A Florida AC-only storage building is equally divided sections each with:

  • a single hallway to provide access to storage units
  • 3 doors (25 feet apart) provide building access into one of the hallways
  • each section is designed the same as the others
  • an AC handler is mounted in the hallway ceiling near the door and a thermostat

The design includes an electronic controller (one single-board computer and electronic relays) to signal for:

  1. cooling (Yellow wire)
  2. fan (Green wire).

Decision point: The relays can be placed either:

  • proximal to the computer: requiring thermostat wiring to be run to the computer
  • proximal to the each air handler: requiring wiring to be run between air handler and the computer

What are the questions & considerations that must be thought-out before making a decision as to where to locate the relays?

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  • I don't know what "rPI -relay wiring" is. What voltage is it? Jan 26, 2020 at 22:24
  • @Harper-ReinstateMonica Wiring between the rPi computer and the relay is low (5V) voltage. Thermostat wiring is 24V. Question amended to remove reference to rPi (Raspberry Pi).
    – gatorback
    Jan 27, 2020 at 2:56
  • Is the computer being powered from a power supply on a separate 120V feed, or from power generated from the HVAC 24VAC system? Jan 27, 2020 at 3:49
  • @ThreePhaseEel: A 5V-DC Raspberry Pi computer can be powered by either the 24V-AC or a separate 120-240V feed: that decision has yet to be made. Each of the 3 Air-Handlers have a 24V line for a thermostat. I suspect that each handler runs off 240V, if not 120V.
    – gatorback
    Jan 27, 2020 at 6:22

1 Answer 1


Here's what you don't want to do.

Try to haul 5 volts any appreciable distance.

So I would lean seriously toward having the 24VAC wiring do the long haul.

However, if you want the gory details, you need to examine each option: determine the current that will flow on the long haul, and grind that through a voltage drop calculator given the expected wire size and distance.

The root of the problem is that a 0.5-volt drop only reduces 24V only 2%, but reduces 5 volts 10%. Much worse, to do the same practical work, 5V needs five times the current, so now you're looking at 2% versus 50%. In other words, voltage drop's practical burden is a function of voltage difference squared, so it is 25 times worse for 5V than 24V.

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