Short Answer: What Conductor Resistance Shows
The DC resistance of a conductor is an integral parameter that depends simultaneously on the actual cross-section, the conductor material and the temperature. If the manufacturer has understated the cross-section, the resistance will be higher than standardised. If the conductor is made of aluminium instead of copper at the same cross-section, the resistance will be approximately 1.6 times higher. The measurement is carried out according to GOST 7229-76 with a single, double or single-double DC bridge with an error of no more than ±0.5%. The obtained value is brought to a temperature of +20 °C and compared with the norm according to GOST 22483-2021 for the corresponding nominal cross-section, material and flexibility class of the conductor.
For a copper conductor with a cross-section of 2.5 mm² of class 2, the standardised resistance is not more than 7.41 Ω per 1 km at +20 °C. For an aluminium conductor of the same cross-section — not more than 12.4 Ω per 1 km. Exceeding the norm means that the conductor does not correspond to the declared cross-section or material. The suitability of the cable for the design load is determined by a separate calculation according to the installation conditions.
What Is Checked and Why
Physical meaning. The electrical resistance of a conductor is directly proportional to the resistivity of the material and the length of the conductor, and inversely proportional to the cross-sectional area: R = ρ · L / S. The resistivity of copper at +20 °C is 0.0175 Ω·mm²/m, of aluminium — 0.028 Ω·mm²/m. When the actual cross-section deviates from the nominal one, the resistance changes inversely. This is precisely what makes resistance measurement a quantitative method for checking the cross-section: it does not depend on how accurately the geometry of the individual wires in the conductor is executed.
Why the measurement is carried out. Determining the DC electrical resistance of current-carrying conductors is part of the acceptance tests of cables. It makes it possible to:
- identify an understated actual conductor cross-section;
- confirm the conductor material (copper or aluminium);
- assess the suitability of the cable for the design load;
- compare the result with the standardised values according to GOST 22483-2021.
Measurement of the DC resistance of conductors is applied to cables of all classes and cross-sections. It does not replace the insulation resistance check, which is performed separately with a megohmmeter. The difference between these measurements is described in the material about measuring cable insulation resistance with a megohmmeter.
When the Check Is Carried Out
Production. Factory acceptance tests include determining the DC electrical resistance of current-carrying conductors. Cables are presented for acceptance in batches. The measurement is carried out according to GOST 7229-76 on construction lengths or on straightened samples with a length of at least 1 m in the measured part. The method does not apply to cable products in an installed state.
Acceptance at the site. Incoming inspection may include selective measurement of conductor resistance to verify compliance with the declared cross-section. More about this procedure — in the article incoming cable inspection: what to check when receiving a batch.
After installation. According to PUE RK, determination of the active resistance of conductors is carried out for lines with a voltage of 20 kV and above. For lines up to 1 kV and above 1 kV up to 35 kV, checking the integrity and phasing of conductors and measuring insulation resistance are carried out without fail, while measuring the DC resistance of conductors is not included in the list of tests for these lines.
Operation and troubleshooting. When a conductor defect is suspected (local heating, break, poor contact in a joint), resistance measurement allows the problem to be localised. Reflectometers and other methods are used to locate the damage. Conductor resistance is also checked during the examination of cable product quality.
How It Is Carried Out
Sampling
The measurement is carried out on construction lengths of cables or on straightened samples with a length of at least 1 m in the measured part, unless another length is specified in the standards or technical specifications for specific types. Sampling is carried out by random selection. The number of samples is specified in the standards or technical specifications for specific products.
Equipment
The measurement of the electrical resistance of current-carrying conductors is carried out with a single, double or single-double DC bridge according to GOST 7165-72 with a measurement error of no more than ±0.5%. It is permitted to use automatic and other equivalent instruments that perform measurements on direct current with the same error. The bridge must contain a built-in or separate null measuring instrument with a sensitivity corresponding to GOST 7165-72.
The choice of measurement scheme depends on the value of the measured resistance. For small resistances (units and fractions of an ohm), a double bridge is used; for large ones — a single bridge. Specific recommendations are given in GOST 7229-76.
Measurement Procedure
- Prepare the sample: the conductor ends are stripped of insulation, oxide film and contamination.
- Connect the bridge to the conductor under test in accordance with the scheme for the selected resistance range.
- Balance the bridge, achieving a zero reading of the galvanometer.
- Record the measured resistance value.
- Repeat the measurement for each conductor of the cable.
The measurement is performed on direct current. The use of alternating current is not permitted, since on long cables an inductive component appears, distorting the result. That is why the standard establishes a method for determining DC resistance.
Reduction to +20 °C
The measured resistance value is recalculated to a temperature of +20 °C using the formula:
R20 = Rt · K
where R20 is the resistance reduced to +20 °C, Ω; Rt is the measured resistance at temperature t, Ω; K is the correction temperature coefficient.
The temperature coefficient of electrical resistance αR for soft (annealed) copper is 0.00393 °C⁻¹, for hard copper — 0.00381 °C⁻¹, for aluminium — 0.00403 °C⁻¹. The values of K for various temperatures are given in GOST 7229-76 and GOST 22483-2021.
How to Evaluate the Result
The resistance reduced to +20 °C is compared with the standardised value according to GOST 22483-2021 for the corresponding nominal cross-section, material and flexibility class of the conductor. The norms are established for each class separately: class 1 (solid), class 2 (stranded for fixed installation), classes 3–6 (flexible).
| Nominal cross-section, mm² | Copper conductor, uncoated, Ω, not more than | Copper conductor, tinned, Ω, not more than | Aluminium conductor, Ω, not more than |
| 1.5 | 12.1 | 12.2 | 22.7 |
| 2.5 | 7.41 | 7.56 | 12.4 |
| 4 | 4.61 | 4.70 | 7.41 |
| 6 | 3.08 | 3.11 | 5.11 |
| 10 | 1.83 | 1.84 | 3.08 |
| 16 | 1.15 | 1.16 | 1.91 |
| 25 | 0.727 | 0.734 | 1.20 |
| 35 | 0.524 | 0.529 | 0.868 |
| 50 | 0.387 | 0.391 | 0.641 |
| 70 | 0.268 | 0.270 | 0.443 |
| 95 | 0.193 | 0.195 | 0.320 |
| 120 | 0.153 | 0.154 | 0.253 |
Source: GOST 22483-2021, tables for class 2. Values for other classes are given in the corresponding tables of the standard. For aluminium alloy conductors, the values are agreed between the manufacturer and the customer.
Evaluation of the result:
- Resistance equal to or below the norm. The conductor corresponds to the declared cross-section and material. The cable is suitable for further use.
- Resistance above the norm. The actual cross-section is understated or the conductor material does not correspond to the declared one. The cable does not comply with the standard. For critical lines, such a cable is not used.
- Resistance significantly above the norm (several times). A break of part of the wires in a stranded conductor, poor contact in a joint, or the use of a material with high resistivity is possible.
When evaluating, it is important to take into account the flexibility class. Flexible conductors of classes 5 and 6 have a higher standardised resistance at the same cross-section due to the increased total length of the wires and the contact resistances between them. The result must be compared with the norm specifically for the class to which the conductor under test belongs. The differences between single-core and multi-core design are analysed in the article single-core or multi-core power cable.
What Can Distort the Measurement
| Factor | How it affects | What to do |
| Conductor temperature | Resistance increases with temperature. Without reduction to +20 °C, comparison with the norm is incorrect. | Measure the temperature and reduce the result to +20 °C using the formula. |
| Contact quality | Poor contact at the connection point of the bridge gives an overestimated resistance. | Strip the conductor ends, ensure a reliable connection. |
| Sample length | Resistance is proportional to length. On short sections, the error is higher. | Measure on construction lengths or samples with a length of at least 1 m. |
| Alternating current | On long cables an inductive component appears, distorting the result. | Measure only on direct current. |
| Condition of the conductor surface | An oxide film on aluminium or contamination increases the contact resistance. | Strip the conductor before connecting. |
| Instrument error | A bridge with an error of more than ±0.5% gives an incorrect result. | Use a calibrated bridge with an error of no more than ±0.5%. |
| Conductor flexibility class | Flexible conductors have a higher standardised resistance. | Compare with the norm for the actual conductor class. |
What to Do in Case of Deviation
Resistance above the norm by 5–15%. The quality of the contacts is checked, and the measurement is repeated on another section. The temperature and the correctness of the reduction to +20 °C are clarified. If the excess is confirmed, an understated cross-section by 1–2 steps is probable. For lines with high current, such a cable will give increased heating and an increased voltage drop.
Resistance above the norm by 1.5–2 times. The probable cause is an understated cross-section or the use of an aluminium conductor instead of copper. The conductor material is checked by density and colour. For critical lines, the cable is not used.
Resistance above the norm by several times. A break of part of the wires in a stranded conductor or destruction of the contact is possible. The integrity of the conductor along the entire length is checked. Reflectometry is used to localise the damage location.
Resistance below the norm. For a standard cable, this means that the cross-section is overstated, or the measurement was performed with an error. The compliance of the type and cross-section with the order is checked. If the cross-section is indeed higher than declared, the cable is suitable, but may not meet the requirements for dimensions and weight.
The measurement results are recorded in a report. If the resistance does not correspond to the norm, the cable is considered to have failed the test. The decision on further use is made by the incoming inspection commission.
Typical Mistakes
Measurement without reduction to +20 °C. The result is compared with the norm without temperature correction. At a temperature of +50 °C, the resistance of copper is about 12% higher, and the cable may be erroneously recognised as non-compliant. At a temperature of 0 °C, it is understated, and the defect is not detected.
Measurement on alternating current. On long cables the inductive component increases the measured value. The standard prescribes measurement on direct current.
Checking only one conductor. The resistance of conductors may differ due to different actual cross-sections or defects. All conductors are measured, not just one selectively.
Using a bridge with insufficient accuracy. An error of more than ±0.5% does not allow a reliable identification of the deviation. The bridge must comply with GOST 7165-72.
Comparison with the norm for a different conductor class. The norm for a flexible class 5 conductor is higher than for class 2. If the result for class 5 is compared with the class 2 norm, the cable will be erroneously recognised as non-compliant.
FAQ
What instrument is used to measure the resistance of a cable conductor?
A single, double or single-double DC bridge according to GOST 7165-72 with an error of no more than ±0.5%. For small resistances (units and fractions of an ohm), a double bridge is used; for large ones — a single bridge. Automatic instruments with the same error are permitted.
What does a conductor resistance above the norm mean?
The actual cross-section of the conductor is understated compared to the nominal one, or the conductor material does not correspond to the declared one (for example, aluminium instead of copper). For a stranded conductor, a break of part of the wires is possible. Under load, such a cable will heat up more than calculated, and the voltage drop on the line will be higher than the design value.
Why is the resistance reduced to +20 °C?
The standardised resistance values are established at +20 °C. At another temperature, the resistance differs. Reduction to +20 °C makes it possible to compare the measured value with the norm regardless of the temperature at the time of measurement. The temperature coefficient for copper is 0.00393 °C⁻¹, for aluminium — 0.00403 °C⁻¹.
Can the conductor cross-section be checked with a caliper?
A geometric measurement gives an idea of the actual cross-section, but for a stranded conductor it is difficult to take into account the packing density of the wires and the contact resistances. The control indicator is the electrical resistance of the conductor, reduced to the standardised temperature and length. It is this that is compared with the standardised value according to GOST 22483-2021.
For which cables is the DC resistance of conductors standardised?
For all cables with copper and aluminium conductors. The norms are established in GOST 22483-2021 for each nominal cross-section, material and flexibility class. For copper and aluminium conductors, the norms differ, since the resistivity of aluminium is approximately 1.6 times higher than that of copper. The differences between copper and aluminium conductors also affect the choice of cable according to installation conditions.
Sources
- GOST 7229-76 “Cables, wires and cords. Method for determining the electrical resistance of current-carrying conductors and conductors” — in force, introduced 01.01.1978 — official text on elec.ru, cn.gostinfo.ru — measurement method, equipment, sampling, reduction to +20 °C.
- GOST 22483-2021 (IEC 60228:2004, MOD) “Current-carrying conductors for cables, wires and cords” — in force — official text on normativ.kontur.ru — standardised resistance values by class, cross-section and material.
- GOST 7165-72 “DC bridges for resistance measurement” — in force — official text — requirements for bridges and measurement error.
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — section 29, paragraph 29 — in force — IPS “Adilet”, online.zakon.kz — composition of cable line tests, requirements for active conductor resistance.
- IEC 60228:2004 “Conductors of insulated cables” — in force — official IEC website, webstore.iec.ch — international standard in relation to which GOST 22483-2021 is modified.
- GOST 31996-2012 “Power cables with plastic insulation for rated voltages of 0.66; 1 and 3 kV” — in force — official text of the standard — requirements for cable design.
