Control wire is sized by more than steady current. Breaker coils need sufficient terminal voltage during short pulses, motors create inrush, long runs add resistance and protective devices must clear faults without damaging conductors.
Learning objectives
Calculate worst-case DC drop, select conductor cross-section and verify terminal, fuse and thermal constraints.
Core engineering principles
Coil operation uses terminal voltage
Battery minimum, cable resistance, contacts and protective devices all reduce voltage. The calculation should use hot-conductor resistance and actual pulse current.
Two-wire loop length governs drop
Outgoing and return conductors both contribute unless a documented common return architecture is used.
Intermittent and continuous loads differ
A trip coil may tolerate thermal duty for milliseconds, while relay supply and heaters require continuous ampacity.
Fault protection must match conductor capacity
MCB or fuse characteristics, DC breaking rating and prospective auxiliary fault current determine protection.
Terminals can limit maximum size
Large conductors may not fit relay or terminal clamps. Intermediate terminals or parallel conductors require approved connection methods.
Engineering application method
- Step 1: Define source minimum/maximum and load current profile.
- Step 2: Calculate loop resistance at service temperature.
- Step 3: Compute terminal voltage for worst-case operation.
- Step 4: Check ampacity, short-circuit protection and device terminal range.
- Step 5: Verify by measuring terminal voltage during FAT.
Practical example
A 220 VDC trip circuit can fail even when the battery reads 198 V if a long cable and aged contacts drop another 35 V under coil current. Measuring only at the battery misses the problem.
Common mistakes
- Using one-way cable length.
- Calculating with cold copper only.
- Sizing from steady coil watts.
- Ignoring contact and fuse resistance.
- Forcing oversized wires into small terminals.
Design and review checklist
- Is worst battery minimum used?
- Does the loop include return path?
- Are peak and continuous duties separated?
- Is protection DC-rated?
- Is terminal voltage measured under load?
Standards basis and official sources
- IEC 60445:2021 + AMD1:2026 — Identification of equipment terminals, conductor terminations and conductors.
- IEC 61439-1:2020 — General rules for low-voltage switchgear and controlgear assemblies where applicable.
- IEC 62271 series — High-voltage switchgear and controlgear framework.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.