Voltage rating
One of the most fundamental ratings assigned to an electric cable is that of voltage rating. The rated voltage of a cable is the reference voltage for which the cable is designed and which serves to define the electrical tests.
The rated voltage of a cable is normally expressed by the combination of two values Uo/U, expressed in volts:
Uo being the Root Mean Squared (r.m.s) value between any insulated conductor and ‘earth’ (the metal covering on the cable or the surrounding medium);
U being the r.m.s value between any two phase conductors of a multicore cable or of a system of single core cables.
There is however a third designated term Um which is used in some cable standards, it is defined as the ‘highest system voltage’ for which the cable and their accessories are designed.
In an alternating current system, the rated voltage of a cable shall be at least equal to the nominal voltage of the system for which it is intended. This condition applies to both the value of Uo and to the value U.
In a direct current (d.c.) system, the nominal voltage of the system should be not higher than 1.5 times the rated voltage of the cable.
Recognised voltage ratings
Within Europe the most popular recognised voltage ratings are
For domestic, office and low power light industrial applications
- 300 / 500 V (sometimes expressed as 0.3/0.5 kV)
- 450 / 750 V (or 0.45/0.75 kV)
The standards covering these cables do not always state an actual value for Um however most do contain the following Note:
NOTE The operating voltage of a system may permanently exceed the nominal voltage of such a system by 10 %. A cable can be used at 10 % higher voltage than its rated voltage if the latter is at least equal to the nominal voltage of the system.
- 600 / 1000 (1200) V (or 0.6/1.0 (1,2) kV)
This particular rating is unique in that most standards covering these cables allow a Um 20% higher than the nominal voltage, so in these cases the following Note is used:
NOTE The operating voltage of a system may permanently exceed the nominal voltage of such a system by 20 %. A cable can be used at 20 % higher voltage than its voltage rating if the latter is at least equal to the nominal voltage of the system.
For large, high power equipment and machinery
- 1800 / 3000 V (or 1.8/3.0 kV)
- 3000 / 6000 V (or 3/6 kV)
- 6000 / 10000 V (or 6/10 kV)
- 8700 / 15000 V (or 8.7/15 kV)
- 12000 / 20000 V (or 12/20 kV)
- 18000 / 30000 V (or 18/30 kV)
The Um assigned to these voltage ratings revert to a 10% increase, so the Note reads:
NOTE The operating voltage of a system may permanently exceed the nominal voltage of such a system by 10 %. A cable can be used at 10 % higher voltage than its rated voltage if the latter is at least equal to the nominal voltage of the system.
See Annex A for the ratings used in the UK
Cables manufactured to American standards have totally different voltage designations, as do some Australian standards, typically Mining Cables, so if in doubt ASK.
Maximum rated voltage and nominal voltage
With the exception of 600/1000V cables all of the Notes use the same wording
A cable can be used at 10 % higher voltage than its rated voltage if the latter is at least equal to the nominal voltage of the system.
What this means is that it is perfectly acceptable to use a cable with a RATED voltage of say 6/10 kV on a system where the NOMINAL system voltage is 6.35/11 kV. This also applies equally to all other cables and voltage ratings.
Earthed and unearthed systems
All the voltages given above are for cables operating on an “earthed” system Uo/U. However there are some systems that are “unearthed” that is to say U/U the voltage between the conductor and “earth” is the same as the voltage between phase conductors for example – 10/10 kV.
In these cases, the cable chosen should have a Uo rating of at least 10 kV, so a 10/15 kV would normally be used.
Note: in the USA they do not use the terms “earthed” and “unearthed” on some enquiries from the USA you will see terms used like “100% insulation level” and “133% insulation level”
For simplicity, it is easier to say for “100% insulation level” read “earthed” and for “133% insulation level” read “unearthed” and choose the applicable voltage rating.
Which type of cable – LV, MV, HV, EHV, UHV?
People often refer to LV cables, MV cables, HV cables, EHV cables and UHV cables but there is no standardised National or International definition of what each term means, this can lead to misunderstanding, so care is needed when using these terms, especially in international markets.
As a general guide, not a definite rule the following can be used
LV – Low Voltage Cable
Low voltage (LV) cables are generally taken to include
- 300 / 500 V
- 450 / 750 V
- 600 / 1000 V
MV – Medium Voltage Cable
Medium voltage (MV) cables are generally taken to include
- 3/6 kV cables up to and including 18/30 kV
This group also includes the cables with the UK voltage designations
- 3.8/6 kV to 19000/33000V
1.8/3 kV and 1.9/3.3 kV cables are the odd ones out, some people refer to them as LV cables, others as MV cables, personally I prefer to class them as MV.
HV – High Voltage Cable
It should be noted that International technical specifications (CENELEC and IEC) refer to values of Um rather than country-specific rated voltages for HV cables and higher voltages
- 36/66 kV referred to as 72 kV Cables
- 64/110kV referred to as 132 kV Cables
- 76/132kV referred to as 145 kV Cables
- 87/150kV referred to as 170 kV Cables
Basically all cables with a U rating of 30 kV up to and including 150 kV
EHV – Extra High Voltage Cables
Extra-High Voltage (EHV) cables are generally taken to include
- 89/154 kV referred to as 170 kV
- 127/220 kV referred to as 245 kV
- 200/345 kV referred to as 362 kV
- 220/400 kV referred to as 420 kV
UHV – Ultra High Voltage Cable
And then there are even Ultra High voltage (HV) cables which are generally taken to include cables rated above 400 kV
- Above 400 kV
Annex A
In the UK the DNO’s and other users assign the following ratings to their cables, but it should be noted that in all cases the radial thickness of insulation and the test voltage is the same as the European cables.
- 600/1000 (1200) V
- 1900/3300 (3500) V
- 3800/6600 (7200) V
- 6350/11000 (12000) V
- 8700/15000 (17500)V
- 12700/22000 (24000) V
- 19000/33000 (36000) V
