How do I calculate voltage drop on a 24V thermostat wire?

Q: Is there an NEC voltage-drop limit for Class 2 circuits?

No. NEC Article 725 governs Class 2 wiring methods but sets no percentage limit on voltage drop. The real constraint is the powered device's operating-voltage tolerance, which is why the Allowable drop is a field you enter (default 10%) rather than a fixed code value.

Q: My low-voltage run drops too much — what do I do?

Size up the conductor or raise the supply voltage. When a run fails your tolerance, the calculator suggests the next larger AWG that would pass, drawing from its small-gauge circular-mil table (22 through 14 AWG). Long thermostat, sensor, and access-control runs on 22 to 18 AWG are the usual culprits.

Question

How do I calculate voltage drop on a 24V thermostat wire?

Accepted Answer

This calculator uses the NEC Chapter 9 Table 8 K-method on a 2-wire loop: Vd = 2 × K × I × L / circular mils, where L is the one-way length. For an 18 AWG copper run it pulls K and the 1,620 circular-mil area, computes the volts dropped, and divides by your supply voltage for percent drop. It then flags whether the result stays within your device tolerance.

How do I calculate voltage drop on a 24V thermostat wire?

Answers