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12 Volt Electric Wire - Length and Current

Maximum length of copper wire with 2% voltage drop

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Maximum Wire Length Calculator 

The calculator can be used to calculate maximum length of copper wires.

Voltage (volts)

Current (amps)

Cross sectional area (mm2) - AWG Wire Gauge vs. mm2

Voltage Drop (%)

Maximum one-way lengths for copper conductors from power source to loads in 12 volt systems with 2% voltage drop are indicated below:

Wire Length - feet

12 volt copper wire maximum length

Wire Length - meter

12 volt copper wire maximum length

  • double the distance if 4% loss is acceptable
  • multiply distance by 2 for 24 volts
  • multiply distance by 4 for 48 volts

Example - Maximum Length of Wire

The current to a light bulb with power 50 W can be calculated with Ohm's law

I = P / U                  (1)


I = current (amps)

P = power (watts)

U = voltage (volts)

(1) with values

  I = (50 W) / (12 V)

    = 4.2 A

From the diagram above the maximum length of the wire should not exceed approximately 8 m for gauge #10 (5.26 mm2). By increasing the size of the wire to gauge #2 (33.6 mm2) the maximum length is limited to approximately 32 m.

Example - Calculate Maximum Wire Length

The electrical resistance in a copper conductor with cross sectional area 6 mm2 is 2.9 10-3 ohm/m. This is close to wire gauge 9. 

In a 12V system with maximum 2% voltage drop - and current 10 amps - the maximum length of the wire can be calculated with Ohm's law

U = R L I                (2)


R = electrical resistance (ohm/m)

L = length of wire (m)

(2) rearranged for L

L = U / (R I)                        (2b)

(2b) with values

L = (12 V) 0.02 / [(2.9 10-3 ohm/m) (10 amps)]

   =  8.3 m   

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