Remote control Class 2 in enclosure with circuit conductors

[UBG]

My customer wants to enable an existing gable exhaust fan to operate simultaneously with the HVAC blower that is in the same building.

He wants simultaneous operation under any condition:

• if the FAU is on because the system is calling for heat or cool or fan only, he wants the gable fan to run.
• if the gable fan is turned on manually, he wants the FAU to run.

CA Title 24 issues aside hmy:, the control circuit to accomplish this is pretty easy, but I want to stay compliant with Art. 725.

725.136(D) addresses the Class 2 control wire of the HVAC being in the same enclosure with the circuit conductors of the gable fan. It seems I either have to:

1. choose an enclosure, and relays, that allow me to maintain a 0.25" separation between the circuit conductors and Class 2 control conductors [725.135(D)(1)] or
2. install the Class 2 control conductors as a Class 1 circuit in accordance with 725.41. [725.135(D)(2)b]

If I take approach 2 above, does this mean

1. the control circuit has to be installed as a Class 1 circuit all the way back to the source transformer, or
2. could I set a separate j-box adjacent to the relay enclosure and splice the Class 2 control cable to a Class 1 method, and then run that into the relay enclosure?

If it has to be Class 1 all the way, do I need OCP for the conductors on the load side of the transformer?
What about the control wiring to the thermostat: would it be allowed to remain in Class 2 cable, or need to be changed?

Thanks in advance, and if I'm missing any other points or approaches, please let me know your thoughts!

 

[UBG]

bump.

<crickets> :?

 

[GoldDigger]

My customer wants to enable an existing gable exhaust fan to operate simultaneously with the HVAC blower that is in the same building.

He wants simultaneous operation under any condition:

• if the FAU is on because the system is calling for heat or cool or fan only, he wants the gable fan to run.
• if the gable fan is turned on manually, he wants the FAU to run.

CA Title 24 issues aside hmy:, the control circuit to accomplish this is pretty easy, but I want to stay compliant with Art. 725.

725.136(D) addresses the Class 2 control wire of the HVAC being in the same enclosure with the circuit conductors of the gable fan. It seems I either have to:

1. choose an enclosure, and relays, that allow me to maintain a 0.25" separation between the circuit conductors and Class 2 control conductors [725.135(D)(1)] or
2. install the Class 2 control conductors as a Class 1 circuit in accordance with 725.41. [725.135(D)(2)b]

If I take approach 2 above, does this mean

1. the control circuit has to be installed as a Class 1 circuit all the way back to the source transformer, or
2. could I set a separate j-box adjacent to the relay enclosure and splice the Class 2 control cable to a Class 1 method, and then run that into the relay enclosure?

If it has to be Class 1 all the way, do I need OCP for the conductors on the load side of the transformer?
What about the control wiring to the thermostat: would it be allowed to remain in Class 2 cable, or need to be changed?

Thanks in advance, and if I'm missing any other points or approaches, please let me know your thoughts!

The wording of the code is pretty clear to me that the Class 1 methods have to be extended to any wires connected in that circuit, including any other wires fed by the same Class 2 transformer. The reason for this is that being in the same raceway with power may result in power being applied to the formerly Class 2 wires and therefore the Class 2 methods you would have been able to use are not appropriate anywhere along the way.
You can only overcome this restriction by putting some sort of power limiting overcurrent protection between the Class 1 and Class 2 parts of the wiring, making them separate (although interconnected) systems.

 

[UBG]

The wording of the code is pretty clear to me that the Class 1 methods have to be extended to any wires connected in that circuit, including any other wires fed by the same Class 2 transformer. The reason for this is that being in the same raceway with power may result in power being applied to the formerly Class 2 wires and therefore the Class 2 methods you would have been able to use are not appropriate anywhere along the way.
You can only overcome this restriction by putting some sort of power limiting overcurrent protection between the Class 1 and Class 2 parts of the wiring, making them separate (although interconnected) systems.

Thanks GoldDigger. That's what I figured the intent was regarding all wires fed from the Class 2 transformer.

I just realized I had a typo when citing one of the code sections in my OP, sorry to all readers for the confusion that must have caused. "725.135(D)(1)" should have read 725.136(D)(1); 725.135 does not exist.

I want to point out that my design, and my question, does not involve Class 1 & 2 conductors sharing the same conduit or raceway, but only sharing the same ENCLOSURE, where the relays will be located. (A relay coil energized by the HVAC system T-stat will turn on the gable exhaust fan, and a second relay coil energized by a timer switch will turn on the exhaust fan and and FAU).

This is where is see the application of 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).....

(D) Associated Systems Within Enclosures. Class 2 and
Class 3 circuit conductors in compartments, enclosures, device
boxes, outlet boxes, or similar fittings shall be permitted
to be installed with electric light, power, Class 1, non?
power-limited fire alarm, and medium-power network powered
broadband communications circuits where they
are introduced solely to connect the equipment connected
to Class 2 and Class 3 circuits, and where (1) or (2) applies:

(1) The electric light, power, Class 1, non?power-limited
fire alarm, and medium-power network-powered broadband
communications circuit conductors are routed to
maintain a minimum of 6 mm (0.25 in.) separation
from the conductors and cables of Class 2 and Class 3
circuits."

So, I would have wires of associated systems, in the same enclosure (but not the same raceway), and it seems to me if I can maintain the minimum 6mm separation btw the Class 1 and Class 2 conductors within that enclosure (including where they terminate on the relays) I will be in compliance...

What do you all think?