What’s the ampacity for 18 AWG wire, at 24 V AC

Question

I’m wiring up some door bells for my company. Have at 120v to 24vAC 50VA transformer. The length of wire is somewhere around 200ft at it longest single length. I’m just wondering what’s the max amp draw that can be out through this wire before it is a safety/fire hazard. I’m not concerned about voltage drop as it’s AC current and they are doorbells so not concerned if they have reduced sound. Just wondering how many amps can be tied on in parallel with this gauge wire.

Answer 1

What’s the ampacity for 18 awg wire, at 24v AC

Safe ampacity does not depend on voltage. However, it does depend on ambient temperature, insulation temperature rating, wire material, and other wiring sharing the same conduit or bundled with the wire, none of which you've told us.

However...

120v to 24vAC 50VA transformer

Your transformer is limited to 2A. In no situation will 18AWG ever be a fire hazard with only 2A on it.

Answer 2

To answer the question you asked:

Assuming you are under NEC, 310.15(B)(16) only lists 90°C for 18AWG insulation (presumably because they only expect to see it inside fixtures) at 14A for not more than 3 current carrying conductors. If your insulation is not rated for 90°C that does not apply.

https://www.engineeringtoolbox.com/wire-gauges-d_419.html suggests 7A for PVC-insulated wiring.

The first example of 18/2 doorbell wire I found that admitted to a temperature rating is 60°C PVC, so 7A is probably a safer bet.

Your 2.08A transformer would, if fully loaded, drop to less than 19V at 200 feet on 18AWG wiring. At 0.94A the drop goes to 10%, at 0.47A the drop goes to 5%. How sensitive your particular doorbells are to voltage may vary - certain types of chime may fail to (audibly) operate at all, rather than operate at reduced volume, if the voltage is too low.

The chart here suggests that for 200 ft on 18AWG, a 24VAC load should be 20W (0.83A) or less.

If you happen to be using CCA doorbell wire, the numbers get worse (copper clad aluminum - cheap, higher resistance.) The above figures are for copper wire.