Current Sensing Switch using neutral

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sandsnow

sandsnow

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Southern California
In a high rise residential project, the dryer duct is installed with a booster fan. The fan is turned on using a current sensing switch rated for the load served. The dryer recep is connected to the box containing the switch with a short nipple. The fan has it's own 120V circuit.
The engineer has the dryer neutral coming into the dryer recep box; going through the short nipple to the switch and back through the short nipple to terminate on the dryer recep.
This seems ok, just like running a hotleg down to a switch and back up in the same cable. I can't find anything in Art 200 or 300 to prohibit it.
Does anyone see a problem here I missed?
Thanks
 
Re: Current Sensing Switch using neutral

Possibly 300.3(B)?

I wonder if the grounded conductor to a dryer always carries current?
 
Re: Current Sensing Switch using neutral

I don?t see a problem if it leaves and returns through the same nipple.
 
Re: Current Sensing Switch using neutral

Is the flow switch UL listed?
 
Re: Current Sensing Switch using neutral

This doesn't sound right?
Why the neutral and not the hot that the motor is on. A problem I see is if these dryers are ever changed to a newer dryer that maybe doesn't use a neutral for the blower motor, Or is wired differently, this blower might build up exhaust gases in the living unit. Why not use a air flow or pressure differential switch that would be more reliable. Like the combustion blowers on a furnace uses.
 
Re: Current Sensing Switch using neutral

I read 300-3(b) also. That got me wondering where in the code it allows a hot and switch leg only in one conduit which we do all the time.
I don't know if the neutral always carries current, good question.

The switch is CSA/us listed.

I don't know why they don't use a pressure differential switch, cost maybe??

Do they make a dryer without a neutral? These are condos and the appliances are provided with the unit and regulated by the HOA.
 
Re: Current Sensing Switch using neutral

Originally posted by sandsnow:
That got me wondering where in the code it allows a hot and switch leg only in one conduit which we do all the time.
Click to expand...
Section 404.2(A) Exception


Do they make a dryer without a neutral?
Click to expand...
I don't think they make dryers without grounded and grounding conductors, but I'm not completely positive the grounded neutral conductor carries current for all dryer operations. There may be some models that only carry neutral current on one cycle or mode. That could be a problem.
 
Re: Current Sensing Switch using neutral

Actually, I think 300.3(B) should be worded differently.

It should say, (or something to the affect) all conductors of the same circuit necessary for the operation of a device or outlet shall be in the same (common) cable or raceway and shall not be in multiple cables or raceways. Of course this is excluding sections and exceptions that would specifically allow it.


If any one conductor is not necessary in a portion of the circuit routing, there is no need to waste natural resources to install it.

Roger
 
Re: Current Sensing Switch using neutral

Originally posted by bphgravity:
Click to expand...
Section 404.2(A) Exception
Thanks, that's 380-2(a) Exc in my world. Forgot all about that little gem

QUOTE]I don't think they make dryers without grounded and grounding conductors, but I'm not completely positive the grounded neutral conductor carries current for all dryer operations. There may be some models that only carry neutral current on one cycle or mode. That could be a problem. [/QUOTE]

i'll have to check that. I thought the neutral runs the internal fan motor w/ one phase, which if true would be why they measure the neutral current rather than try and pick the correct phase wire. Thanks, your input got me thinking.
 
Re: Current Sensing Switch using neutral

As long as they are using the neutral for current sensing and not power for the blower motor, there should be no problem. The ultraviolet light inside and dryer timer usually operate on 120 volts, so there is current going back to be sensed. We have seen small blower motors put in series with the neutral!!!
 
Re: Current Sensing Switch using neutral

All dryers that I know of use the neutral for the motor (120V), therefore there is always a neutral current when the dryer is running.
 
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