undersized neutral for hvac equip

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tyha

Senior Member
Location
central nc
Is it permissable to use a #10 neutral for a air handling unit on a 35 amp 3p breaker. the 3 phase wires will be # 8. min cir amp for unit is 27.3 max ocpd is 40.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Yes, if the unit even needs a neutral at all.

Beyond that, have your electrician post questions here; DIY's are specifically not allowed here. Sorry.
 

tyha

Senior Member
Location
central nc
The neutral is required due to control voltage for the thermostat. or do they not allow you adjust the temp where you are from. As far as your DIY I not only hold an unlimited contracting license in nc I work for myself. I only asked the question because of the #10 30 amp breaker requirement and to see if anyone knew of any other requirement that I was overlooking. since I can see that you aren't really sure what youre talking about then I will seek advice elsewhere.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
tyha said:
The neutral is required due to control voltage for the thermostat.
Obviously, if there is a neutral terminal, a neutral connection is required. However, most line-to-line equipment is equipped with a transformer with a suitable primary voltage.

You seemed to be unaware that it's permissable to use #10 wire for the entire circuit, even with a 40-amp breaker, as long as its above the minimum-circuit-ampacity requirement.

Nobody was trying to give you a smart-assed answer. Be patient and stick around. Some of us do know what we're talking about. You may not know everything, either.
 

Smart $

Esteemed Member
Location
Ohio
LarryFine said:
tyha said:
The neutral is required due to control voltage for the thermostat.
Obviously, if there is a neutral terminal, a neutral connection is required. However, most line-to-line equipment is equipped with a transformer with a suitable primary voltage.
FWIW - Many air handlers have an internal service receptacle--which requires a neutral.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Smart $ said:
FWIW - Many air handlers have an internal service receptacle--which requires a neutral.
Interesting; never seen one, but you'd be correct, as long as it's ahead of the unit disconnect.
 

Smart $

Esteemed Member
Location
Ohio
LarryFine said:
Interesting; never seen one, but you'd be correct, as long as it's ahead of the unit disconnect.
The one's I've seen have been and of those, the equipment had an integral disconnect in the form of a 3P breaker with one phase of line side tapped for receptacle via OCPD.
 

tyha

Senior Member
Location
central nc
Larry,

I do apologize for my response. I used to use this site years ago when I was up and comming and trying to learn everything possible. but know that I have my own company I am so absorbed with pricing work and dealing with customer issues and making sure everything and everyone on the job is running efficiently I lose site of the technical aspect of the work. But I greatly apreciate your input. The reason I was asking is because in the drawing it shows 4 #8 's and a #10 grnd and when I looked at the unit I found the other info. Where can I site that I can use #10 for the entire circuit. Because even on the panel schedule it shows to be a 35 amp not a 40 amp.
 
If you are tapping on the line side of the disco, that is permitted. There are limitations though, due to the size of the overcurrent device supplying that circuit. If the circuit is rated 30 amps or larger, than that will preclude tapping that circuit for the receptacle that supplies a 15 or 20 ampere rated receptacle - 210.21(B)(3).
I myself have not seen this receptacle that is integral with the A/C equipment tapped off the line side of the disconnect, but I have seen them where a circuit is installed to them, or there is a small transformer in the enclosure to supply them (usually 277/480v supplies).
 

Smart $

Esteemed Member
Location
Ohio
Pierre C Belarge said:
If you are tapping on the line side of the disco, that is permitted. There are limitations though, due to the size of the overcurrent device supplying that circuit. If the circuit is rated 30 amps or larger, than that will preclude tapping that circuit for the receptacle that supplies a 15 or 20 ampere rated receptacle - 210.21(B)(3).
The tap had it's own OCPD ahead of the receptacle.

210.21(B)(3)? I don't think that applies here since it refers to a branch circuit supplying two or more receptacles... and there is only one involved here.

Pierre C Belarge said:
I myself have not seen this receptacle that is integral with the A/C equipment tapped off the line side of the disconnect, but I have seen them where a circuit is installed to them, or there is a small transformer in the enclosure to supply them (usually 277/480v supplies).
I've seen those types, too... in addition to ones that don't have any! :D
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
tyha said:
Larry, I do apologize for my response.
Not necessary, but thanx.

Where can I site that I can use #10 for the entire circuit.
I don't have the exact code sections in front of me, but it's based on the fact that almost all HVAC compresors have internal current-limiting/overload protection.

You must meet minimum circuit ampacity (27.3a in this case) with the conductors (#10) and maximum OCP (40a) with the breaker/fuse (if HACR-rated).
 
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