Ampacity reduction applied to the published value in accordance with the less favorable installation conditions compared to the reference conditions used for the published value. Conductor temperature is increased by higher ambient temperature, closely spaced conductors, inadequate air movement, enclosure mounting, conduit, thermal insulation and some load profiles and the allowable continuous current often needs to be reduced to maintain the conductor within the temperature limits. A cable is more likely to be used if its size or the current listed in a catalog are the only criteria. The final selection must conform to the actual load, the heat dissipation route, the cable’s temperature rating, the voltage drop requirements and protective device coordination.
What Is Cable Current Derating?
Cable current derating is the process of applying a reference current carrying capacity to the thermal conditions the cable will actually experience, otherwise known as “ampacity adjustment” or “application of correction factors”. The exact factors and the sequence of application of them to the reference value is determined by the governing standard (NEC, IEC 60364-5-52, BS 7671 or manufacturer data for the particular construction). There is no one all-encompassing factor that applies to every cable or jurisdiction.
Ampacity and Allowable Current-Carrying Capacity
Ampacity is the maximum value of current that can flow through a cable for a continuous period without exceeding the maximum insulation or conductor temperature as specified. This is not the same as normal load current, short circuit withstand, fuse, or breaker ratings, or voltage drop limits. These are independent design verifications.
Why Heat Determines Cable Current Capacity
Electric currents passing through a conductor generate heat. This heat has to be conducted outwards through the insulation, through the jacket and into the surrounding air, tray, conduit, or soil. The more the current, the greater the temperature rise in the surrounding environment, and the smaller the air circulation, the larger the temperature rise. For instance, a bunch of servo and power cables is installed in a control cabinet adjacent to a drive bank within a factory, and the temperature rises rapidly; the cables are no longer able to dissipate heat as readily as they would in the open air.
Reference Conditions and Real Installation Conditions
The published ampacity tables are based on certain reference conditions—typically an ambient temperature (usually specified at 30 °C in air for many ampacity tables and IEC tables) a small number of conductors that are loaded, a defined conductor spacing, and a specific installation method. Before using a catalog figure, identify those reference conditions. A cable designed for open air free space will not be able to carry the same current in conduit, trunking, insulation or inside a crowded enclosure.
Ambient Temperature and Cable Ampacity
The higher the ambient temperature, the narrower the temperature difference between the environment and the maximum operating temperature of the cable. The nearer that the ambient air is to the insulation limit of the cable, the less additional heat will the cable generate and the lower will be the load current limit.
How Hot Environments Reduce Allowable Current
A cable with a maximum allowable current in a cool environment could have a lower maximum allowable current in an environment where the air is already hot. The appropriate factor for the insulation system, conductor temperature rating and installation method is given in the correction tables published in the applicable standard or by the cable manufacturer. Use those tables instead of a generalized formula.
Temperature Rating Is Not the Same as Ambient Temperature
Don’t assume that a 90 °C conductor rating allows a cable to neglect heat dissipation. The temperature increase available is still 45 K in a 45 °C cabinet, grouping, current loading and limited airflow still play a role. The rating is merely the maximum temperature for continuous use.
Heat Sources, Enclosures, and Ventilation
Local air temperature is elevated by motors, drives, transformers, heaters and process equipment. Poorly ventilated sealed enclosures, thermal insulation and lack of air flow also limit heat removal. Measuring or carefully estimating the ambient temperature at the cable location, and not taking a general room ambient temperature.
Cold Environments and Temperature Correction
There are some standards that permit higher correction factors if the ambient temperature is below the reference value, but the minimum operating and installation temperature of the cable shall not be violated. Some typical scenarios that involve reviewing the upper and lower temperature limits include cold-storage rooms, outdoor mobile machinery and refrigerated warehouses.
Cable Bundling, Grouping, and Mutual Heating
If multiple cables are loaded and the cables are close together, they will heat each other and the surface area available for heat transfer will be decreased. As the arrangement is more restricted, the grouping or formation factor becomes increasingly important.
Why Grouped Cables Run Hotter
All conductors of current generate heat. The heat will not dissipate as easily in a tight bundle or multi-layer tray as it will from a free air isolated cable. The size of the effect is influenced by the number of conductors loaded, their distance apart, their configuration, their installation and their ventilation.
Loaded and Unloaded Conductors
Normally, only conductors which are energized for the conditions being evaluated are counted for grouping purposes. The neutral, protective-earth, control, signal, and spare conductors can be handled differently in accordance with the standard. Always verify definition from the relevant table prior to using a factor.
Bundles, Cable Trays, Conduit, and Trunking
The most effective method of spacing heat dissipation is open air. If cables touch in a bundle, single or multi-layer tray, conduit, trunking and enclosed equipment wiring, then it will alter the thermal path and may need to be corrected in a different way. The exact factor will be taken from the standard with a geometry and cable type that matches.
Duty Cycle, Continuous Loads, and Intermittent Operation
The duty cycle is related to conductor temperature, which depends on the load profile, but the duty cycle should be taken with care. There is the possibility of a different thermal steady state with intermittent loading than with continuous loading; however the proper evaluation will depend upon the peak current, the duration of the cycle, the cooling time, the construction of the cable, and the applicable standard. An average current discount is not generally enough on its own.
Continuous Current vs Intermittent Current
A continuous loading is a loading that is more or less constant for long durations. Intermittent loading consists of short periods of high current followed by lower or no current—such as motor starting, servo acceleration, welding cycles, or solenoid actuation. Both peak current and heat build-up over cycles are important.
Duty Cycle in Motors, Servo Systems, and Automation Equipment
The amount of current that draw in a Servo drive and multi-axis machine varies with acceleration, deceleration, holding and idle. The cable must be sized for the actual current waveform, and not just the nameplate continuous rating, and the thermal time constant of the installation.
Why Intermittent Operation Does Not Automatically Eliminate Derating
Conductor temperature is still raised even with short high current events. If the off time is not long enough, heat will build up in the cable. High cycle or exceptional duty cycles require manufacturer advice, thermal calculation or an examination by a qualified engineer.
Peak Current, Average Current, and Thermal Memory
Peak current causes instantaneous voltage drop and heating. Long-term temperatures are affected by average current. The cable also has a “memory” of the past cycles, the surrounding temperature, and grouping. A machine which has to accelerate and decelerate repeatedly can thus be hotter than might be thought from an average current calculation.
Installation Method and Heat Dissipation
Different currents can be allowed to flow in the same cable when it is installed in free air, conduit, tray, trunking, insulation, soil or in an enclosure. Thermal path from the conductor to the outside is control.
Open Air vs Enclosed Installation
The best heat dissipation is provided by open air spacing. The temperature and air circulation within cabinets, ducts or closed machines are higher. The evaluation must take into account the temperature of the cabinets and the movement of the air inside.
Conduit, Trunking, and Cable Carrier Conditions
The heat transfer is limited in restricted space. The mechanical flex life is not an independent requirement when moving cable carriers, but rather, both thermal and mechanical conditions must be met.
Thermal Insulation and Poorly Ventilated Spaces
Heat dissipation is influenced by soil temperature, thermal resistivity, depth of burial, moisture content, and surrounding circuits. Note that buried-cable calculations are based on different reference conditions and standards from those of the above-ground machine wiring and are included here for completeness sake.
Buried or Soil-Installed Cables
Soil temperature, thermal resistivity, burial depth, moisture, and adjacent circuits all affect heat dissipation. Buried-cable calculations follow different reference conditions and standards from above-ground machine wiring and are mentioned here only for completeness.
Derating Is Not the Same as Voltage Drop or Protection Sizing
A current-derating check is just the first to pass — the rest of the cable design is not satisfied.
Current-Carrying Capacity vs Voltage Drop
Derating is used to provide protection against overheating of the cable. The resistance of conductors and the length of the conductors are the primary factors that control voltage drop. A cable could be thermally OK but give too much voltage drop on a long motor run or be too sensitive on a DC control circuit.
Cable Ampacity vs Short-Circuit Withstand
Normal ampacity tables are not used to determine the short circuit or fault current withstand of the cable. Other calculations and standards are used to determine those limits.
Cable Rating vs Breaker or Fuse Selection
Protective-device coordination is taking into account the final cable rating, the load current (including the starting current), fault conditions and the applicable electrical code. There are no universal rules for sizing the breakers, the sizing depends on the relevant standard of the designing professional in charge.
Common Mistakes When Applying Cable Derating Factors
Using the Catalog Ampacity Without Checking Installation Conditions
Published values are based on definite ambient temperature, spacing, number of conductors loaded and installation method. It is a common cause of overheating to use the figure without confirming those assumptions.
Applying Only the Ambient-Temperature Factor
One way to correct is to raise the ambient. Other factors may include grouping, enclosure, insulation, installation method, ventilation and load profile. List all possible factors.
Counting Every Physical Conductor as a Loaded Conductor
Standards differentiate continuously loaded power conductors from neutral, earth, control and spare conductors. Use the definition given in the governing table.
Ignoring the Hottest Part of the Installation
There could be multiple thermal environments along a cable route. The limiting condition is the most constraining part of the run, not the coolest open tray at the beginning of the run.
Assuming Intermittent Loads Need No Derating
Even with shorter cooling periods, repeated peak loads can still lead to some heat buildup. Check the duty cycle and peak current.
Choosing a Larger Cable Without Reviewing the Whole System
The larger the conductor size, the lower the resistance, the lower the voltage drop, but the size may be different, bend radius may be different, the size of the connectors may be different, flexibility may be different, routing space may be different, and cost may be different. In some instances, a changed cable design is more viable than simply choosing the next higher cable cross-section.
How to Calculate and Document Cable Derating for an OEM Project
The procedure started with the base ampacity and works sequentially through each required correction factor as per the applicable standard or manufacturer’s data which controls the cable.
Step 1 — Define the Load and Operating Profile
Recognize normal current, peak current, continuous and intermittent operation, duty cycle, starting current and load diversity. Sizing the cable based on the actual equipment profile.
Step 2 — Define the Cable and Reference Rating
Detail conductor material and size, conductor insulation temperature rating, number of conductors loaded, cable construction, and the reference installation condition for which the conductor rating was published. Refer to the manufacturer’s technical data of that specific product.
Step 3 — Identify the Correction Factors
List ambient temperature, grouping, spacing, installation method, enclosure, thermal insulation, soil conditions (if applicable), and ventilation. Obtain the numerical factors from the applicable standard or verified manufacturer tables.
Step 4 — Apply the Factors and Check the Result
Record ambient temperature, grouping, spacing, installation method, enclosure, thermal insulation, soil conditions (where applicable) and ventilation. Use standard or manufacturer verified tables for numerical factors.
Step 5 — Check Voltage Drop, Protection, and Physical Installation
If allowed in the governing method, multiply the reference ampacity by the product of the factors. Compare the allowable current obtained with the design load. Any non-standard or safety critical installations should be assessed by a suitably qualified engineer.
Cable Derating Must Reflect the Real Installation
The cable current capacity is not only affected by the conductor size but also by the ambient temperature, grouping, installation, ventilation, load profile, and the thermal limit of the insulation system. A good industrial design utilizes proven data, proven correction factors, actual duty-cycle information and a thorough review of voltage drop and protection. If the thermal/environmental/and mechanical demands for an OEM machine cannot be satisfied by standard cables from the catalog, then application specific cable engineering support is the next practical course of action for power, servo, motor and control circuits.
