New 28 V DC feeder: voltage drop versus breaker size
You are installing a 28 V DC feeder to a new 20 A continuous-load unit in the aft equipment bay. The route measures 11 m one-way, so the conductor loop is 22 m. The cable drawn from stores is 8 AWG (8.37 mm²), conductor resistance 2.06 mΩ/m at 20 °C. It runs in a bundle in an unpressurised area where the maximum conductor temperature in service is 70 °C, which gives a resistance multiplier of 1.20. The maintenance manual requires a total feeder voltage drop of not more than 1.0 V, and a circuit breaker rated at least 125 % of the continuous load but not more than the cable's rating in this bundle (8 AWG: 45 A). A 25 A breaker is proposed. The technician estimates 'about 0.9 V' and considers the job done.
1. Calculate the voltage drop at maximum conductor temperature for the 8 AWG cable and state whether the installation meets the manual.
Read the worked answer
Loop resistance at 20 °C: 22 m × 2.06 mΩ/m = 45.3 mΩ
At 70 °C: 45.3 mΩ × 1.20 = 54.4 mΩ
Voltage drop: 20 A × 0.0544 Ω = 1.09 V
That is 3.9 % of 28 V and it exceeds the 1.0 V limit - the installation does not meet the manual.
The technician's 0.9 V is the 20 °C figure. Copper resistance rises with temperature, and a feeder in an unpressurised bundle will run hot; the hot case is the one that must be shown to comply, because it is the worst case the equipment will ever see. The margin is only 9 %, but 9 % over the limit is a non-conformance, not a rounding argument.
2. Select the next standard cable size and verify it, then check the breaker coordination.
Read the worked answer
Next size up: 6 AWG (13.3 mm²), 1.30 mΩ/m at 20 °C.
- Loop resistance at 20 °C: 22 × 1.30 = 28.6 mΩ
- At 70 °C: 28.6 × 1.20 = 34.3 mΩ
- Voltage drop: 20 × 0.0343 = 0.686 V (2.4 % of 28 V) - comfortably inside the 1.0 V limit.
Breaker check:
- 125 % of 20 A = 25 A, so a 25 A breaker is the minimum acceptable rating.
- 6 AWG in this bundle is rated above the 8 AWG figure of 45 A, so 25 A protects the cable with margin.
- The 25 A breaker must also be acceptable to the equipment manufacturer's approval basis (some equipment specifies a maximum protective device); verify before fitting.
- Note the breaker protects the wire; the voltage-drop requirement is a separate check on conductor size, and both must pass independently.
3. The technician suggests fitting a 30 A breaker 'to give some margin'. Is that acceptable? Explain.
Read the worked answer
No. A larger breaker does nothing for voltage drop - the drop is set by current and conductor resistance, and both are unchanged. It buys no margin at all; it removes margin.
Specifically:
- 30 A is 150 % of the continuous load, outside the 125 % design basis, so the circuit is over-protected relative to its design intent and the equipment's approval basis.
- The breaker must protect the smallest conductor in the circuit. If any downstream wire or equipment internal wiring is smaller than the feeder, a 30 A device may allow a sustained overload that the small wire cannot survive - that is the classic EWIS fire scenario.
- The correct fix is to attack the cause: increase conductor size (6 AWG), shorten the route, or reduce the load. Where a component is replaced, the change must be recorded against the wiring diagram and the load analysis updated.
- Any change of protective device rating also requires approved data and an update to the circuit schedule, not an ad-hoc substitution.