Every current-carrying cable has an ohmic resistance and dissipates part of the energy as heat. Over a year, this power loss costs real money. With the Calculate cable power loss you can find out in seconds how many watts are lost in your cable and what that means energetically.
The formula for power loss
The power dissipated in the cable follows Ohm's law: PL = I² · R. The resistance depends on length, cross-section and material. Since current always flows through both the outgoing and return conductor, twice the length counts:
- R = 2 · L · ρ / A (L = single cable length)
- Resistivity ρ: copper 1/56, aluminium 1/35 Ω·mm²/m
- A = conductor cross-section in mm²
Copper conducts much better than aluminium, so for the same cross-section it produces less loss.
Worked example: 16 A over 20 m
Take a typical cable with I = 16 A, a single length of L = 20 m and a cross-section of A = 2.5 mm² copper:
- R = 2 · 20 · (1/56) / 2.5 ≈ 0.286 Ω
- PL = 16² · 0.286 ≈ 73 W power loss
These 73 W are converted into heat in the cable. Relative to the apparent power U · I = 230 V · 16 A, that is about 2 % loss. Over the operating hours per year this results in an energy loss in kWh and, multiplied by the electricity price, a concrete cost. It is best to calculate the exact annual cost directly in the calculator, as it depends heavily on your usage.
Reducing losses
Power loss can be significantly reduced with simple measures:
- Larger cross-section: resistance is inversely proportional to the area. Going from 2.5 to 4 mm² noticeably lowers the loss.
- Shorter routes: resistance rises linearly with length. Short, direct cable routes save energy.
- Copper instead of aluminium: copper has the lower resistivity and therefore less loss.
Power loss is not the voltage drop
An important distinction: the calculator gives the loss in watts, i.e. the power dissipated as heat and the resulting energy costs. This is different from the voltage drop ΔU along the cable, which reduces the voltage arriving at the load. For sizing according to the permissible 3 % or 5 % voltage drop there is a separate calculator. Here we deal purely with the energetic losses.
