Protecting the Low Voltage Wires to the AC on a Central Air Conditioner

Protecting the Low Voltage Wires to the AC on a Central Air Conditioner

That brown-sheathed, low voltage wire from the air handler to the AC unit outside tells the contactor when to engage and disengage the AC unit itself.
This allows the high voltage to pass from one side of the contactor to the other, flowing on to the compressor and condenser fan motor. Without this low 24-volt communication, the AC won’t start.
So, shouldn’t we protect that wire as much as possible from potential damage and UV rays?
Isn’t it in the electrical code that we have to use some sort of propective conduit with wiring outside the house?
Now, we’ve never heard of any low voltage wire that we use in residential heating and air conditioning that is rated for outdoors, including wet or damp conditions.
So why when we service equipment and go on HVAC inspections around the Red Deer area, do we find dried up, brittle sections of thermostat wire that were simply taped to the suction line from the wall to the AC?

We spent hours researching this online and we were having the hardest time finding it in the National or Canada’s Electrical Code citing when we need to protect the low voltage wire in outdoor conditions such as the installation of an air conditioner.
Article 725 of the National Electric Code talks about this type of control wiring. And we can’t find one part in there that says “Class 2 wire (like the 24 volt thermostat wire we use in residential HVAC) must be protected by or enclosed in conduit.”
On one side of this conversation, the stat wire is not rated for outdoor use, let alone wet or damp conditions which we feel leaves it exposed to damaging elements like landscapers who use weed eaters, a dog’s incessant need to chew up things in the yard, the ultraviolet rays coming from the sun, and the arid nature of summer and winter which dries up just about anything that stays outside long enough.
On the other side, installing stat wire inside a liquid-tight conduit technically doesn’t make it a dry environment either. And according to what we’ve found (and not found) in our research, a dry environment isn’t even needed for class 2 wiring anyways.
But, ever since our first HVAC installation, we were required by our foremen to protect the stat wire with ½ seal-tight conduit, so we’ve always taught our techs to do the same.
It undeniably protects the wire better than just strapping it to the suction line without seal-tight and leaving it exposed to the elements. It’s also in the best interest of the customer to ensure the stat wire lasts as long as the AC itself.
If the stat wire dries up and becomes brittle, it takes almost nothing, like a bump by the lawnmower to expose the bare wire within the sheathing and have the wrong wires touch each other.

This shorts out the low voltage system, rendering it inoperable.

This requires the homeowner to call a service technician to come out to troubleshoot and fix the issue. But it’s not in the Electrical code books. So, when we see a newly built residential neighborhood with exposed stat wire at the AC, we cringe, but we have to remind ourselves it’s not required.
If it’s not required, why do so many inspectors write up correction letters to us for doing retrofit change outs, and not protecting the stat wire with some sort of conduit?
The answer may be, “that’s the way they want it.” Remember, local jurisdictions can tighten the rules as they deem necessary. And the tightest provision of any code is the one that gets enforced.
If you wanted to push the issue, you could ask the code inspector (nicely) where you could find the source of their local rules. One that lists their requirements which are more restrictive than the National Electric Code.
We get it, there are several sections in the code book that say wiring must be protected from potential damage, but never mentioning it specifically when it comes to Class 2 control wiring.
With that said, let’s take a look at what it would take to upgrade your customer’s low voltage wiring to a more protected state. It doesn’t take much work to do, and the cost of the parts is minimal compared to the protection you’re giving the stat wire for the future.
Taking the old dried up stat wire off the existing suction line insulation and cutting it back to about six inches from the wall, will allow you to splice on new wiring to run through the conduit.

Once it’s been wire nutted and taped for protection, leave a little bit of the colored wires there so if a future technician comes and has to troubleshoot, they can search back to your splice and easily see the wires that are connected giving them the possibility of using that third wire as an alternate if needed.


Shove the new wire ties into the penetration of the wall where it comes out and slip the new wire through the conduit. Fasten the conduit to the unit and strap it to the rest of the line set and high voltage conduit going to the AC.





This makes for neat and clean workmanship of your repair, which is required by the National Electrical Code.

So, the next time you see exposed thermostat wire coming from the wall to the AC, think about your customer and what’s right for them. If you’re a homeowner, this job shouldn’t be too expensive to have done to your system by your local HVAC company.
And, as always, whether dealing with high or low voltage electricity, there are inherent dangers and mechanical failures that can happen when dealing with them. So, let’s leave it to the professionals.
With that said – feel free to call us at Red Deer Heating and AC and we’ll make sure your AC is in top notch condition.