A cable is sold as "600 volt" or "0.6/1 kV" and everyone reads it as the voltage the cable can handle. That reading is close enough to be useful and wrong enough to cause trouble, because the number is a class designation rather than a working limit, and in the two-number form the pair describes two different duties measured against two different references.
One of those references is decided by the installation rather than by the cable.
The rating is a class, not an operating voltage
The clearest statement of this comes from a definition rather than from a cable standard. The National Electrical Code defines nominal voltage like this:
A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.
Two things follow from that sentence. The value is assigned for the purpose of designating a class, which makes it a labeling device as much as a technical quantity. And the voltage a circuit actually runs at is expected to differ from it, within a range that still permits satisfactory operation. The same definition carries an informational note pointing to ANSI C84.1, Voltage Ratings for Electric Power Systems and Equipment, as the reference for those ranges.
So "600 volt" is the name of a class of system. A cable rated for that class is not thereby declared fit to sit at 600 volts indefinitely, in the way that a temperature rating is not a working temperature.
The two references a cable’s insulation can be measured against
Voltage is defined between two conductors
The next definition is worth reading carefully, because it decides what a single number can even mean:
Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.
Greatest, between any two conductors. On a three-phase four-wire system the same installation contains circuits at different voltages, and the definition’s own informational note says as much, listing three-phase four-wire, single-phase three-wire and three-wire direct current as systems that may have various circuits of various voltages.
That is the reason a cable cannot be described by one voltage figure without ambiguity. The figure has to be paired with a statement of what it is measured against, and there are two candidates.
The number that changes with the grounding arrangement
The third definition is the one that catches people out, because it has two branches and the branch that applies is a property of the installation:
Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.
Read the two branches side by side. In a grounded circuit, the reference is the grounded point, which is the arrangement most people picture when they say "to ground." In an ungrounded circuit there is no grounded point to measure to, so the same term is defined as the greatest voltage between the given conductor and any other conductor.
System arrangement | What "voltage to ground" is measured against |
|---|
Grounded circuit | The point or conductor of the circuit that is grounded |
|---|
Ungrounded circuit | The greatest voltage between the given conductor and any other conductor |
|---|
The consequence is practical rather than academic. An insulation duty expressed as a voltage to ground is not a fixed property of the cable. It is a property of the cable in a particular system, and moving the same cable into an ungrounded arrangement changes what the requirement is asking of it. Anyone specifying a cable from a number labeled "to ground" needs to know which arrangement the number was written for.
The same insulation duty, measured against a different reference depending on how the system is grounded
The two-number form expresses the same two duties
The IEC convention writes the voltage designation as a pair, U0/U, and the meanings line up with the two references above. U0 is the rated voltage between a conductor and earth. U is the rated voltage between conductors.
That is why a low-voltage power cable appears as 0.6/1 kV. The first figure is the earth-referenced duty, 600 volts. The second is the conductor-to-conductor duty, 1000 volts. The form is used consistently in manufacturer catalogs, where products are listed by the designation rather than by a single figure.
Designation | What the first number is | What the second number is |
|---|
U0 | Rated voltage between a conductor and earth | |
|---|
U | | Rated voltage between conductors |
|---|
0.6/1 kV | 600 V to earth | 1000 V between conductors |
|---|
There is a caution attached to that table. The two figures do get transposed in secondary and republished material: at least one published buying guide describes the same 0.6/1 kV construction as 600 volts phase-to-phase and 1000 volts phase-to-earth, which reverses the definition. The pair is short, the numbers are similar in magnitude, and the error survives proofreading because both readings look plausible. A drawing that states which figure is which, rather than relying on the reader to know the convention, removes the ambiguity at no cost.
Where the two systems meet
Buyers rarely work in one convention. A specification may arrive as an IEC-style designation while the installation it goes into is described in North American terms, and the two describe the same insulation duty with different amounts of information attached to them.
The Code’s own nominal-voltage definition shows the inconsistency in its examples. Two of them are written as pairs, 120/240 volts and 480Y/277 volts, which carry a relationship between two voltage references inside the designation itself. The third is written as a single figure, 600 volts. The amount of arrangement captured in a designation therefore varies with the designation, and the one carrying a single number is the one carrying the least.
The instruction is the same in either direction. Ask for the conductor-to-earth figure and the conductor-to-conductor figure as separate items, whatever convention the rest of the document uses, and ask which arrangement the first one was written against. That turns a label into two statements that can be checked against a system, and it costs one line on a drawing.
What the rating does not tell you
Five things are commonly read into a voltage rating and are not there.
It is not a maximum working voltage. The nominal-voltage definition above is explicit that the actual operating voltage varies from nominal, and the rating is the designation of a class rather than a ceiling for the insulation.
It is not a test voltage. The voltages applied during routine testing are set by the cable standard, and they are not equal to the rating. A test is performed at a level chosen to reveal defects, over a specified duration, and the cable is expected to survive it.
It does not set the installation limit on its own. The wiring rules decide what may be installed where, and a cable with an adequate insulation rating can still be inadmissible for a location for reasons that have nothing to do with voltage.
It says nothing about current. Voltage rating and current rating are separate properties on separate mechanisms. The conductor’s cross-section and its thermal environment decide the current a cable can carry, and the insulation’s voltage class is not part of that calculation.
It does not survive contact with the termination. A cable’s insulation rating and the rating of whatever it connects to are separate declarations, and an installation is limited by the lower of the two. A cable rated for a voltage class can sit in a system of that class and still be capped by a component at either end with a lower rating.
What to put on the drawing
Item | Why it needs to be explicit |
|---|
The designation in full, with both numbers | 0.6/1 kV rather than "1 kV" leaves no room to transpose the pair |
|---|
Which number is conductor-to-earth and which is conductor-to-conductor | The convention is standard; the reading of it is not universal |
|---|
The system’s grounding arrangement | It changes what a voltage-to-ground requirement is measured against |
|---|
The standard the designation comes from | The designation is defined by the standard that publishes it, not by the numbers alone |
|---|
The test regime the rating invokes | Routine test levels are set by that standard and are not the same as the rating |
|---|
The temperature rating alongside it | Two independent ratings, often printed together, and neither one implies the other |
|---|
Reading the designation: which number is which, and where the transposition happens
FAQ
Is a 600 volt cable safe to use at 600 volts?
The rating designates a class of system rather than a working ceiling. Under the Code’s own definition, the actual voltage at which a circuit operates is expected to vary from the nominal, and a cable rated for the class is the cable intended for systems of that class. Whether it may be used in a specific installation is a question for the applicable wiring rules.
What does the first number in 0.6/1 kV mean?
The first number is U0, the rated voltage between a conductor and earth, which is 600 volts in that designation. The second number is U, the rated voltage between conductors, which is 1000 volts. Published material transposes the two, which is why a specification should state which is which.
Why does voltage to ground have two definitions?
Because the term is defined against different references depending on the system. For grounded circuits it is measured to the grounded point of the circuit; for ungrounded circuits, where there is no such point, it is the greatest voltage between the given conductor and any other conductor of the circuit.
Is the voltage rating the same as the insulation test voltage?
No. The rating is part of the designation of the cable, while test levels and durations are set by the cable standard that publishes the designation, and the two figures are not the same number.
Conclusion
A voltage rating is a name for a class of system, applied to a cable that has to work in one. Reading it that way explains why the two-number form exists, why the second number is not simply a higher version of the first, and why the definition of voltage to ground splits into two branches that a grounded and an ungrounded installation answer differently.
The number tells you which class the cable belongs to. The arrangement of the system tells you what the insulation is actually being asked to withstand. That is the reason the pair belongs on the drawing in full, rather than abbreviated to whichever figure a reader is expected to infer.
Voltage designation and insulation ratings
ZONG Cable supplies HDMI, USB, network, patch cord, fiber, power cord and communication cable, and can state the voltage designation, the conductor-to-earth and conductor-to-conductor figures, the standard it comes from and the temperature rating separately on the drawing rather than as an undifferentiated pair. If a project crosses between an IEC-style designation and a North American one, or between grounded and ungrounded arrangements, send us the system arrangement and the standard you are working to and we will set the figures out where each one can be checked. The range sits under cable, the batch evidence is described on our test and inspection page, and OEM support covers how ratings are fixed for private-label programs.
Sources
- NFPA 70, National Electrical Code, 2011 edition, Article 100 definitions. Definitions read directly for this article: Voltage (of a circuit), including its informational note on systems having circuits of various voltages; Voltage, Nominal, including the example designations and the informational note referring to ANSI C84.1, Voltage Ratings for Electric Power Systems and Equipment (60 Hz); and Voltage to Ground, including both its grounded-circuit and ungrounded-circuit branches. Full text published by Public.Resource.Org. https://archive.org/details/gov.law.nfpa.nec.2011
- Manufacturer and catalog descriptions of the U0/U designation for low-voltage power cable, giving U0 as the rated voltage between a conductor and earth and U as the rated voltage between conductors, and listing low-voltage power cable as U0/U 0.6/1 kV. https://jindacablegroup.com/cable-vs-wire-difference/ and https://www.cabledatasheet.com/pdf/Power_and_Control_Cables.pdf
- ZONG Cable range. https://zh-zong.com/cable/
- ZONG Cable test and inspection. https://zh-zong.com/test-inspection/
