for the clarifications.
Industrial v home
OK, thanks for the info. Just as a point of order, in industrial applications we have not built circuits like this for a long time, at least where I work, for personnel safety requirements. I think it is a good idea to kill both hot legs for safety whether it is in a homeowners yard or in a large scale industrial setting. Just my point of view. Thank you for the clarifications.
jumper
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Why do you see this as unsafe? The controller is not the disco.
Most the 240V appliances in your house are wired this way ie stove, dryer, HVAC, etc.
Yes, but in a machine, where I have spent most of my career in, we don't allow two means of isolation, as in the single pole situation, where the disconnect and a controlling element are not of equal state, for personnel protection. I was mixing my uses. But, with that out of the way if by some set of circumstances this driveway melter is inadvertently accessed by unqualified personnel, and this is what we tried to prevent in our overkill situation in a facory, that being the human equation, a contactor is a cheap piece of insurance.
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Training makes things safe, no one should be touching circuits not shut off at the disconnecting means.
As an industrial guy it might interest you to know that even today the NEC allows the use of a two pole motor starter to control a three phase motor. As a rule I don't see anyone doing this anymore but it is good to know this so you will not be surprised by it.
jumper
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:thumbsup:
For what it's worth, whether allowed by code or not, I agree with you.
I've never been a fan of "not breaking all ungrounded legs through a contact" device either.
If I run a 240v heat blanket through a thermostat you can bet there will be a contactor involved somewhere breaking all current carrying legs going to it.
JAP>
just the cowboy
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Control circuits in machines
Control circuits in machines
You don't break both sides of the coil with a stop button on 240V coils.
Control circuits in machines
You don't break both sides of the coil with a stop button on 240V coils.
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The only reason it would be less safe to only break one pole is if untrained persons are working on it.
Typically when I wired 240 electric baseboard I would choose a DP T-stat with marked off position so it can serve as the disconnecting means but that was a choice to avoid putting breaker locks on the panel.
Okay, the training on the home and commercial side verses the industrial side is different from what I can make out from the conversation. My perspective is like this:
We have had to find ingenious means to keep personnel out of circuitry and here is why. My coworker has their production supervisor breathing down his or her respective neck to "make the numbers". The machine breaks just after the super chewed out my coworker. The coworker, fearful for her job, opens the control cabinet and starts pushing reset buttons and accidentally contacts a live circuit. She was just trying to do her job and all of the OSHA regulations in the world did not stop her from trying to keep her livelihood in tact and her supervisor happy. I therefore am ethically bound to do all in my power to make it human-nature proof as I can. And that sums up the training programs and engineering standards put in place becasue this type of incidence has happened enough times to warrant a $300 contactor.
Modern interlocks have been engineered to remove this as much as possible, for safety/personnel protection. But, human nature is to outsmart something in its way of achieving success or a goal. I leave this to everyone's contemplation becasue the code is about protecting life and property only.
This scenario can bleed over into the homeowner realm just as easily. Say the homeowner hires a handy service who has some electrical training. The "some" part is the key element in this version. How the heck do you control the qualified personnel in a homeowner situation?
I have not seen a 2-pole mcc setup used for 3 phase circuits. My plant goes back to the mid 80's for some of its equipment and have not come across this usage. But, I bet that this would be a "Qualified Personnel" situation due to a 277V shock potential.
I'd have to gently disagree and say that not having that one leg energized at the point of use would be safer.
Not so much on an automated control as your talking about but just in general.
JAP>
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