12 Volt  Maximum Wire Length vs. Current
Maximum length of copper wire with 2% voltage drop.
Maximum Wire Length Calculator
The calculator can be used to calculate maximum length of copper wires. Note that
 for a typical electrical circuit with two wires  one back and one forth  this is the length of the two wires together. The maximum distance between the source and the equipment is half the calculated distance
 in a car where equipment can be grounded to the chassis  the body of the car acts as the negative wire. The electrical resistance in the chassis can normally be neglected and the maximum distance equals the calculated distance
Maximum lengths for copper conductors from power source to loads in 12 volt systems with 2% voltage drop are indicated below:
Wire Length  feet
Download and print 12 Volt Electric Circuit Chart
Wire Length  meter
Download and print 12 Volt Electric Circuit Chart
 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)
where
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 total wire back and forth should not exceed approximately 8 m for gauge #10 (5.26 mm^{2}) . By increasing the size of the wire to gauge #2 (33.6 mm^{2}) 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 mm^{2} 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 total length of the wire back and forth can be calculated with Ohm's law
U = R L I (2)
where
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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