electrical academy · lesson 3 · about 15 minutes
How long can a cable be?
A longer cable resists fault current more. If the fault current gets too small, the protection may not trip quickly. This lesson shows the idea with a worked example.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.
The idea
- In a short circuit between a live conductor and neutral or earth, the current flows out along the cable and back again. This path is called the fault loop.
- The longer the cable and the thinner its conductor, the more resistance in the loop and the less current flows in a fault.
- Protective devices trip fast only above a certain current. If the cable is too long, the fault current can fall below that level and the device trips slowly or not at all.
A worked example
- Take a copper cable of 4 mm² and 100 m, with a 230 V supply to earth and a 32 A circuit.
- Loop resistance is about 2 × 0.0225 × 100 ÷ 4 = 1.1 ohms. Fault current is about 230 ÷ 1.1 = 200 A.
- A breaker of this size may need roughly 160 to 320 A to trip instantly. At 200 A it may trip slowly, so the cable is near the limit.
- This is an illustration with rounded numbers. Real designs use the supply source resistance, warmer cable resistance, and the maker's tables.
What sets the longest safe length
- The cable cross-section: thicker conductors allow longer runs.
- The protective device: its type and its rating.
- The supply voltage, and the room temperature.
- The number of cores and how the cable is laid.
A second check: voltage drop
Even when protection works, a long cable drops voltage under load. Motors and electronics need near-rated voltage, so check voltage drop separately from the fault-current check.
If either check fails, use a thicker cable, a shorter route, or a device with a lower trip level.
check yourself
Three quick questions
Think of your answer first, then open the question.
Why does a longer cable reduce fault current?
More loop resistance.
Name two things that decide the longest safe cable length.
For example conductor cross-section and the protective device.
Is the fault-current check the same as the voltage-drop check?
No. Do both.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.