Electrical Basics · Cabling · Ampacity · Load Current
Cable Size for Load Current: 5 Costly Mistakes Engineers Must Avoid
A cable rated for 20 amps on a cool day is not necessarily rated for 20 amps in a hot, crowded conduit. This guide explains how to choose cable size for load current properly, including a graph showing how ampacity drops with temperature, and a live cable sizing calculator.
What Ampacity Actually Means
Ampacity is the maximum current a cable can carry continuously without exceeding its insulation's safe temperature limit. A cable's ampacity is not a single fixed number, it depends heavily on the conditions it is installed in, including ambient temperature and how many other current carrying conductors sit nearby in the same conduit or tray.
Picking the right cable size for load current is one of the most consequential decisions in electrical design, since an undersized cable overheats, degrades its insulation, and can become a fire risk long before any breaker trips, especially once fault current is added to the equation.
The 5 Costly Mistakes to Avoid When Sizing Cable
The Two Main Derating Factors
As surrounding air or ground temperature rises above the table's reference point, the cable has less thermal headroom before reaching its insulation limit.
Typical impact: ampacity can drop 10 to 30 percent or more in hot environments.
Each additional current carrying conductor bundled nearby adds heat that the group must dissipate together, lowering the safe current for every cable in the bundle.
Typical impact: ampacity can drop significantly once more than three or four conductors are grouped.
How Ampacity Falls as Temperature Rises
The Cable Sizing Formula
Adjusted Ampacity = Table Ampacity × Temperature Derating Factor × Grouping Derating Factor
Where:
Load Current = actual current the circuit will carry
Safety Margin = typically 1.25 for continuous loads
Temperature and Grouping Derating Factors = from manufacturer or code derating tables
Example: Load current 40 A, margin 1.25, base table ampacity 63 A Temperature derate 0.82, grouping derate 0.80 Required ampacity = 40 × 1.25 = 50 A Adjusted table ampacity = 63 × 0.82 × 0.80 ≈ 41.3 A, which is not enough In this example, the cable that looked adequate before derating falls short once temperature and grouping are applied, meaning a larger conductor size is needed to safely carry the required current under real installation conditions.
Common Insulation Types and Temperature Rating
| Insulation Type | Typical Max Conductor Temperature | Common Use |
|---|---|---|
| PVC | 70°C | General purpose building wiring |
| XLPE | 90°C | Industrial and higher current circuits |
| EPR | 90°C | Flexible cabling, harsher environments |
| Mineral insulated | Higher, fire rated applications | Fire alarm and emergency circuits |
Where Careful Cable Sizing Matters Most
Continuous, high current loads make derating especially important here.
High ambient temperature reduces usable ampacity significantly in these spaces.
Grouping derating becomes significant once many cables share the same tray.
Direct sun exposure adds real heat load beyond the surrounding air temperature.
Soil thermal resistivity and burial depth both affect underground cable ampacity.
Voltage drop, not just ampacity, often drives the real cable size decision here.
Sizing Cable Correctly
- Apply both temperature and grouping derating: together, not just one or the other.
- Add a safety margin above bare load current: for continuous loads and future growth.
- Check voltage drop separately from ampacity: a thermally adequate cable can still fail on voltage drop.
- Match insulation temperature rating to the correct ampacity table: mixing tables gives a wrong answer.
- Don't size cable using nameplate current alone: without checking real installation conditions.
- Don't ignore bundling in a crowded tray or conduit: heat builds up faster than it looks.
- Don't assume a cable rated for a cool climate works everywhere: ambient temperature genuinely changes the safe rating.
- Don't skip rechecking cable size after adding more circuits nearby: new neighbors change the grouping derate.
Cable Sizing Calculator
Enter load current and derating factors to check if a candidate cable's ampacity is adequate.
Quick FAQs: Cable Size for Load Current
External References
- Wikipedia: Ampacity
- NFPA 70: National Electrical Code, Conductor Ampacity Tables
- IEC 60364: Electrical Installations Standard
What we learn today
- Ampacity depends heavily on installation conditions, not just the cable's raw table value.
- Ambient temperature and grouping or bundling are the two main derating factors that reduce a cable's safe current.
- Choosing cable size for load current correctly means applying a safety margin and both derating factors, then checking voltage drop separately.
- Matching insulation temperature rating to the correct ampacity table is essential, since different insulation types allow different limits.