Quick answer: Cable ampacity is the current a specified cable can carry continuously under stated installation and environmental conditions without exceeding its permitted conductor temperature. Conductor size alone does not determine an ampere rating. Start with a rating for the actual cable and installation method, then apply only the corrections required by the governing standard or manufacturer data. Finally check terminations, voltage drop, protective-device coordination and short-circuit duty.
This guide is for engineers and buyers specifying industrial power cables. It explains the calculation workflow and the information TEBAOFLEX needs for a project review; it is not a universal ampacity chart or a substitute for an installation design.
What Determines Cable Current-Carrying Capacity?
The conductor produces heat while the insulation, sheath and surroundings determine how readily that heat escapes. A copper conductor generally has lower resistance than an aluminum conductor of the same nominal area, but conductivity cannot be converted directly into a fixed ampacity multiplier. The complete cable construction and installation conditions matter.
| Input to confirm | Why it matters to ampacity | What to record |
|---|---|---|
| Conductor and cable construction | Material, cross-section, number of loaded cores and cable layers affect loss and heat flow. | Product model, conductor size and core arrangement |
| Permitted conductor and termination temperatures | The lowest applicable equipment or connection limit can constrain the usable rating. | Cable marking or datasheet, terminal rating and governing rule |
| Installation method | Air, tray, conduit, duct and buried routes have different reference conditions. | Route drawing, enclosure, spacing and burial details |
| Ambient conditions | Air temperature or soil conditions change heat dissipation. | Design temperature; soil thermal resistivity if buried |
| Grouping and load profile | Adjacent loaded circuits and duty cycle affect heating. | Loaded conductor count, spacing and continuous/intermittent duty |
| Electrical system | AC losses, screens and armour can matter, especially in medium-voltage designs. | AC/DC, frequency, voltage, bonding and sheath arrangement |
Do not infer a current rating from the insulation name alone. EPR, XLPE, PVC and silicone constructions can have different permitted temperatures and installation rules; the exact cable datasheet and connection limits must be checked. Similarly, stranding class by itself does not justify a universal percentage increase or decrease in ampacity.
How to Calculate Cable Ampacity for a Real Installation
Step 1: Select the applicable calculation route
For many low-voltage installations, the applicable edition of IEC 60364-5-52 or the local code provides reference installation methods and current-carrying-capacity tables. For a project-specific steady-state cable calculation, IEC 60287-1-1:2023 sets out current-rating equations and loss calculations. In North America, the applicable NFPA 70 Article 310 ampacity workflow uses installation-specific tables, correction and adjustment rules. These routes are not interchangeable tables for the same cable.
IEC 60287 accounts for construction, losses, thermal resistances and surroundings. The often-shown expression based only on conductor-to-ambient temperature difference, resistance and one overall thermal resistance is a teaching simplification, not the complete IEC 60287 equation for every cable or installation. Screen, sheath and armour losses and dielectric losses may have to be included where applicable.
Step 2: Establish a reference rating and its conditions
Record the exact source of the reference ampacity: cable datasheet or standard table, conductor material and size, number of loaded conductors, installation method, ambient temperature, and permitted conductor temperature. A number copied from a different construction or installation route is not a valid starting point.
Step 3: Apply only the relevant corrections
Where the selected method permits it, an adjusted rating may be expressed as Iadjusted = Ireference × ktemperature × kgrouping × …. The factors and order of application must come from the same applicable design method. Some installation conditions require a different base table or a full calculation rather than a simple multiplier; do not combine IEC and NEC factors.
Arithmetic illustration only: If a hypothetical reference rating is 125 A and a project’s verified method specifies independent factors of 0.90 and 0.80, the arithmetic gives 125 × 0.90 × 0.80 = 90 A. These assumed numbers are not a rating or prescribed correction factors for any TEBAOFLEX product. The designer still has to verify whether the chosen factors, termination limits and protection rules apply.
Step 4: Check the complete circuit
Compare the corrected capacity with the design load using the applicable continuous-load and protective-device rules. Then check voltage drop for the full cable run, short-circuit withstand, installation constraints and mechanical duty. A cable that passes the thermal check may still need a larger conductor because of voltage drop or fault duty. For the broader sizing sequence, see amps to cable size.
Derating Factors: Which Conditions Must Be Checked?
There is no universal ambient-temperature or grouping table that can be pasted into every cable specification. A factor is meaningful only with its reference method, installation geometry, insulation temperature column and edition. Use the relevant tables or project calculation for the actual configuration.
| Check | Evidence needed | Common selection error |
|---|---|---|
| Air or soil temperature | Design temperature and the reference temperature used by the rating source | Applying an air factor to a buried-cable rating |
| Grouped circuits | Loaded circuits, cable spacing, tray/duct geometry and route length | Using a touching-cable factor for a spaced arrangement |
| Terminations | Equipment instructions and terminal temperature limits | Assuming a high-temperature insulation rating controls the whole circuit |
| Buried route | Soil thermal resistivity, depth, duct arrangement and nearby heat sources | Using a free-air rating without a burial calculation |
| AC cable construction | Frequency, screens, armour and bonding arrangement | Ignoring applicable sheath, armour or dielectric losses |
Ampacity Is Not the Same as Short-Circuit Rating
A continuous current rating controls steady operating temperature. A short-circuit check addresses the much higher current for a limited fault-clearing time. The adiabatic relation I²t ≤ k²S² can be useful when its assumptions are met, but the permissible k value depends on conductor material, initial and final temperatures and insulation system. Large conductors, parallel paths and longer clearing times may require a more detailed method. Confirm the prospective fault current and protective-device clearing time rather than applying one set of k values to every cable.
Industrial Cable Examples: Start With the Product, Not a Generic Chart
TEBAOFLEX’s DLO 2 kV flexible power cable, RG7H1R airport-lighting primary cable and JZ-500 PVC control cable illustrate different constructions and duties. Their nominal voltages, conductor arrangements and installation contexts differ, so an ampacity copied from one product cannot be transferred to another. Use the selected product’s confirmed data and project route as the inputs to the governing calculation.
What to Send for a Cable-Sizing Review
Send the load current and duty cycle, system voltage and AC frequency or DC supply, cable length, conductor preference, installation drawing, maximum ambient or soil conditions, grouped-circuit details, terminal limits and fault-clearing data. State the required standard and destination market. This lets TEBAOFLEX review a suitable product configuration and identify the calculations that the project engineer must approve. For related reading, use the cable size selection guide and cable resistance formula.
Frequently Asked Questions
Can cable size alone tell me its ampacity?
No. Conductor area is important, but the cable construction, installation, ambient conditions, loaded cores, terminations and applicable rules determine the usable current rating.
Can I multiply any published ampacity by a derating factor?
No. First confirm the published rating’s reference conditions and whether the governing method calls for a factor, a different base table or a project-specific calculation.
Is a 90°C insulation system always usable at a 90°C ampacity?
No. Termination and equipment limits, cable marking and the applicable electrical rules may impose a lower usable rating.
Sources and Project Review
Technical framework: IEC 60287-1-1:2023, IEC 60364-5-52 and NFPA’s NEC ampacity workflow. The complete applicable standards, equipment instructions and product datasheet govern the final design.
For a project-specific cable proposal, send TEBAOFLEX your installation and load requirements. We can review the product construction and the information needed for engineering approval; a quotation or current rating should not be inferred from this guide alone.
