Cable Sizing for Instrument and Motor Circuits

9 min read · updated 2026-08-14

A cable must pass three independent checks, and the one that governs varies with the circuit. Instrument circuits reverse the priorities entirely — which is why applying power-cable habits to a 4-20 mA loop produces cable that is simultaneously oversized and inadequate.

The three checks

Every power cable has to satisfy all three of these, and passing two is not passing:

  1. Thermal capacity. Can the cable carry the current without exceeding its insulation temperature limit?
  2. Voltage drop. Does enough voltage arrive at the load for it to work properly?
  3. Protection coordination. Will the protective device disconnect a fault before the cable is damaged?

They fail in different circumstances. Thermal capacity usually governs short runs at high current. Voltage drop governs long runs — and it governs far more often than people expect, which is why a cable sized purely on ampacity tables can leave a motor stalling on start.

Check one: ampacity, and why the table value is never the answer

Published ampacity assumes reference conditions: roughly 30 °C ambient, one circuit, a defined installation geometry. Real installations are hotter and more crowded, so the tabulated figure gets corrected:

I_z = I_tabulated × k_ambient × k_grouping

The two factors compound harshly. A 40 °C ambient factor of 0.87 with a four-circuit grouping factor of 0.65 leaves 57% of the tabulated rating. A 6 mm² cable rated 46 A becomes 26 A — and if someone fitted protection based on the table value, the cable can overheat without the breaker ever noticing. The cable ampacity and derating calculator applies both corrections and flags the coordination rule.

Check two: voltage drop

Voltage drop scales with current, length and conductor resistance. Typical limits are 3% for lighting and 5% for power, though local codes and equipment specifications vary. On long runs this routinely demands a conductor two or three sizes above what thermal capacity alone would require — use the voltage drop calculator and check at the actual run length rather than the straight-line distance.

Motor circuits: size for running, verify for starting

Motor cable sizing starts from full load current, which is not the nameplate kW divided by voltage. Nameplate rating is shaft output, so both efficiency and power factor enter the calculation:

I = P_shaft ÷ (√3 × V × PF × η)

A 15 kW motor at 415 V, PF 0.85, efficiency 0.90 draws 27.3 A — not the 20.9 A that skipping efficiency and power factor would suggest. The motor full load current calculator handles both single and three phase.

Then verify against starting conditions. Direct-on-line starting draws five to seven times full load current for several seconds, and the resulting voltage dip must stay within what the motor needs to actually accelerate its load. Size the cable for running current; confirm it against starting current.

Instrument circuits play by different rules

A 4-20 mA loop carries twenty milliamps. Thermal capacity is never the constraint — a 0.5 mm² pair could carry the current a hundred times over. What actually governs:

  • Loop voltage budget. Every element in the loop drops voltage, and what remains at the transmitter must exceed its minimum at 20 mA. Check with the loop voltage drop calculator.
  • Lead resistance, for RTDs. The instrument cannot distinguish cable ohms from sensor ohms. Two-wire connection over a long run reads several degrees high — quantify it with the RTD lead wire error calculator before choosing between 2, 3 and 4-wire.
  • Noise immunity. Screening, twisted pairs and segregation from power cabling matter more than cross-sectional area. Millivolt-level thermocouple signals are especially exposed.
  • Thermocouple extension cable must match the thermocouple type all the way to the cold junction compensation point. Copper anywhere in that path silently relocates the reference junction.

A working sequence

  1. Establish design current — full load current for motors, actual load for other circuits.
  2. Select a trial size from tabulated ampacity for the installation method.
  3. Apply ambient and grouping derating; confirm derated capacity still exceeds design current.
  4. Check voltage drop at full run length; increase size if it fails.
  5. Confirm protection coordination: design current ≤ device rating ≤ derated capacity.
  6. For motors, verify starting dip is tolerable.

Buried and duct installations add soil thermal resistivity and depth factors beyond this sequence, and harmonic-rich loads need neutral sizing considered separately — both cases call for the full tables in the standard that applies where you are.

Calculators used in this guide: cable ampacity & derating · voltage drop · motor full load current · loop voltage drop · RTD lead wire error