Grounded conductor (Neutral) at electric coil fan powered (VAV) boxes

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Stranberg

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Kent, Washington
I have an electric coil fan powered (VAV) box that is rated at 208v, 3 phase, 12kw. The nameplate shows a minimum circuit ampacity of 56.3 amps and a maximum overcorrect protection of 60 amps. The heater amps is 33.3 amps which is consistent with the wattage rating of the unit. The motor is 3/4hp, 11.8 FLA at 115v, which is tapped off of the ?a? phase at the terminal block inside the unit which is consistent with the minimum circuit ampacity of both combined loads x 125%. I have an existing 1? EMT conduit which is feeding (2) of these units. Table C.1 shows that I can get (7) #6 conductors in a 1? EMT conduit, but I would need (8) #6, (4) each if the grounded conductor is rated at 100% of the circuit.


What I would like to do is to downsize the grounded conductor to #10, the same size as the EGC, and sufficient for the unbalanced load of 11.8 amps. Looking in 210, 220, and 424, I can not find anything that tells me this is acceptable. There are similar conditions like household cooking appliances where this is allowed, as stated in 210.19(a)3.


Does anyone have any experience or insight as to where this would be considered acceptable or not.


Thank you in advance for your help!
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I have an electric coil fan powered (VAV) box that is rated at 208v, 3 phase, 12kw. The nameplate shows a minimum circuit ampacity of 56.3 amps and a maximum overcorrect protection of 60 amps. The heater amps is 33.3 amps which is consistent with the wattage rating of the unit. The motor is 3/4hp, 11.8 FLA at 115v, which is tapped off of the ?a? phase at the terminal block inside the unit which is consistent with the minimum circuit ampacity of both combined loads x 125%. I have an existing 1? EMT conduit which is feeding (2) of these units. Table C.1 shows that I can get (7) #6 conductors in a 1? EMT conduit, but I would need (8) #6, (4) each if the grounded conductor is rated at 100% of the circuit.


What I would like to do is to downsize the grounded conductor to #10, the same size as the EGC, and sufficient for the unbalanced load of 11.8 amps. Looking in 210, 220, and 424, I can not find anything that tells me this is acceptable. There are similar conditions like household cooking appliances where this is allowed, as stated in 210.19(a)3.


Does anyone have any experience or insight as to where this would be considered acceptable or not.


Thank you in advance for your help!

I don't believe there is an exception as there is for Ranges to downside the neutral for a branch circuit. Now if you have a disconnect that has overcurrent protective device then the conductors are feeders and art.215.2(A)(2)

(2) Grounded Conductor. The size of the feeder circuit
grounded conductor shall not be smaller than that required
by 250.122, except that 250.122(F) shall not apply where
grounded conductors are run in parallel.
Additional minimum sizes shall be as specified in
215.2(A)(2) and (A)(3) under the conditions stipulated.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
210.19 states that branch circuit conductors shall have an ampacity not less than the load served.
IMO that gives you the permission to size your grounded conductor per the load.
 

Little Bill

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Tennessee NEC:2017
Occupation
Semi-Retired Electrician
210.19 states that branch circuit conductors shall have an ampacity not less than the load served.
IMO that gives you the permission to size your grounded conductor per the load.

Along with that, exception 2 would apply if the grounded conductor is not on an OCPD.
OP mentioned "125%" for the motor/continuous loads, so the grounded conductor wouldn't have to be considered at 125% which would help in reducing it.

Exception No. 2: Grounded conductors that are not connected
to an overcurrent device shall be permitted to be
sized at 100 percent of the continuous and noncontinuous
load.

I am still on the 2008 where this excerpt is from. I noticed after I posted that the exception is not in the 2011. I can only assume it was taken out as most grounded conductors are not connected to an OCPD anyway.
 
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