Understanding how cables safely carry electrical current is essential for reliable installations. Derating factors adjust a cable’s current rating so it stays safe under different conditions like high temperatures, being buried underground, or grouped with other cables. Without proper derating, cables can overheat, leading to damage or even fire risks.
This guide explains why derating is necessary, when it must be applied, and how to ensure that chosen cable sizes will handle their loads safely. Readers will also learn about key ratings, common adjustment factors, and where to find official guidance.
Key Takeaways
Derating factors help prevent overheating by adjusting cable ratings.
Choosing the correct cable size involves checking installation conditions.
Using the right tables and correction factors is vital for safety.
What Derating Means
Derating is the process of reducing the rated capacity of a cable to ensure safe operation under real-world conditions. A cable’s current rating is usually based on standard or ideal situations, but these are rarely met in practice.
The derating factor is a multiplier applied to the cable’s rated capacity. This adjusts the cable’s allowable current to reflect the actual installation environment, such as high temperatures or grouped cables.
For example:
Condition | Example Derating Factor |
Ambient temp: 35°C | 0.94 |
Cable in thermal insulation | 0.80 |
Multiple cables grouped | 0.85 |
If more than one factor applies, they are multiplied together. For instance, two derating factors of 0.9 and 0.85 would be combined as: 0.9 × 0.85 = 0.765.
Derating is needed because factors like ambient temperature, burial depth, soil type, and proximity to other cables can raise cable temperature. Higher temperatures reduce a cable’s ability to carry current safely. Without derating, cables might overheat, causing damage or even electrical hazards.
In short, the derating factor ensures that cables operate within their thermal limits by adjusting the current rating for actual conditions, keeping installations safe and reliable.
When You Need To Derate
Cables must be derated when real-world installation conditions differ from the standard assumptions used in current rating tables. If these conditions are not considered, the cable may overheat and fail.
Common reasons to apply derating factors include:
High ambient temperatures
Cables grouped closely together
Buried cables with poor soil thermal properties
Cables installed in enclosures or trays
Presence of direct sunlight
Additional harmonic currents
Even small changes in environment can affect a cable's ability to carry current safely. For example, a cable that is buried deeper or run through insulation will lose heat less effectively.
A simple table highlights some situations where derating is likely required:
Condition | Typical Impact |
High ambient temperature | Lower current rating |
Multiple cables bunched | Lower current rating |
Buried at greater depth | Lower current rating |
Installed in conduit | Lower current rating |
Exposed to sunlight | Lower current rating |
Short, isolated cable runs | Often, no derating needed |
In some cases, derating may not be necessary. For instance, if a cable operates at less than 35% of its rated capacity or is only grouped over a very short distance, the standard rating can be used.
They should always check the installation conditions against the standard tables and guidelines. Applying the correct derating factors helps prevent overheating and ensures long-term safety.
What Can Go Wrong If You Don’t
Ignoring derating factors can cause several safety and performance problems for electrical cables. Overloading cables without proper adjustment for conditions like high temperature or multiple groupings may lead to insulation breakdown. This can increase the risk of short circuits or electric shock.
Cables that carry more current than allowed can heat up quickly. Over time, this extra heat can damage cable materials, leading to a shorter service life and possible failure during operation. If cables overheat, their protective device, such as a fuse or circuit breaker, might not trip as intended because it’s sized for a higher rating under standard conditions.
Possible issues include:
Excessive power loss in the cable, which wastes energy and increases running costs.
Higher chance of fires if cable temperatures rise beyond safe limits.
Difficulty in fault finding, as damaged cables often fail in unpredictable ways.
Reduced efficiency of electrical systems due to voltage drop in overloaded cables.
Table: Problems Caused by Not Applying Derating Factors
Issue | Possible Result |
Overheating | Insulation damage, fire risk |
Protection device mismatch | Delayed fault clearing |
Extra power loss | Higher electricity costs |
Cable lifespan shortened | More frequent replacements |
Without proper derating, problems may not appear right away, but the risk remains. Proper planning helps keep equipment safe and reliable.
The Basics You’ll Use
When choosing a cable, it’s important to know its current rating. The current rating tells how much electrical current a cable can safely carry without overheating.
Derating factors adjust the current rating to account for real-life conditions. If a cable is surrounded by insulation, placed in a hot environment, or bunched with other cables, derating is needed.
Some of the most common derating factors include:
Ambient temperature
Number of cables grouped together
Installation method (buried, in air, in conduit)
Depth of burial
Soil thermal resistivity (for underground cables)
Here is a simple example table for reference:
Factor | Typical Derating Value |
Ambient temp (40°C) | 0.87 |
3 cables grouped | 0.70 |
Buried at 0.7 m depth | 0.93 |
To use derating factors, multiply the standard current rating by each relevant factor. For example, if a cable’s current rating is 50A and the combined derating factors are 0.87 and 0.70, use 50 × 0.87 × 0.70 = 30.45A.
Voltage drop is also important. If the voltage falls too much along the cable, equipment may fail or operate unsafely. Regulations set maximum limits for voltage drop, so both derating and voltage drop checks are essential during design.
The Key Ratings (Ib, In, Iz)
There are three main ratings used in cable current calculations: Ib, In, and Iz.
Ib is the design current. This is the current that the connected equipment is expected to draw during normal operation.
In is the nominal current rating of the protective device (such as a fuse or circuit breaker). It should be selected based on the design current (Ib), and must not be less than Ib.
Iz is the current-carrying capacity of the cable under the installation conditions. This is the maximum continuous current that the cable can carry without overheating or damaging its insulation.
A summary table is shown below:
Symbol | Description | Meaning |
Ib | Design Current | Expected current of the circuit load |
In | Nominal Rating of Protective Device | Current setting for fuse or breaker |
Iz | Current-Carrying Capacity of Cable | Maximum allowable current for the cable |
The basic rule is as follows:
Ib ≤ In ≤ Iz
Cable selection uses these ratings to help ensure safety and performance. Derating factors are used to adjust Iz, accounting for installation and environmental conditions, so the chosen cable is suitable for the actual conditions on site.
Accurate calculation and comparison of Ib, In, and Iz is key to proper cable design. This protects the circuit and maintains reliable operation.
Correction Factors And Where To Find Them (BS 7671 Appendix 4)
Correction factors, also known as rating factors, change the current-carrying capacity of a cable. These factors adjust cable ratings to suit real-world installation conditions, making them vital for safe and reliable electrical design.
BS 7671 Appendix 4 is the main source for these correction factors. It contains tables and guidance for calculating rated current capacity under different environments.
Some common rating factors found in Appendix 4 include:
Adjustments are often required if cables are close together or installed in hotter surroundings, as heat affects their performance.
A typical formula used is:
It = Iz × Ca × Cg × Cs × Cd
Where:
It = Tabulated current rating (from Appendix 4 tables)
Iz = Required cable current-carrying capacity
Ca, Cg, Cs, Cd = Correction factors based on conditions
The tables in Appendix 4 show these correction factors alongside the standard values for different cable types. Electricians and designers consult these tables during cable selection to ensure compliance with BS 7671.
Using Appendix 4 helps users select the right cable size and apply all required derating. It also helps avoid overheating, voltage drop, and other problems that result from incorrect cable choice.
The Factors That Reduce A Cable’s Rating
The ability of a cable to carry current safely depends on various real-world conditions. Factors like how a cable is installed, surrounding temperature, the number of cables close together, and cable material can all lower its current rating.
How It’s Installed (Clipped, Conduit/Trunking, In Insulation, Buried)
How a cable is installed changes its heat dissipation and maximum safe current. A cable clipped to a wall in free air allows more heat to escape, so it can carry more current.
Cables run in conduit, trunking, or ducting have less air flow around them. This limits heat loss and lowers their current rating. The effect is even greater with cables embedded in or passing through thermal insulation because insulation traps heat around the cable.
Buried cables face extra challenges. The soil’s ability to transfer heat (thermal resistivity) and the cable's burial depth both matter. Deeper burial means less heat can escape. For example, single core and multicore cables at a 0.5 metre burial depth may require a significant derating factor in calculations.
Temperature Around The Cable (Air And Soil)
Ambient temperature affects how cables handle heat. Higher air or soil temperatures mean the cable can’t release heat as efficiently. When the temperature near a cable is higher than the standard assumed in tables (often 30°C for air, 25°C for soil), the cable's rating must be reduced.
Table: Example of Derating Factors for Ambient Temperature
Ambient Temp (°C) | Typical Derating Factor (PVC Cable) |
20 | 1.08 |
25 | 1.04 |
30 | 1.00 |
35 | 0.94 |
40 | 0.87 |
Soil with high thermal resistance also causes buried cables to run hotter. Wet soils (low thermal resistance) are better at drawing away heat, while dry soils need more derating.
Cables Grouped Together
When several cables run close together, such as on trays, in bunches, or in multicores, they heat each other up. This mutual heating can be significant with large numbers of cables, requiring derating by a grouping factor.
For example, installing four single core cables together on a tray may require multiplying the current rating by as little as 0.65 for each cable. The effect is stronger if many current-carrying conductors are grouped, or if cables are bunched inside a conduit or buried together.
Grouping factors are detailed in standards, and must be used for both single and multicore cables, as well as multiple circuits installed in parallel.
Cable Type And The Load (PVC Vs XLPE, Continuous Duty, Harmonics)
The cable material sets its heat limits. PVC cables have a lower maximum working temperature (often 70°C), while XLPE cables can usually work up to 90°C. XLPE cables may therefore be rated higher for the same size and conditions.
The nature of the load also matters. A cable carrying its maximum current all the time (“continuous duty”) may need more derating than one loaded only part of the time. Where cables carry harmonic currents, such as with non-linear loads (computers, LED drivers), more heat is generated within the cable. This is especially important with third harmonics, which increase joule losses and require extra derating.
Variables like duty cycle, harmonics, and cable insulation all need to be considered to ensure cables are not overloaded.
Picking A Size That’s Safe
Choosing the right cable size means considering how the real-world environment affects how much current the cable can safely carry. Using derating factors makes sure the cable is not overloaded under different conditions, which helps reduce the risk of overheating and damage.
Simple Step-By-Step
To select a safe cable size, follow a sequence of steps based on the expected current load and the specific installation situation:
Find the Standard Rating
Use the manufacturer’s table or national standard to get the base current rating for the cable under ideal conditions. Make sure to note the cable type and installation method.
Determine Key Derating Factors
Identify all relevant derating factors, such as high ambient temperature, thermal insulation, grouping with other cables, or if the cable is buried. Each factor usually has a number (for example, 0.85 for insulation).
Apply the Derating Calculation
Multiply all applicable derating factors together. Then, multiply this total derating factor by the standard cable rating.
Final Rating = Standard Rating × Total Derating Factor
Check Against the Load
The cable’s final amperage rating must be greater than or equal to the expected load. If not, move up to a larger cable size and repeat the process.
Tip: Use a table to keep track:
Factor | Example Derating |
High Temperature | 0.90 |
Grouping | 0.80 |
Insulation | 0.85 |
Total | 0.61 |
Quick Example: EV Charger On A Grouped Run
Imagine running an electric vehicle (EV) charging cable along with two other circuits on a cable tray in a garage. The base rating of the chosen cable is 40 A.
Grouping: Since three circuits are together, use a grouping derating factor (e.g., 0.70).
Ambient Temperature: If the garage reaches 35°C, apply a temperature derating, say 0.95.
Multiply these factors:
0.70 (group) × 0.95 (temperature) = 0.665 total derating.
Calculate the derated current:
40 A × 0.665 ≈ 26.6 A.
If the EV charger draws 32 A, this cable would not be safe. The installer should select the next cable size up and repeat the calculation. This ensures the cable stays within safe temperature limits, preventing premature ageing and safety hazards.
Avoid These Common Mistakes
Applying derating factors incorrectly can lead to overheating, early cable failure, or even safety hazards. The most frequent mistakes occur when installers overlook specific installation conditions or misapply standard tables.
Using Clipped-Direct Ratings In Insulation
Clipped-direct current ratings are for cables fixed directly to a wall or surface, exposed to free air on at least one side. Some mistakenly use these ratings for cables run through insulation or enclosed in insulated voids, which is incorrect.
Insulation traps heat, reducing a cable’s ability to cool. If a clipped-direct rating is used in this situation, the cable may overheat. British Standard BS 7671 requires a lower rating when cables are surrounded by thermal insulation. For example, a 2.5 mm² copper cable can carry about 27 A clipped-direct, but this may drop to only 13.5 A when entirely surrounded by insulation.
Key points to check:
Confirm the method of installation
Refer to tables for cables under insulation, not free-air ratings
Always apply the manufacturer’s specified derating factors where insulation is present
Incorrect use of clipped-direct ratings in insulated spaces can be dangerous and is a common cause of overheating and damage.
Ignoring Grouping In Trunking Or Conduit
Cables run together in trunking or conduit generate more heat due to mutual proximity. Overlooking grouping factors leads to the risk of exceeding safe current levels, since the heat cannot easily escape.
When several cables are bunched or grouped:
The total heat from all cables increases
The heat is not dispersed as efficiently
Each cable’s current-carrying ability must be derated
Typical grouping correction factors are found in IEC and BS tables. As an example, with six circuits bunched together, the correction factor may reduce the current rating to 0.57 of the original.
It is important to use the correct factor for the specific number of cables and the arrangement inside trunking or conduit. Failure to account for this can over-stress both cables and containment, potentially leading to faults, insulation breakdown, or fire.
Frequently Asked Questions
What is the derating factor of cable rating?
The derating factor is a multiplier used to adjust the rated capacity of a cable. This adjustment is needed when real-world conditions, like temperature or installation method, differ from standardised test conditions. Applying the correct derating factor helps ensure cables do not exceed their temperature limits.
What are the factors affecting current rating of cables?
Current rating is affected by ambient temperature, burial depth, soil thermal resistivity, the presence of thermal insulation, exposure to sunlight, harmonic currents, and whether cables are bunched together. These factors can reduce or occasionally increase how much current a cable safely carries.
How is the derating factor calculated?
Derating factors are typically taken from standards or manufacturer tables based on observed conditions. To get the overall derating, each relevant factor is multiplied together. For example, temperature, grouping, and installation method each have their own factor, which are then combined.
What are the factors for derating ampacity?
The main factors for derating ampacity are ambient temperature, depth of burial, soil thermal resistivity, mutual heating from grouped cables, installation in enclosures or trays, and thermal insulation. Each can influence heat dissipation and change the safe current capacity.
What is the formula for derating a cable?
To calculate the derated current rating:
Derated current = Standard current rating × All applicable derating factors
First, find the standard (tabulated) current rating. Then multiply by each derating factor, such as for temperature or grouping.
How to select cable size from current rating?
To select a cable size, start with the load current and the standard current rating tables from the relevant standard. Apply all necessary derating factors based on actual installation conditions. Choose a cable with a derated current rating equal to or just above the calculated requirement.
Can I use a 2.5mm cable on a 32amp?
A typical 2.5mm² cable should not be used for a continuous 32-amp load. Under most installation conditions, 2.5mm² cable is rated for much lower current, especially when factors like temperature and grouping are considered. Using undersized cables can lead to overheating and safety hazards. Always check the current rating and apply the correct derating factors before use.
