How do I determine the rating of a cable?
The continuous current rating of a cable is determined by the ability of the cable to dissipate the heat generated by the current passing through its conductor. It depends on a number of parameters, but the most important are the:
- Conductors’ DC resistance;
- Thermal resistance of the insulating sheathing materials; and
- Ambient conditions of the environment where the cable is installed (for example the surrounding air temperature).
For standard cables such as PVC building wire, XLPE SDI or circular cables (all rated 0.6/1kV), the ratings have all been calculated and are tabulated in Australian Standard AS/NZS 3008.1.1. This standard includes various circuit configurations such as single and three phase and various installation arrangement such as “in air”, “direct buried in ground”, “in ducts”, etc.
Olex has included the same tables from AS/NZS 3008.1.1 in the Standard Product Search our customer’s convenience.
For ratings of HV XLPE cables, please see the attached HV PDF in the products sections.
If ratings are required for non-standard cables or for cables in non-standard installation or environmental conditions, Olex have specialised software available for performing the rating calculation to suit specific customer requirements.
How do I measure the amount of current a cable can carry over a particular distance?
A current which flows in a conductor will cause a voltage drop over the conductor’s length. This voltage drop is due to the resistance of the conductor. The Wiring Rules AS/NZS 3000:2000 states a maximum limit for voltage drop of 5% for low voltage systems. Therefore there is a simple calculation which can be done that relates the percentage volt drop, the cable length, and the voltage drop factor for a particular cable. This calculation and voltage drop factors (in mV/A.m –milliVolt per Amp metre) are given in AS/NZS 3008.1.1. The formulae for how much current is:
I = 5 x V x 1
100 L x Vc
Where:
I = cable current that produces the maximum voltage drop
V = system voltage (ie normally 415V for 3 phase and 240V for single phase)
Vc = mV/A.m volt drop factor for cable from AS/NZS 3008.1.1
L = circuit length
Note: In addition to performing this voltage drop calculation, the continuous current rating of the cable must be checked.
How do I determine the size of cable required for a particular current of a distance?
A current which flows in a conductor will cause a voltage drop over the length of conductor due too its resistance. The cable size can be determined to find Vc as shown below:
Vc = 5 x V x 1
100 L x I
When Vc is found, it is then necessary to look up the appropriate table of volt drop factors in AS/NZS 3008.1.1 to find a cable size which has a lower voltage drop factor than that just calculated. After obtaining the cable size, the continuous current rating must again be checked. If the cable size meets or exceeds the required current, I, then this is the answer. If not, the cable size must be increased until the current, I, is obtained.
Are orange circular PVC cables suitable for use outdoors exposed to sunlight? Is the orange PVC “UV stabilised”?
Yes, orange circular PVC are suitable for use outdoors, since they have an outer sheath which protects the core insulation. Whilst some fading of the colour of the sheath is expected over time, this is restricted to just the outer surface layer and does not penetrate into the bulk of the sheath material. Therefore the sheath will continue to provide protection of the core insulation. In addition AS/NZS 3000:2000 states in Note 1 to Clause 3.3.12 - “Sheathed cables exposed to direct sunlight do not require further protection from ultraviolet radiation as the sheath is considered to provide the necessary protection.”
The term “UV stabilised” has an understanding in the cable industry to mean the addition of a minimum amount of carbon black to a material (2%). In reality, this is only essential for Polyethylene exposed to UV, such as XLPE Aerial Bundled cable or in high density PE in High Voltage cables. This will obviously render the material black and is regarded as the best means of UV protection of a material. Even black PVC sheathed cables, using only 0.5% carbon black, will not suffer from fading. The orange circular PVC cable does not contain carbon black, but the material is still “UV resistant”.
What is the correct lug for use on compacted conductors?
Compacted conductors have a smaller diameter compared to standard stranded conductors, but their nominal cross sectional area is equivalent and a lug designated for the same cross sectional area must be used on the compacted conductor. Even though the lug will appear to fit more loosely, when crimped in accordance with the lug manufacturer’s recommendations, the end result is the equivalent to the crimping of a standard conductor. Compacting the conductor by the cable manufacturer is just the action of pushing all the wires together, getting rid of the air gaps, ahead of time.
Note: The general practice is for lug manufacturers to manufacture a range of lug sizes that suits both the compacted and standard conductors. However, if there are any concerns, please consult your lug manufacturer.
What is the correct lug for use on flexible conductors?
Flexible conductors have a larger diameter compared to a nominal stranded conductor of the same nominal cross-sectional area. Since the diameter of the flexible conductor is larger, it is difficult to insert into the barrel of a lug of the same cross-sectional area.
One method of terminating is by using the next largest lug size and crimping with a half hex and flat dye. Alternatively, a special lug suited to flexible conductors can be sought.
Further information on crimping techniques can be obtained from lug manufacturers.