When it comes to electrical installations, selecting the correct cable size is crucial for ensuring both safety and efficiency. Choosing the right cable type isn’t just about picking a length and diameter – it involves considering a range of factors such as current capacity, voltage drop, installation conditions, and more.
Whether you’re an experienced electrician or a DIY enthusiast, understanding how to properly calculate cable size is essential for any electrical project.
In this guide, we’ll walk you through the importance of cable sizing, the key factors to consider, and provide you with a step-by-step process for determining the right cable size for your needs.
What is cable size?
To a novice, cable size may just sound like the length and diameter of a cable. However, those electrically-minded will tell you that cable size actually refers to the physical and electrical characteristics of an electrical cable, which determine its suitability for specific applications.
Why is cable size important?
You may be wondering why cable size is important. Finding the proper cable size that suits the specific application of the cable ensures:
1. Safety
Accurate cable size helps to avoid electrical fires and ensures proper cable insulation.
2. Efficiency
Proper cable size supports minimising energy loss, due to resistance.
3. Longevity
Calculating the correct cable size prevents premature cable wear, and helps to reduce maintenance costs.
4. Compliance
Using a cable with the correct cable size for its application meets electrical code requirements, such as BS 7671: The IET Wiring Regulations.
Key factors in cable sizing
Cable sizing, as we previously mentioned, refers to the physical and electrical characteristics of an electrical cable. This includes:
1. Load current
Load current refers to how much current (measured in amps) the cable must carry.
2. Voltage drop
Voltage drop is the reduction in electrical voltage that occurs as electric current flows through the cable due to its resistance. Excessive voltage drop can cause equipment to operate inefficiently or malfunction.
3. Cable length
Longer cable runs have higher resistance, which increases voltage drop. To compensate, a larger cable size (with a lower resistance per meter) may be required to meet voltage drop limits.
4. Ambient conditions
When calculating cable size, you need to consider the conditions of the cable in use, such as temperature, buried cables, or exposure to direct sunlight. Here, you may need to look into high temperature cables or harsh environment cables.
5. Conductor material
The material used in the cable conductor can have an impact on cable size – such as copper vs. aluminium – due to their conductivity levels and cost implications.
6. Installation method
How and where is the cable going to be installed? Different installation methods, such as open-air, conduit, and buried methods, can have an effect on cable sizing.
How to calculate a cable size
If you’re not an electrician and just want to figure out what size cable you need, here’s a simplified step-by-step guide:
Step 1: Figure out the power of what you’re plugging in
First, look at the devices or appliances you’re using. Every device has a power label, usually in watts (W) or kilowatts (kW).
Example: a heater might say 2000 W, and a fridge might say 500 W.
Add up the total power of all the devices you’ll use at the same time.
Example: Heater (2000 W) + Fridge (500 W) = 2500 W.
If the power is in watts, divide it by the voltage in your house (usually 230V in the UK) to get the current in amps: Power (watts) / Voltage (volts) = Current (amps).
Example: 2500 W / 230 V = 10.87 amps. This then tells you the cable needs to handle around 11 amps.
Step 2: Measure the distance
Then, you need to measure how far the cable will go, in meters. If the cable goes back, like in some circuits, double the distance.
Example: From your circuit breaker to the appliance might be 30 meters. If the cable goes back, double it. So, 30 x 2 = 60 meters.
Step 3: Check voltage drop
As electricity travels through the cable, it “loses” a bit of power. This is called voltage drop. Long cables tend to lose more power, as do thin cables.
You don’t want your appliance to get less electricity than it needs. So, you’ll need to pick a cable that keeps this loss small, usually less than 5% of your voltage (e.g. 11.5 V for 230 V).
Step 4: Pick a cable material
Cables are usually made of copper or aluminum. Copper tends to be better and smaller, but more expensive, whereas aluminum tends to be cheaper but thicker. For most home setups, copper is the way to go.
Step 5: Use a cable size chart
You don’t need to calculate everything yourself. Electricians and cable manufacturers use charts or a BS 7671 cable size calculator that tell you the cable size (in mm²) based on:
- Current (amps): How much electricity your devices need.
- Distance (meters): Longer distances need bigger cables.
- Type of installation: Is the cable in the air, buried, or in a wall?
Example: If your device needs 11 amps, and the cable runs 60 meters, you might need a 4mm² copper cable. If it’s a shorter distance, say 10 meters, a smaller 2.5mm² cable might work.
Step 6: Adjust for heat or other factors
If the cable is in a hot place (like an attic) or bundled with other cables, it gets hotter. Hot cables carry less electricity safely, so you may need a bigger size. Refer to the chart or calculator, or professional, for advice.
Step 7: Double-check the rules
In the UK, cables must meet the electrical code BS 7671: The IET Wiring Regulations. This code tells you the minimum cable size to use for safety. Always make sure your choice meets the code.
Example in simple terms
- Device: 2000 W heater.
- Voltage: 230V.
- Current = 2000 / 230 = 8.7 amps
- Cable length: 50 meters one-way (100 meters total).
For 8.7 amps over 100 meters, you might need a 4mm² copper cable. If it’s a hot attic, you might choose a bigger 6mm² cable for safety.
Common mistakes to avoid when calculating cable size
Properly calculating cable size is crucial to ensure safety, efficiency, and compliance with regulations. Here are common mistakes to avoid when calculating cable size and why they can lead to problems:
1. Ignoring voltage drop
When not accounting for voltage drop, especially for long cable runs, excessive voltage drop can lead to equipment malfunction, reduced efficiency, or failure to meet regulatory limits. For example, motors may overheat or fail to start if the voltage at their terminals drops too low.
2. Overlooking current-carrying capacity
When selecting a cable based solely on its cross-sectional area (the mm²) without checking its current-carrying capacity, cables may overheat. This is because they are not rated to handle the current load, which can cause insulation degradation, fire hazards, or circuit failures.
3. Failing to consider installation conditions
Using current ratings from tables or charts without adjusting for installation conditions can reduce the cable’s ability to dissipate heat, decreasing its current-carrying capacity and risking overheating.
4. Neglecting short-circuit performance
If you fail to check whether the cable can withstand short-circuit currents for the required duration, it can fail catastrophically during a short circuit, causing damage or safety hazards.
5. Incorrect calculation of load demand
Underestimating the total load or not accounting for future load increases can have hazardous effects. Under-sizing cables leads to overheating and potential failure, whilst over-sizing can increase costs unnecessarily.
6. Disregarding reactive loads
By ignoring the effects of power factor in circuits with inductive or capacitive loads, reactive loads can increase the apparent current, which may require larger cables to handle the increased current.
7. Using Incorrect standards or codes
Not referring to the appropriate standards (BS 7671 in the UK) can result in unsafe installations, legal penalties, or the need for costly rework.
8. Misjudging cable length
Underestimating the length of the cable run and its impact on resistance and voltage drop causes higher resistance, leading to excessive voltage drop if not accounted for.
9. Overlooking economic factors
When prioritising initial cost savings by choosing smaller cables, smaller cables can lead to higher energy losses (due to increased resistance), resulting in greater long-term operating costs.
Frequently asked questions about cable sizing
How do I account for future load expansion in cable sizing?
To account for future load expansion in cable sizing, add a safety margin to the current load when sizing cables. For example, if the current load is 80% of the cable’s capacity, this leaves room for future expansion.
What is the difference between single-core and multi-core cables?
Single-core cables have one conductor and are used for straightforward circuits like power distribution. Multi-core cables, on the other hand, contain multiple conductors, typically used for complex installations like control panels or devices requiring multiple connections.
Do harmonics affect cable sizing?
Harmonics from nonlinear loads (such as variable frequency drives and LED lighting) increase the effective current in the cable. This may require upsizing the cable to prevent overheating and energy loss.
What happens if I oversize a cable?
Oversizing cables increases material costs and installation difficulties. Whilst it may improve voltage drop and energy loss performance, it is generally unnecessary unless future load increases are expected.
When it comes to cables and cable manufacturing, we’re the experts. To learn more about what type of cable you need, explore more resources like this available on our blog, or contact our helpful team today for tailored advice and support.
