Cable voltage ratings: what 0.6/1 kV and 8.7/15 kV mean

Two numbers, two different stresses

IEC cable standards express the rated voltage as U0/U. U0 is the voltage between the conductor and earth or the metallic screen; U is the voltage between any two phase conductors. A 0.6/1 kV cable is therefore designed for 0.6 kV from conductor to earth and 1 kV between conductors. IEC 60502-1 adds a third value in brackets, 0.6/1 (1.2) kV, where 1.2 kV is the highest system voltage the cable may see in service.

The split matters because the insulation is stressed in two ways. Between conductor and earth, the stress sits across one insulation wall. Between two conductors, it sits across two walls in series. A cable rated U0/U is built so both stresses stay inside what its insulation thickness and compound can handle over its service.

The common mistake is to read 0.6/1 kV as "for systems from 600 V to 1,000 V". Another is to compare a US 600 V rating directly with an IEC 0.6/1 kV rating. They come from different systems, use different test regimes and are not interchangeable on a project without the engineer's agreement.

Ratings you will see on quotes

The reference constructions on this site follow the same pattern: the building wire reference is 450/750 V, the flexible cord 300/500 V, the LV power cable 0.6/1 kV and the MV cable 8.7/15 kV. See the building wire and LV power cable pages for the full constructions.

What 8.7/15 kV means in practice

Medium-voltage ratings follow the same U0/U logic. An 8.7/15 kV cable is designed for 8.7 kV between each conductor and its metallic screen and 15 kV between phases. IEC 60502-2 lists the standard pairs 3.6/6, 6/10, 8.7/15, 12/20 and 18/30 kV. The partner plant catalogue used for our references covers ratings up to 8.7/15 kV; projects needing 12/20 or 18/30 kV are matched to a different plant with the right extrusion and test equipment.

Two points catch buyers out. First, the U0 value matters as much as U. Which U0 suits a network depends on the system voltage, how the system is earthed and how quickly earth faults are cleared, so the project engineer selects the rating from the network data, not from the nominal system voltage alone. Second, the insulation thickness rises steeply with voltage: the reference 8.7/15 kV 3x95 mm2 cable has 4.5 mm of XLPE against 0.9 mm on the 0.6/1 kV 4x25 mm2 reference. That extra material, plus the screens, is why MV cable costs far more per metre at the same conductor size. The MV power cables page shows the construction.

Wiring ratings below 1 kV

Below 1 kV, the ratings in IEC 60227 and the harmonised codes look small but matter just as much. A 450/750 V single-core wire such as H07V-R is intended for fixed wiring inside conduit or trunking; a 300/500 V cord such as H05VV-F is intended for appliances and light-duty connections. The lower rating comes with thinner insulation and sheath walls: the reference 3x1.5 mm2 cord has 0.7 mm PVC insulation and a 0.9 mm sheath. Using a 300/500 V cord where the design calls for 450/750 V cable is a non-compliance even if the circuit voltage is only 230 V, because the standards tie each rating to specific uses and installation conditions.

Control and instrumentation cables to GB/T 9330 are typically rated 450/750 V as well. They carry signals and control circuits rather than power, but the rating still has to suit the highest voltage present in the panel or tray. Where control cores share a route with 0.6/1 kV power cables, the project engineer decides whether segregation or a higher-rated cable is needed. Mixed BOQs are common on engineering projects, and the desk checks that each line carries the rating the drawings specify.

How ratings are tested

A rating is only meaningful if the cable is tested against it. Every drum of every voltage class gets a voltage test or spark test on the insulation during or after production. Medium-voltage cables add more: IEC 60502-2 sets a routine voltage test at 3.5 U0 for 5 minutes and a routine partial discharge test, measured by the method in IEC 60885-3, where discharge at 1.73 U0 must not exceed the declared sensitivity of 10 pC or better. Type tests, done once per construction, include longer and harsher voltage tests and ageing. The testing and inspection page lists what the desk checks per order.

Choosing the rating for your order

A practical sequence for buyers who are not the design engineer:

1. Take the rating from the drawings or BOQ: the designer has already chosen U0/U for the network. If the BOQ says only "1 kV cable", ask whether 0.6/1 kV to IEC 60502-1 is meant.
2. Match the standard to the market: a 0.6/1 kV cable to IEC 60502-1, BS 5467 or a national standard may differ in sheath, marking and test details.
3. Do not up-rate or down-rate without approval: a higher rating means thicker insulation, a larger diameter and different glands; a lower one is not permitted.
4. For DC solar circuits, use a DC-rated cable: H1Z2Z2-K is rated DC 1.5 kV under EN 50618; see the solar PV cables page.
5. Write it on the sheath: the rated voltage should be printed in the sheath legend so the installer and inspector can read it.

Voltage rating is separate from current rating. Conductor size for a given load depends on installation method and is set under IEC 60364-5-52, BS 7671 or NEC as applicable, confirmed by the project engineer. When both are fixed, send the designation and quantities through the quote form and the desk confirms the construction in writing before any plant books copper.