What is voltage rating?

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 is 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 is 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 its 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 of U. 

what is voltage rating ? It's equal to Uo/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 system’s nominal voltage.

  • 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 an 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 a 20 % higher voltage than its voltage rating if the latter is at least equal to the system’s nominal voltage.

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 system’s nominal voltage.

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 system’s nominal voltage.

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, some systems 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 cable type – 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:

  • 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)

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. Someone refers to them as LV cables, others as MV cables. We prefer to class them as MV.

Some network rails use 12.7 kV. to 22 kV cables.

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 DNOs 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 are 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

Conductor Temperature Rating

All Thermoplastic and Thermoset materials used to make cables will slowly age and deteriorate over time; the rate of deterioration will accelerate as the temperature increases (Arrhenius Law). It is, therefore, usual to state a temperature at which the material will continue to operate normally and give an acceptable life; for electric cables, this is known as the “continuous conductor operating temperature” rating.

The continuous conductor operating temperature for the popular insulation materials used by Tratos are:

  • Ordinary PVC 70 C
  • High-temperature PVC 85/90 C*
  • Cross-linked Polyethylene 90 C
  • EPR/HEPR 90 C
  • Thermoplastic Zero halogen low smoke 70 C
  • Thermoset Zero halogen low smoke 90 C

* Some PVC compounds are rated at 105oC but this is usually a short life rating used in some American Standards.

These “continuous conductor operating temperature” ratings are based on the proven experience and reliability of cables over many years of operation.

The “continuous conductor operating temperature” rating is important because, as well as being directly related to the acceptable life of a cable, it is directly related to the Current Rating of a cable.

Current Ratings

Someone once said, “Electric cables are inefficient electric fires”, and when one looks closely at the subject, they are not all that wrong!

All conductors have a measurable value of resistance. When a cable is energised, and a potential difference (voltage difference) exists between points in a circuit, then a current will flow. The mere act of this flow of current having to overcome the resistance of the conductor is a form of work; this work generates heat in the conductor, and this heat must go somewhere.
The insulations, coverings and finishes that we apply to conductors act as heat barriers – thermal insulation – and, to a greater or lesser degree, prevents this heat from freely escaping, radiating, from the conductor. The installation also has an important bearing on the current rating of a cable; any installation where the heat generated cannot freely radiate away from the cable, such as cable in conduit or cables under thermal insulation in attics, must be derated accordingly.

Different sizes of conductors have different resistances, and the various materials we use to cover them have different maximum continuous conductor temperature ratings.

Therefore there is a direct relationship between:

  • Conductor size
  • Insulation and coverings
  • The current Rating of a particular cable

An important point to note is that the maximum continuous conductor temperature is derived from, initially, the ambient air temperature surrounding the cable, to which must be added the temperature rise caused by the flow of current.

It must also be remembered that although the foregoing is true for all types of cable, in practice, it is only applicable to power cables or cables carrying appreciable amounts of current, 10’s or even 100’s of amps. Data, Control and Instrumentation cables, which usually only carry only fractions of an amp, are not normally sized on the current that they will actually carry but on other factors such as voltage drop and mechanical considerations.

Power over the ethernet (POE) is now causing people to look more closely at what current data cables can carry safely without deterioration in transmission characteristics.

Tratos catalogues and web pages contain useful information regarding the temperature rating and current rating of the various cables that we produce; Tratos Engineers are also always available if specific advice is required.

Tratos Energy Cables

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