Cable sizing in 50 °C ambient: the derating mistakes that cost a cable size

By Eng. Mannan Maqsood, Electrical EngineerPublished 5 November 20265 min read

The current-carrying capacity tables in IEC 60364-5-52 and BS 7671 Appendix 4 are based on a reference air temperature of 30 °C and, for buried cables, a ground temperature of 20 °C. Nobody in Dubai, Riyadh or Doha installs cables at 30 °C. Gulf projects commonly use a design air ambient of around 50 °C and a ground temperature in the mid 30s, and many authority regulations and project specifications state these values directly. If the correction factors are not applied, or are applied to the wrong number, the cable runs hotter than its insulation is rated for. It may not fail on day one, but insulation life drops quickly once the conductor runs above its rated temperature.

The rule you already know, applied properly

The basic selection rule is the same everywhere: the design current Ib must not exceed the rating of the protective device In, and In must not exceed the installed capacity of the cable Iz:

Ib ≤ In ≤ Iz, where Iz = It × Ca × Cg × Ci (and any other factors that apply)

Here It is the tabulated rating for the installation method, Ca corrects for ambient temperature, Cg for grouping with other circuits and Ci for thermal insulation. Turned around, the minimum tabulated rating you must look for is:

It ≥ In ÷ (Ca × Cg × Ci)

Most sizing mistakes come from using Ib instead of In in that line, from forgetting one factor, or from applying the 30 °C table directly.

How big the ambient correction really is

At 50 °C air, the ambient correction factor from the IEC and BS 7671 tables is about 0.71 for 70 °C PVC-insulated cables and about 0.82 for 90 °C XLPE-insulated cables. That single factor removes almost 30% of a PVC cable's rating. It is also why XLPE cables are the default for feeders in the region: the higher conductor temperature leaves more margin above a hot ambient.

For buried cables the reference is 20 °C ground. At about 35 °C ground, XLPE is corrected by roughly 0.89 and PVC by roughly 0.84. Soil thermal resistivity matters just as much. The tables assume a value of 2.5 K·m/W. Dry desert sand and poorly compacted backfill can be worse. If the soil report shows a higher value, or the trench is not backfilled with selected material, apply the soil resistivity factor as well.

Grouping: the factor that grows with the project

Grouping factors depend on how many loaded circuits run together and how they are arranged. For circuits bunched together, the tables give roughly 0.80 for two circuits, 0.70 for three, 0.65 for four, 0.57 for six and 0.50 for nine. Single-layer arrangements on perforated tray or ladder with spacing do better. The trap is that the electrical designer often sizes each feeder as if it were alone, while the containment drawing produced later puts eight feeders on one tray in a riser. The cable sizes do not get updated when the containment is coordinated.

Check grouping again when the tray layout is fixed, not only at concept design. The Cable Tray & Containment Lab shows the fill and the number of circuits per tray, which is the number you need for Cg.

Sun on roofs and exposed routes

Cables on roof trays, external walls and car park ramps can sit in direct sunlight. Solar gain raises the cable surface temperature well above the shade air temperature. The standards say this must be taken into account. In practice the options are to shade the tray with a cover, route through shaded areas, or add margin to the ambient used for those sections. A roof run to rooftop chillers, PV inverters or AHUs is the most common place this is missed.

Worked example

A feeder to a rooftop AHU control panel has a design current Ib of 180 A. The protective device is a 200 A MCCB, so In = 200 A. The cable is a multicore copper XLPE cable on perforated tray. It shares the tray with one other loaded circuit, and the air ambient is 50 °C. These numbers are for illustration only.

• Ca for XLPE at 50 °C: 0.82
• Cg for two circuits (taking the bunched value, conservatively): 0.80
• No thermal insulation: Ci = 1.0

Minimum tabulated rating: It ≥ 200 ÷ (0.82 × 0.80 × 1.0) = 305 A.

Without derating, a designer reading the 30 °C table would pick the first size rated above 200 A. With derating, the cable must be the first size whose tabulated rating for this installation method is at least 305 A. In most tables that is two standard sizes larger. If the tray is in direct sun on the roof and not shaded, the margin shrinks again.

Then check the rest. A bigger cable usually helps voltage drop, but the voltage drop calculation should use the resistance at the operating temperature, not at 20 °C. Also confirm that the cable can withstand the prospective fault current for the disconnection time of the MCCB (the adiabatic check), and that the earth fault loop impedance at the far end allows disconnection in the required time.

The mistakes we see most often

1. Sizing from Ib, not In. The cable must be protected by the device that is actually installed. If the breaker is 200 A, the cable must carry 200 A after derating, even if the load is 150 A.
2. Using the 30 °C table and a note saying "derated". Show the factors in the cable schedule. Reviewers and authorities increasingly ask for them.
3. Different ambients for different sections, one cable size. A feeder that passes through a 50 °C plant room and a 35 °C air-conditioned corridor must be sized for the worst section along its route.
4. Breaker size increased on site, cable unchanged. When a nuisance-tripping breaker is replaced with a larger one, the cable protection is lost. Any change of In needs the cable check repeated.
5. Parallel cables not equal. Parallel runs must be the same type, size, length and route, or the current will not share equally and one cable will overheat.
6. Terminal temperature ignored. A 90 °C XLPE cable connected to a breaker terminal rated for 70 °C must be sized as if it were a 70 °C cable, unless the device data allows otherwise.

A short checklist before you issue the cable schedule

• Ambient temperature for every section of the route, including roofs and plant rooms
• Installation method that matches the containment drawing
• Number of loaded circuits grouped together, from the coordinated tray layout
• Ground temperature, depth and soil resistivity for buried sections
• Protective device rating In (not just Ib) used in the check
• Voltage drop at operating temperature, added to the upstream drop
• Fault withstand and earth fault loop impedance at the far end
• Terminal temperature limits of the connected equipment

Practise it in the labs

The Cable Sizing & Voltage Drop lab applies the ambient and grouping factors and shows the effect on each check. If you already have a load schedule in Excel, the Load Schedule Auditor flags breakers that are larger than the cable they feed. Always confirm the final values with the edition of the regulations your authority has adopted and with the cable manufacturer's data.

Disclaimer. This article is general technical information for education only. It is not professional engineering, design, legal or safety advice for any specific project, and it does not replace the judgement of a qualified, registered engineer who has reviewed the actual installation. Worked examples and figures are illustrative and do not describe any identified project, client or site. Codes, authority requirements and product data change, so confirm every value against the edition that applies to your project, the authority having jurisdiction and the manufacturer's data. Any testing or work on live systems must be carried out only by competent, authorised people under the approved procedure. The views are the author's own and do not represent any employer, client or authority. To the extent permitted by law, the author, the editor and MEPVirtual accept no liability for any loss, damage or injury arising from the use of, or reliance on, this content. See the full disclaimer or report a correction.