Copper Cable Ampacity Chart (0.6/1 kV) — Reference Table

What an ampacity number is telling you

The current rating of a cable is the steady current that brings the conductor to its maximum permitted operating temperature, from the stated ambient, in the stated installation. Nothing more. It is a heat balance: the I²R loss generated in the conductor has to flow out through the insulation, the bedding, the armour if there is any, the sheath, and then into the air or the soil. Any layer in that path that gets worse lowers the rating.

That is why the conditions matter more than the cable. A 50 mm² XLPE cable in free air at 30 °C carries 192 A. The same cable buried in dry sand carries far less, and the same cable in a duct bank with four other loaded circuits carries less again. The number in the table is not a property of the cable; it is a property of the cable plus its surroundings. These reference figures come from IEC 60364-5-52, Tables B.52.5 and B.52.9, for three- or four-core copper cable with XLPE insulation, and they assume a single circuit and a 30 °C ambient in air or 20 °C in the ground.

• 30 °C ambient air, or 20 °C ground temperature, at the reference conditions
• A single circuit, not touching anything else that generates heat
• Free air, installation method E, for the in-air column
• Direct burial at 0.7 m depth in 2.5 K·m/W soil, for the buried column
• Conductor at its maximum operating temperature: 90 °C for XLPE, 70 °C for PVC

Reference table: XLPE 90 °C copper, 0.6/1 kV

Use this table for YJV, YJV22 and their LSZH equivalents. The A per mm² column is the useful one for a sanity check: it falls from 15.3 A/mm² at 1.5 mm² to 2.2 A/mm² at 240 mm², because the surface area available to shed heat grows far more slowly than the cross-section that generates it. If you ever find yourself mentally applying a single current density to every size, this column is the correction.

The buried column is lower than the in-air column for everything above about 6 mm², and the gap widens with size: 95 mm² loses 17 % of its rating when it goes underground, 240 mm² loses 24 %. Small cables do slightly better buried than in air because 20 °C soil is a better heat sink than 30 °C air. Every figure here also assumes the cable is not grouped with any other loaded circuit.

The same sizes in PVC 70 °C

VV and VV22 work to a 70 °C conductor limit, and that one difference costs about 20 to 25 % of the rating in every size. The comparison of the two insulations side by side, with the reasons behind it, is in the YJV versus VV article in this series. The figures below are the PVC in-air and buried ratings for the sizes most often used in building and light industrial work.

There is one trap in this table worth stating separately: a 90 °C cable can only be loaded to its 90 °C rating if the terminations and the connected equipment can take that temperature. Terminal blocks, lugs and many devices are rated 70 °C, and in that situation the XLPE column is not usable and the PVC column becomes the real limit for both cable types.

Factor 1: ambient temperature

The tables are built on a 30 °C ambient. Where the air around the cable is warmer, the available temperature rise is smaller and the rating falls. The 90 °C column is more tolerant than the 70 °C column, which is one of the practical reasons XLPE is preferred in hot climates and in the Middle East, Africa and Southeast Asia.

Two cautions. The ambient for the calculation is the temperature of the air immediately around the cable, not the shade temperature on the weather report: inside a sun-loaded steel conduit on a roof it can be 20 K higher. And where the cable is protected by a device, the ambient factor applies to the device too, so a cabinet in the same room may need derating as well.

Factor 2: grouping of circuits

Two cables touching each other each get in the way of the other's heat loss, so the middle of a bundle runs hottest. The reduction factor depends on how many circuits are in the group and on the installation method, and it applies whether the circuits are three-phase or single-phase. Note that this is a factor on the number of circuits, not on the number of cores: two 4-core cables is two circuits.

There are two ways to avoid the penalty. Spacing the cables one cable diameter apart, centre to centre, removes the factor entirely, which on a wide ladder rack is usually free. The other is to apply a load factor: if the grouped circuits are known not to be fully loaded simultaneously, IEC 60364-5-52 permits a less severe factor, but the assumption has to be documented and defensible, because it will be inherited by whoever adds a circuit to that tray in five years.

Factor 3: burial, soil resistivity and depth

The buried ratings assume soil with a thermal resistivity of 2.5 K·m/W, which is typical of normal damp ground. Dry sand, chalk and rock are far worse at carrying heat away, and a buried cable in dry sand can lose more than a third of its rating. Measuring or estimating the soil resistivity is a genuine part of the design on any buried route of consequence; where the ground is known to be dry for part of the year, design for the dry condition.

Depth and spacing matter for the same reason. The standard tables assume 0.7 m, and deeper burial is not automatically worse because the soil temperature is more stable at depth, but it does increase the thermal resistance of the path to the surface. Cables in a group in a trench need a spacing factor as well, and cables drawn into ducts are worse again, because the air gap inside the duct is an extra thermal barrier.

Worked example: three factors at once

A 100 A load is fed by a 4-core XLPE copper cable laid on a ladder rack in a plant room at 45 °C ambient, in a group of three circuits, none of which will be fully loaded at the same time. Take the factors in order: ambient 45 °C gives 0.87, three grouped circuits gives 0.70.

Combined, that is 0.87 × 0.70 = 0.609. Try 25 mm²: 127 × 0.609 = 77.3 A, well short of 100 A. Try 35 mm²: 158 × 0.609 = 96.2 A, still short, and only 4 % below the requirement, which is not a margin to design to. Try 50 mm²: 192 × 0.609 = 116.9 A, which clears it. The selected size is 4 × 50 mm².

Now check the protective device. With In = 100 A for a moulded-case breaker, the requirement Iz ≥ In is met, since 116.9 A exceeds 100 A. The installation would then be 4 × 50 mm² XLPE, and if the run is long the voltage drop still has to be checked separately, because none of this arithmetic says anything about it. If the route had been 45 °C but the three circuits were each spaced apart by one cable diameter, the factor would have been 0.87 alone, and 25 mm² would have carried 110.5 A and been adequate: one rack width saves two size steps.

Why two published tables disagree

Engineers working across markets notice that the same cable size appears to have three different ratings, and conclude that somebody is wrong. Usually nobody is: the tables are built on different assumptions, and the number is a description of the assumption as much as of the cable. When you receive a datasheet, the first thing to check is which standard and which installation conditions it was tabulated under.

The practical consequence for imported cable is simple. A cable selected to an IEC table and installed to NEC rules will be undersized, because the NEC raceway conditions are more onerous, and a cable selected to NEC tables for an IEC-designed job will be oversized and more expensive than it needs to be. Match the table to the standard the project is being built to.