Cable loss running cost
What a cable size up saves in energy, cost and CO2.
Reads AS/NZS 3008.1.2:2017 (NZ) and AS/NZS 3008.1.1:2017 (AU). Tables checked 24 Sept 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- What a size up saves on a feederThe yearly saving of 120 over 95 mm² on a 200 A feeder that never stops.
- Running cost of a heat pump circuitThe energy a 6 mm² heat pump circuit loses a year, against 10 mm².
- Payback of a larger submainHow fast 35 mm² pays back over 25 mm² on a busy 80 m submain.
- CO2 saved by a larger aluminium cableThe kWh and CO2 a year of 120 over 95 mm² aluminium at 150 A.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Jurisdiction
- NZ · AS/NZS 3008.1.2
- Phases
- Three phase
- Running current, rms
- 100 A
- Running hours a year
- 4,000 h
- Route length
- 60 m
- Conductor
- Copper
- Insulation
- XLPE 90 °C
- Size that passes
- 35 mm²
- Size up to compare
- 50 mm²
- Energy tariff
- 0.3 $/kWh
- Emission factor
- 0.1 kg/kWh
- Extra cost of the size up
- 800 $
Cost saved a year
$375.84
- Energy saved a year
- 1,252.8kWh
- CO2 saved a year
- 125.3kg
- Payback of the extra cost
- 2.1years
- Energy lost a year, 35 mm²
- 4,809.6kWh
- Energy lost a year, 50 mm²
- 3,556.8kWh
This result is a design aid. Verify it against the current standard.
Calculation steps
- 35 mm² a.c. resistance at 90 °C0.67Ω/km
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- 35 mm² loss: 3 × 100² × R × 60 m1,202.4W
- 35 mm² energy lost a year, over 4000 h4,809.6kWh
- 35 mm² cost a year, at $0.30 a kWh$1,442.88
- 50 mm² a.c. resistance at 90 °C0.49Ω/km
AS/NZS 3008.1.2:2017 Table 30 (Xc), Tables 34–35 (Rc) p.98, p.102–103
- 50 mm² loss: 3 × 100² × R × 60 m889.2W
- 50 mm² energy lost a year, over 4000 h3,556.8kWh
- 50 mm² cost a year, at $0.30 a kWh$1,067.04
- Energy saved a year1,252.8kWh
- Cost saved a yearResult$375.84
- CO2 saved a year125.3 kg
- Payback of the extra cost2.1 years
About this calculator
How this is calculated
Every cable turns some of the energy it carries into heat. The loss is the current squared times the resistance of the conductors that carry it. A larger cable has less resistance, so it wastes less. This tool puts a yearly figure on that waste for two sizes, so you can see what the size up saves.
The tool reads the a.c. resistance of each size from AS/NZS 3008.1, in ohms per kilometre, for the conductor metal and insulation you pick. It reads the table at the conductor's full-load temperature. On a single-phase circuit the current flows out and back, so two conductors carry the loss. On a balanced three-phase circuit the neutral carries no current, so three do.
The loss in watts is the number of loaded conductors times the current squared times the resistance times the length. The tool multiplies it by the running hours a year to get kWh. The tariff turns kWh into dollars, and the emission factor turns kWh into kilograms of CO2. Both are figures you set from your own contract and grid.
The difference between the two sizes is the saving a year. If you enter what the size up costs over the smaller cable, installed, the tool divides that by the saving to give the payback in years.
Enter the rms current over the running hours, not the mean or the peak. Loss grows with the square of the current, so a cable that runs near full load for long hours gains the most from a size up.
| Step | Table or clause | What it decides |
|---|---|---|
| Resistance of each size | Tables 34 and 35 | The ohms per kilometre at the full-load temperature |
| Loss | n × I² × R × L | The watts each size wastes |
| Energy a year | Loss × running hours | The kWh each size wastes |
| Cost and CO2 | Tariff and emission factor | Dollars and kilograms a year, from your figures |
| Payback | Extra cost ÷ saving a year | The years for the size up to pay for itself |
Worked examples
- A 100 A feeder of 60 m in 35 or 50 mm² XLPE saves $375.84 a year with the size up. That is 1,252.8 kWh a year, and the extra $800 pays back in 2.1 years.
- A 200 A feeder of 120 m that never stops, 95 or 120 mm² saves $1,892.16 a year. That is 6,307.2 kWh a year, and the extra cost pays back in 1.3 years.
- A 25 A heat pump circuit of 40 m in 6 or 10 mm² PVC saves $68.40 a year. That is 228 kWh a year, and the extra cost pays back in 2.2 years.
- A 150 A aluminium feeder of 100 m, 95 or 120 mm² saves $870.75 a year. That is 2,902.5 kWh a year, and the extra cost pays back in 1.7 years.
Limits
The resistance is the table figure at the full-load temperature. A cable at part load runs cooler, so the real loss is a little lower than the tool shows.
The tool does not size the cable. Size it first for current, voltage drop and fault with the cable size tool, then compare the size that passes with the next size up. It does not count the time value of money, a tariff that changes through the day, or harmonic currents, which add loss.
See also: Cable size, Voltage drop and Three phase power.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do you calculate the energy lost in a cable?
Multiply the number of loaded conductors by the current squared, the resistance per metre and the length. That is the loss in watts. Times the running hours over 1000 gives kWh a year. The tool reads the resistance from AS/NZS 3008.1.
Does a bigger cable save money?
A bigger cable has less resistance, so it loses less energy. On a long run that carries a high current for many hours, the saving can pay back the extra cable in a few years. On a short or lightly used run it rarely does.
Why is the loss an upper bound?
The tool reads the resistance at the cable’s full-load temperature, 75 °C for PVC and 90 °C for XLPE. A cable at part load runs cooler and its resistance is lower, so the real loss is a little less. So is the saving, and the payback is a lower bound.
What tariff and emission factor should I use?
Use the energy rate in your supply contract, including any time-of-use split that matches the hours. For CO2, use your grid’s published emission factor. Both are assumptions you set. AmpSize holds neither.
Does this replace the cable size check?
No. Size the cable for current, voltage drop and fault first, with the cable size tool. Then compare the size that passes with the next size up here.
Clauses and tables this tool reads
- AS/NZS 3008.1.2:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
Every clause and table, with how each was verified
Tables last checked on 24 Sept 2026.
Related tools
- Cable size calculatorFind the smallest cable size that passes each check.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Voltage drop calculatorCalculate voltage drop on a cable run and check the limit.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
- Cable fault rating calculatorCheck that a conductor can withstand the fault current.AS/NZS 3008.1.2AS/NZS 3008.1.1AS/NZS 3000
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