How to use this calculator
- Enter the base ampacity. Use the conductor ampacity from the adopted code table, manufacturer data or project schedule before correction factors.
- Set the terminal cap. Enter the maximum ampacity allowed by the terminal, equipment or temperature-rating limit.
- Enter load and multiplier. Use actual load current and the required load multiplier for the screening case.
- Apply derating factors. Enter ambient-temperature correction, current-carrying conductor adjustment and any other project derating factor.
- Check utilization and margin. Use the usable ampacity, design load, utilization and margin to decide whether the upstream conductor selection still clears the screen.
How it works
Ampacity derating is a multiplier stack. Start with the base conductor ampacity from the adopted table or manufacturer data, then multiply by the correction and adjustment factors that apply: I_adj = I_base x F_temp x F_count x F_other The usable ampacity is then limited by the terminal or equipment rating: I_use = min(I_adj, I_term)
The calculator also multiplies the entered load current by your load multiplier: I_design = I_load x M_load It compares that design load with usable ampacity and reports utilization, remaining ampacity margin, total derating percentage and the maximum load current that would fit under the same multiplier.
Use this after selecting a candidate conductor from the applicable table. Then run the voltage drop calculator for long runs, the conduit fill calculator for raceway area, the box fill calculator for enclosures and the transformer kVA calculator for source capacity. If the same route needs bends, use the conduit bend offset calculator and conduit pull tension calculator.
Worked example
Verified against the live calculator
Suppose the selected conductor has a base ampacity of 100 A, the
ambient correction factor is 0.91, the conductor-count adjustment
factor is 0.80, no other derating applies, and the terminal cap is
also 100 A. The adjusted ampacity is
100 x 0.91 x 0.80 = 72.8 A. A 55 A load with a
125% multiplier needs 68.75 A, leaving about
4.05 A of margin in this arithmetic screen.
Reference data
This page intentionally does not publish a conductor ampacity table. Enter the table value and factors from the adopted code edition, equipment instructions or manufacturer data, then use this calculator for the multiplier arithmetic.
| Input | Role |
|---|---|
| Base ampacity | Table or manufacturer value before correction/adjustment. |
| Ambient correction factor | Temperature correction factor from the adopted source. |
| Conductor-count factor | Adjustment for current-carrying conductors in a raceway, cable or bundle. |
| Other derating factor | Optional project, rooftop, cable-tray, manufacturer or local factor. |
| Terminal/equipment cap | Upper limit from terminal or equipment temperature rating. |
Source: User-entered factors from the adopted electrical code, equipment listing or manufacturer data.
Frequently asked questions
What is ampacity derating?
Ampacity derating reduces a conductor ampacity by multiplying the base table ampacity by correction and adjustment factors, such as ambient temperature and current-carrying conductor count. This calculator multiplies the factors you enter and compares the result with your load.
Why does this calculator ask for base ampacity instead of wire size?
Wire-size ampacity is code-edition, conductor-type, insulation-temperature, termination-temperature and installation dependent. Entering the base ampacity keeps the calculator from pretending one frozen table applies to every jurisdiction and installation.
How do I use the terminal or equipment cap?
Enter the maximum ampacity allowed by the equipment termination or temperature column. The usable ampacity is the smaller of the derated conductor ampacity and that terminal/equipment cap.
What load multiplier should I use?
Use 100% for a simple non-continuous load screen or the multiplier required by your design method for continuous load, future allowance or project rules. The default 125% is only a common screening value.
Does this produce a code-approved conductor size?
No. It is a derating arithmetic screen. Final conductor size, insulation type, ampacity table, terminal rating, overcurrent protection, grounding, bundling, raceway conditions and local amendments need the adopted code and qualified review.
Method & assumptions
- Uses user-entered base ampacity and user-entered derating factors; it does not select a wire size.
- Uses the smaller of the derated conductor ampacity and the entered terminal/equipment cap.
- Does not size overcurrent protection, grounding, parallel conductors, neutral counting, rooftop adders, cable-tray details or raceway thermal conditions.
- Final work needs the adopted code edition, local amendments, listed equipment instructions and qualified electrical review.