How is DC cable sizing and voltage drop calculated?
Easy PV returns the smallest cable cross-sectional area that achieves a voltage drop less than 1% of the voltage across the cable (or the largest cable if none do).
The voltage drop along a cable is the product of the current running through it and its resistance. We calculate cable resistances using the stranding class 1/2 values from the IEC 60228/BS 6360 stranding chart.
Example calculation
In general, we want the cable resistance such that:
I * R < 1% change
In this example, we have current at 12 A. Voltage at 157V so 1% is 1.57V So we have:
12A * R < 1.57V
R < 1.57 / 12 = 0.1308
For the cable sizes in Easy PV (4, 6, 10 mm2), we have the following:
| Resistance at 20 | Resistance at 40 | Ohms per m (rather than km) |
| 4.61 | 4.972346 | 0.004972346 |
| 3.08 | 3.322088 | 0.003322088 |
| 1.83 | 1.973838 | 0.001973838 |