Loop resistance of a primary AGL cable from conductor size, length and temperature, with a field-test comparison.
Resistance is the starting point for every cable calculation in a series circuit. Enter each cable section, and the tool gives the resistance at 20 °C and at your operating temperature, shows how it moves with temperature, compares the other conductor sizes, and checks a measured reading against the expected value.
A series circuit runs out to the last fixture and back, so the conductor length is the loop.
| Section / description | Conductor | Length (m) | Resistance (Ω) |
|---|
Copper resistance rises about 0.393 % per °C above 20 °C. Use the expected conductor temperature.
Used by any section set to "Custom". Take it from your cable data sheet.
If entered, the comparison shows the longest loop each conductor can be for that resistance at your temperature. Leave 0 to skip.
Leave 0 to skip. The reading is corrected to 20 °C and compared with the calculated value.
Your own acceptance rule.
| Temperature (°C) | Factor | Resistance (Ω) | Change from 20 °C |
|---|
The whole loop length re-run in a single conductor size at your temperature.
| Conductor | Ω/km at 20 °C | Ω/km at temp. | Loop resistance (Ω) | Longest loop for target (m) |
|---|
Every step is shown so the result can be checked by hand.
Each conductor has a nominal resistance per km at 20 °C. It is scaled by the loop length of the section, then the sections are added.
The coefficient is the usual figure for copper. A warm cable in summer or a buried cable in winter can differ noticeably from the 20 °C value.
Here k_T is the temperature factor from step 2. It gives the longest loop of one conductor size that stays at or below the target.
The corrected reading is compared with the calculated R₂₀ and shown as a percentage difference against your tolerance. A reading well above expected can point to a poor joint or a longer route than assumed. A reading below expected can point to a larger conductor or a shorter route. Both need checking on site.
| Item | Value |
|---|---|
| #8 AWG, 3,000 m loop | 2.061 Ω/km |
| Resistance at 20 °C | 6.183 Ω |
| Temperature factor at 40 °C | 1.0786 |
| Resistance at 40 °C | 6.669 Ω |
| Measured 6.35 Ω at 30 °C | 6.11 Ω at 20 °C |
| Difference from 6.183 Ω | −1.2 % |
It does not calculate voltage drop or loss (see Calculator #15), include connectors, splices or cable reactance, or cover aluminium conductors. Resistance values are nominal DC figures, so confirm them against your cable data sheet.
Quick answers on the loop resistance of a primary AGL cable and the field-test comparison.
It calculates the loop resistance of a primary AGL cable from conductor size, length and temperature. It gives the resistance at 20 degrees C and at your operating temperature, shows how it moves with temperature, compares the other conductor sizes, and checks a measured reading against the expected value. Resistance is the starting point for every cable calculation in a series circuit.
Either. A series circuit runs out to the last fixture and back, so the conductor length is the loop. You can enter the total loop length, or the one-way route length, which is doubled. Lengths can be in metres or feet, and you add one row per cable section. Each section's resistance at 20 degrees C is its resistance per km times its length divided by 1,000, and the sections are added.
The resistance at temperature is the resistance at 20 degrees C times 1 plus 0.00393 times the temperature minus 20, which is the usual coefficient for copper, about 0.393 percent per degree C. A warm cable in summer or a buried cable in winter can differ noticeably from the 20 degrees C value, so use the expected conductor temperature.
If you enter a target loop resistance, the conductor comparison shows the longest loop each conductor size can be for that resistance at your temperature. The length is the target divided by the resistance per km at 20 degrees C times the temperature factor, divided by 1,000. Leave it at 0 to skip.
The measured loop resistance is corrected to 20 degrees C using the temperature at the test, and compared with the calculated value as a percentage difference against your own tolerance. A reading well above expected can point to a poor joint or a longer route than assumed. A reading below expected can point to a larger conductor or a shorter route. Both need checking on site.
A #8 AWG loop of 3,000 m at 2.061 ohms per km has a resistance of 6.183 ohms at 20 degrees C. At 40 degrees C the temperature factor is 1.0786, which gives 6.669 ohms. A measured 6.35 ohms at 30 degrees C corresponds to 6.11 ohms at 20 degrees C, a difference of minus 1.2 percent from 6.183 ohms.
It does not calculate voltage drop or loss (see Calculator #15), include connectors, splices or cable reactance, or cover aluminium conductors. Stranded conductors read slightly higher than the solid-wire figures.
No. They are nominal DC figures for copper, so confirm them against your cable data sheet. Final design shall be verified against the cable manufacturer's data and the applicable project specification.
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