Wire Gauge Selection – The Do’s and Don’ts

How to correctly size wires and why it is important

The cross-sectional size of a wire is a very important factor which needs to be considered when designing electrical circuits, especially when said wire needs to conduct large quantities of current. This is due to the fact that any physical wire will have some measure of resistance and will thus dissipate energy in the form of heat. It is thus the job of any circuit designer that this dissipated energy is within safe levels and that the load being supplied will operate efficiently.

 

Selecting an Appropriate Gauge for your Current Rating

As stated before the cross sectional area of your chosen wire will determine how much current a cable can safely handle. This is due to the following relationship between resistance (R), length of cable (l), resistivity (ρ) and cross-sectional area (A).

From the equation it is easy to determine that at larger cross-sectional areas, a cable will have a smaller resistance and thus dissipate less energy.  It is thus a generally accepted consensus that larger diameter wires can conduct larger values of current. This brings us to wire gauge tables. These tables are used to determine the size of a cable needed for a certain application.

Referring to the below table we can see that if we have a current rating of 27A or smaller we can use the 2.5mm2 wire gauge.

[Data obtained from South Ocean’s Bare Copper Earth Wire]

Sizing the cable for Voltage Drop

Just as in any resistor, the wire will also have a voltage drop developed across it. This voltage drop can thus hinder certain loads if it is too large. The general rule of thumb is to size a wire such that it will have a voltage drop of less than 2.5% of the source voltage. The size of the voltage drop depends on the length of cable as well as the magnitude of the current carried by the cable. To calculate the voltage drop developed, we simply take the voltage drop constant given by the table, multiply it by the current and length of the cable.

For example if we have a cable which is 10m long and conducts 20A and has a cross-section of 2.5mm2. We have a voltage drop of 3.6V.

(Voltage Drop = 20A x 10m x 0.018V

Working Example:

We want our cabling system to supply a load of 10kW at a voltage of 230V and a cable length of 10m. This gives us a current of 43.5A when we divide the power by the voltage.

We should however design the cable to carry an extra 20% of current in the case of an emergency. We thus design the cable to carry a current of 52.5A.

The choice of a 10mm2 wire should be sufficient for our needs.

We now need to see if the voltage drop is within sufficient limits.

Let’s calculate the voltage drop across the wire:

The voltage developed across the cable is thus within acceptable limits as it is less than 2.5% of the source voltage.

For further information contact Joshb@switchman.com

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.

Jannes is completing a 6 week learnership at Switchboard Group.

jannes9000@gmail.com

 

How to Select the Right Circuit Breaker for your Installation?

Selecting the correct Circuit Breaker (CB) for your distribution panel is crucial  for the longevity of the installation as well as the safety of those maintaining and occupying the premises. This article addresses the selection of key breaker attributes such as voltage, current and kA rating.

Rating Considerations:

Circuit Breaker Voltage Rating

The voltage rating of a CB is determined by the highest voltage that can be applied  across any two conductors in the circuit.  It is important to select a circuit breaker with enough voltage capacity to meet the end application. A single phase AC circuit in South Africa is generally rated at 230V  and a single pole CB rated at 230V can be used. A 3 phase AC circuit operates at 400V and requires a Triple Pole CB rated at 400V.

Circuit Breaker Current Rating

The next rating to consider is the amperage or ‘operating current’ of the breaker. CB’s are designed to operate at 100 percent of the required load . However,  in order to offset the effects of heat generated by the system, it is good practice to select a CB at approximately 125 percent of the required load.

For example: If a supply of 250A is available from the transformer, the breaker of choice for the main incomer should be rated at 250A in order to protect the transformer. However, the feeder breakers feeding a 25A load should be rated at 32A.

[Photo Taken at Switchboard Manufacturers Johannesburg]

Circuit Breaker kA Rating

Finally the ‘kA rating’ or ‘fault level’/’rupturing capacity’ of the CB should be taken into account. The kA rating of the CB indicates the maximum short circuit current that the CB can withstand without arcing or catastrophic failure. This current can be upwards of 100 times the required load and has the potential to  cause major damage to property and personnel.

For Example: A circuit breaker rated at ‘6kA’ means that the circuit breaker can withstand 6,000 amps of current during the brief time it takes to trip.

Why is it so important to choose the correct kA rating?

If the short circuit current is greater than what the CB can withstand, the contacts in the CB can weld together,  preventing it from tripping.  Another possibility is that the CB can explode, spewing dangerous plasma.

Under Rated Circuit Breaker

[Breaker fitted to a DB with an Under rated fault level]

So how do I calculate the correct kA?

The maximum current that can flow through a circuit is determined by the size of the transformer feeding the circuit as well as the length of the cable run from the transformer. This is often called the downstream short circuit current. This will determine the maximum kA rating required for the main circuit breaker.

For example: A 500kVA transformer that has a short circuit current of 35kA at its terminals. The cable run from the transformer to the main breaker is 10m and is run with 90mm2 cable. The resistance in the cable limits how much current comes from the transformer, and so after calculations it was determined that the short circuit current at the end of the cable would be 26kA. In this case, a 20kA circuit breaker cannot be used in the installation.

Switchboard Manufacturers
Distribution Panel

[Photo Taken at Switchboard Manufacturers Johannesburg]

SABS Approved Dealers:

When selecting a CB, it is vital for it to be SABS or IEC approved. This provides the assurance that the CB’s have been tested to strict quality standards and will operate in a safe manner as required. Well known brands such as ABB, Schneider and CBI are all SABS approved and are regarded as high quality devices. Switchboard Group is a registered supplier of these products and the leading manufacturer of LV panels is South Africa.

Conclusion:

In conclusion a CB should be selected based on the nominal current, kA rating, number of poles required and whether the CB is SABS approved.

Author: Brendon Swanepoel

2nd Year Electrical Engineering Student, University of the Witwatersrand

Brendon is completing Switchboard Group’s 6 week Learnership and Training program offered to students looking to further their practical skills.

Empowering South Africa’s youth.