Guide
24V DC Budget Guide for PLC Panels
Budget, headroom, branch, drop.
List every 24V load by simultaneous-on watts, add headroom of at least 25 percent, then step up to a standard supply size. Split loads into fused branches and check the longest run for drop.
Formulas
Sized supply
\[P_{sized} = P_{load} \times (1 + headroom)\]
/headroom is a fraction; step the result up to the next standard supply.
DC drop
\[V_d = \frac{2 K I L}{cmil}\]
L is one-way feet; keep the load end above dropout voltage.
Worked example
Budgeting a small panel
Inputs: PLC 40W, HMI 25W, sensors 30W, valves 48W, 25 percent headroom.
Calculation: Load is 143W; sized is 143 x 1.25 = 178.75W, so a 240W supply at about 5.96A on the 24V bus.
Lesson:
Frequently asked questions
How much headroom is enough?
25 percent covers inrush and small growth on most PLC panels. Valve-heavy or wide-temperature panels often need more.
Why do 24V devices reset on long runs?
Drop on the common conductors lowers the far end below dropout during inrush. Shorten the run, upsize copper, or move the supply closer.
Sources and further verification
- NFPA 79, Electrical Standard for Industrial Machinery National Fire Protection Association
- UL 508A, Industrial Control Panels UL Standards
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