Separating high and low voltage in the same box

crtemp

Senior Member
Location
Wa state
I started working for a new builder and they are claiming that the building inspector is requiring that they make the gas furnace be able to turn on the exhaust fan in the utility room a couple times a day for whole house ventilation. This is fine with me but the HVAC crew is running a 18-2 conductor to a junction box directly under the switch that controls the exhaust fan. They are claiming that as long as I junction the 18-2 into a 14-2 NM-B before it goes into my switch box then I don't have to separate the high and low voltage in the box. I have never heard of this. Can anyone confirm that this is true?
 

crtemp

Senior Member
Location
Wa state
What I would use is an Arlington combo box. http://www.aifittings.com/catalog/specialty-boxes/power-and-low-voltage-box-for-new-construction/
We have used these extensively where you have a Low Voltage fire place ignitor & wish to have the switch next to the light switch. Or as you stated make the splice from 18-2 thermostat to 14-2 Romex ahead of the final switch box.
I can't use this as the high voltage and low voltage connect to the same switch. Basically what I'm asking is can a 24 volt wire and 120 volt wire be in the same device box if they are both 14 gauge wire.
 

John120/240

Senior Member
Location
Olathe, Kansas
I can't use this as the high voltage and low voltage connect to the same switch. Basically what I'm asking is can a 24 volt wire and 120 volt wire be in the same device box if they are both 14 gauge wire.
If you send 120 V to that 24 V fan your going to let all of the Magic Smoke out. The second sentence Yes.
 

crtemp

Senior Member
Location
Wa state
If you send 120 V to that 24 V fan your going to let all of the Magic Smoke out. The second sentence Yes.
I'm not sending 120 volts to anything that is 24 volts. From what I understand from the Hvac guys it is a switch that gets a signal from the furnace and then tells the exhaust fan to turn on.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
There is nothing in the codebook that gives me a bigger headache than Art. 725 so I'm not going to try and look anything up, but if this set up is allowed then splicing 18 gauge wire to 14 gauge romex is a terrible thing to do. It would be better to run the 18 gauge right into the box.
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor
I think your answer is Yes the HVAC people are correct, if you need to do some 24V and 120V switching in the same box and obviously the circuits are not connected.
the 24V 18/2 is a class 2 circuit. You need 18/2 rated 600V or splice to a wire rated 600V like 14/2.

See the NEC:
nec 2014 said:
300.3 (C) Conductors of Different Systems.
(1) 1000 Volts, Nominal, or Less. Conductors of ac and dc
circuits, rated 1000 volts, nominal, or less, shall be permitted to
occupy the same equipment wiring enclosure, cable, or raceway.
All conductors shall have an insulation rating equal to at least
the maximum circuit voltage applied to any conductor within
the enclosure, cable, or raceway.
See the 18/2 wire is only rated for 150V:
http://www.southwire.com/products/ThermostatWire.htm
14/2 is rated for 600V so they cant occupy the same box.
 

crtemp

Senior Member
Location
Wa state
I think your answer is Yes the HVAC people are correct, if you need to do some 24V and 120V switching in the same box and obviously the circuits are not connected.
the 24V 18/2 is a class 2 circuit. You need 18/2 rated 600V or splice to a wire rated 600V like 14/2.

See the NEC:


See the 18/2 wire is only rated for 150V:
http://www.southwire.com/products/ThermostatWire.htm
14/2 is rated for 600V so they cant occupy the same box.
Perfect. The exact explanation I was looking for. Thank you
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
The circuit in question is only 120 volts. Because the stat wire is rated 150 volts and the 14-2 nmb is rated 600 volts can they both occupy the same box?
They can be in the same raceway, they can be in the same equipment, a 24V relay switching a 120V load for instance, I'm not sure about about your situation though. What is this switch you are wiring up?
 

crtemp

Senior Member
Location
Wa state
They can be in the same raceway, they can be in the same equipment, a 24V relay switching a 120V load for instance, I'm not sure about about your situation though. What is this switch you are wiring up?
They have yet to give me the make and model of the switch. I'll have it tomorrow though
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor
I'm not seeing where 600V is being applied anywhere.
Good point since the 24V system is class 2 we are directed to NEC 725.136
(A) General. Cables and conductors of Class 2 and Class 3
circuits shall not be placed in any cable, cable tray, compartment,
enclosure, manhole, outlet box, device box, raceway, or similar
fitting with conductors of electric light, power, Class 1, non–
power-limited fire alarm circuits, and medium-power network-
powered broadband communications circuits unless permitted
by 725.136(B) through (I).
Perhaps 725.136(D) the 1/4 in separation. The others look bleak
 

iwire

Moderator
Staff member
Location
Massachusetts
Article 725 prohibits this unless you run the entire circuit as class 1 and reclassify the circuit from class 2 to class 1.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
I think your answer is Yes the HVAC people are correct, if you need to do some 24V and 120V switching in the same box and obviously the circuits are not connected.
the 24V 18/2 is a class 2 circuit. You need 18/2 rated 600V or splice to a wire rated 600V like 14/2.

See the NEC:
nec 2014 said:
300.3 (C) Conductors of Different Systems.
(1) 1000 Volts, Nominal, or Less. Conductors of ac and dc
circuits, rated 1000 volts, nominal, or less, shall be permitted to
occupy the same equipment wiring enclosure, cable, or raceway.
All conductors shall have an insulation rating equal to at least
the maximum circuit voltage applied to any conductor within
the enclosure, cable, or raceway.
See the 18/2 wire is only rated for 150V:
http://www.southwire.com/products/ThermostatWire.htm
14/2 is rated for 600V so they cant occupy the same box.
In general the rules in Article 725 over ride the one in 300.3. Class 2 wiring cannot be installed with wiring of higher voltage systems no matter what insulation you use on the Class 2 wiring. There is even an Informational Note following 300.3(C)(1) sending you to 725.136.
That being said, 725.136(D) would permit your Class 2 wiring to enter the enclosure to connect to the switch. It does require a 1/4" separation between the power wiring and the Class 2 wiring.
 

GrayHair

Senior Member
Location
Nashville, TN
14 gauge CL2 and CL3 cable is usually 300V rated (marking not required) but AFIK, you still can't use it as part of a non-power-limited circuit. I did the ¼" separation many times and don't remember ever having been called on it.



If the fan is powered by power-limited 24V from the furnace, you might not need be involved at all.
 

kwired

Electron manager
Location
NE Nebraska
I started working for a new builder and they are claiming that the building inspector is requiring that they make the gas furnace be able to turn on the exhaust fan in the utility room a couple times a day for whole house ventilation. This is fine with me but the HVAC crew is running a 18-2 conductor to a junction box directly under the switch that controls the exhaust fan. They are claiming that as long as I junction the 18-2 into a 14-2 NM-B before it goes into my switch box then I don't have to separate the high and low voltage in the box. I have never heard of this. Can anyone confirm that this is true?
The furnace 24 volt control circuit is likely a class 2 circuit. Doesn't matter what kind of conductor you use it is still a class 2 circuit and rules for class 2 wiring apply to it.

It could be reclassified, but that means all components of they 24 volt system need reclassified, you can't just pick and choose isolated sections to reclassify. You can use relays to isolate class 2 from class 1 or from power and lighting circuits - which is probably happening somewhere in this scheme anyway.

Why not just use a timer controller to cycle the fan a few times a day? Mild days when furnace doesn't run will never cycle the whole house ventilation anyway.
 

tom baker

First Chief Moderator
Staff member
I can't use this as the high voltage and low voltage connect to the same switch. Basically what I'm asking is can a 24 volt wire and 120 volt wire be in the same device box if they are both 14 gauge wire.
No, even though 300.3(C)1 would seem to allow it, and most electricians will state if the low voltage has 600 volt insulation, then its OK.
Take a look at the FPN to 300.3(c)1
 

GrayHair

Senior Member
Location
Nashville, TN
I see it as no different than having a 120V bell power and a class 2 power limited fire alarm initiating circuit in the same waterflow switch on a sprinkler system [725.136(D)1], with 1 possible problem. The waterflow is made DP by adding a 2nd microswitch; the common body in a DP light switch could be an issue for the inspector.
 
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