Certainly, this is no less the case in the UK, where compliance with the IET Wiring Regulations – otherwise commonly referred to as “BS 7671” – is of the utmost importance.
One of the most frequently seen pitfalls in such installations is voltage drop. This term refers to the gradual loss of electrical potential along a cable as a result of its inherent resistance.
In this article, then, we will take a closer look at why voltage drop matters, and how it can impact on your projects. Then, we will outline what steps are necessary to calculate the right cable thickness to keep voltage drop in check.
An Introduction to Voltage Drop, And Why It Happens
Voltage drop is the reduction in electrical potential (voltage) that occurs as current flows through a circuit, due to the resistance of components like wires, connectors, and loads.
This resistance converts some electrical energy into heat. This means the voltage available at the far end of the circuit is lower than at the beginning.
Some people find it useful to think of voltage drop like water pressure reducing as it travels through a long and narrow pipe, with the flow becoming weaker as the water continues its journey.
Voltage drop can result in devices underperforming relative to expectations, such as lights dimming or motors running slowly.
Excessive voltage drop can be a safety risk, too. This is why the aforementioned Wiring Regulations stipulate maximum permissible drops to make sure circuits operate efficiently. These limits include 3% for lighting circuits, and 5% for other power circuits such as sockets and heaters.
Why Does Proper Cable Thickness Matter So Much for Safe Wiring?
When we use the term “cable thickness” in this article, we are referring to its cross-sectional area, or “CSA”. In accordance with UK metric standards, this is typically measured in square millimetres (mm2).
A given wire having a larger CSA means there is more copper (or aluminium) to carry the current with a lower level of resistance. This helps minimise voltage drop and any associated buildup of heat.
By getting the cable sizes right for your project where voltage drop could otherwise be an issue, you will be able to unlock advantages such as:
- Improved safety, because of a decreased risk of overheating leading to fire
- Enhanced efficiency, as it will enable your devices to run at full voltage, thereby cutting energy waste
- Greater longevity, as it will help prevent premature wear on appliances.
What Are the Basic Steps of Calculating Voltage Drop?
To determine the right CSA for your project, you will need to use the relevant voltage-drop formula for single-phase AC circuits, as are common in UK homes at 230V.
This formula can be expressed as VD = 2×L×I×(mV/A/m) / 1000, where:
- VD = voltage drop (in volts)
- L = cable length (one-way, in metres)
- I = load current (in amps, from device ratings)
- mV/A/m = millivolt drop per amp per metre (from BS 7671 tables, based on CSA and material)
The manual calculation process, then, is as follows:
- Determine your load. Sum the current draw – for example, a 2kW heater at 230V will be approximately 8.7A.
- Measure the run. Double for return path in the formula, or use two-way length.
- Consult tables. Use Appendix 4 of BS 7671 for mV/A/m values. Start with a guessed CSA – such as 1.5mm2 for lights – and iterate.
- Check VD. If it is over the limit, increase CSA (such as to 2.5mm2) and recalculate.
- Factor extras. Add 20% to 30% for futureproofing; derate for grouping or ambient heat.
If it is a three-phase or DC circuit that you’re dealing with, make sure you adjust the formulas accordingly, while verifying with the regulations.
The Right Online CSA Calculator Will Help You Ensure Safe Wiring
Once you have used the voltage drop formula or a full cable sizing tool to calculate the required CSA for your application, you will need to select a real-world cable that matches.
The great news is that you can help ensure safe wiring with the cable size and cross-sectional area calculator on the website of electrical component supplier, RS.
This free online tool will help you convert between cable specifications, especially for the multi-strand flexible cables that see widespread use in UK installations. It isn’t a full voltage drop calculator. However, it does complement your sizing by verifying the exact CSA from strand count and diameter.
Drawing upon the capabilities of this online calculator, then, would allow you to ensure your chosen cable is the appropriate thickness to keep resistance low.
