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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.

Math Keeps Me Busy
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Dalton
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    "I’m not concerned about [v]olt drop as it’s ac current" That's... not how Ohm's law works. V=IR, whether the power is AC or DC. – nobody Oct 31 '23 at 12:59
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    @nobody that's not actually true. well, of course ohm's law is true, but AC has to deal with skin effect, inductance, power factor, etc. voltage drops, as a general rule, are not the same for AC and DC. (but of course all of that is entirely irrelevant for the purpose of this question) – ickybus Oct 31 '23 at 13:02
  • At 200 feet you will have quite a voltage drop, likely more than you want. 12 gauge would keep it to perhaps 10% voltage drop. – Jon Custer Oct 31 '23 at 13:57
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    Skin effect at 50-60 hz is trivial to the point of being completely a non-issue, unless your conductor is the size of a bridge cable...there is no practical difference for resistance from DC-60hz. – Ecnerwal Oct 31 '23 at 16:04
  • expect 6.385 ohms per 1000 feet. So at 400 feet (gnd+v), expect 2.55 ohms. From a 24v supply, that limits current to 9A, and a voltage drop of ~2.5v per amp of load. – dandavis Oct 31 '23 at 18:01
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    @ickybus - At 60 Hz he skin depth in copper is 8-1/2 mm, or 1/3 inch. Do you really think you're going to see any skin effects in an 18 gauge wire? – SteveSh Oct 31 '23 at 18:20
  • Let's be real here: if someone is ringing your bell long enough to overheat your wires, even drastically under-sized wires, you have other problems. Wire charts are typically for always-on 24/7 current maximums, you can safely drastically exceed those for intermittent short-term loads as there's thermal mass to consider and the radiative cooling of a twisted pair of bell wires is relatively quite high (compared to buried or conduit power lines). Add a slow-blow fuse rated at design current if you worry about a stuck button. – dandavis Oct 31 '23 at 18:57
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    @nobody It's not V=IR in an AC system, it's V=IZ where Z is complex and equal to R+jX. You need another factor, the reactance, and that propagates to all the equations. Even in a purely resistive system, we've actually introduced a factor of sqrt(2) to voltage and current to get the numbers to work right, those have been hidden in the measurements. If you have nonlinear loads and harmonics, then V=IR only applies in an instantaneous sense. – user71659 Oct 31 '23 at 21:09
  • Rethink your layout, the transformer could be mounted in a NEMA box in a closet near the chime. Powered from the closet light. – user176647 Nov 01 '23 at 01:58
  • <2nd-order and beyond> Everyone will have to recalculate if the doorbell button(s) incorporate lights and thus represent an (almost) 100% duty cycle. Then the effect of changes in the solar wind and local magnetic fields ... . 2nd-order and beyond> – HABO Nov 02 '23 at 01:56

2 Answers2

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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.

ickybus
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  • With these gauge wires would I be good if I switched to a 24v 100VA transformer. – Dalton Nov 01 '23 at 14:17
  • @dalton I would be concerned about voltage drop at 200' and 4A, which the other answer touched on. I have no idea what doorbell tolerances are in terms of voltage. – ickybus Nov 01 '23 at 17:52
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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.

Ecnerwal
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  • do your table's (conservative imho) amp ratings consider the sub-1% duty cycle, or is it focused on constant power? – dandavis Oct 31 '23 at 19:03
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    @dandavis: You don't have a safety critical rating on the thing limiting the duty cycle, so you have to assume 100%. It would be bad if a short circuit in the doorbell button burned down your house, because you did your design calculations for a low duty cycle. – Ben Voigt Nov 01 '23 at 16:19