Overhead conductor ampacity
Preview Version 0.1.0How much current a bare overhead conductor can carry in given weather, by the IEEE 738 heat balance.
Steady-state ampacity
1,026A
The line can carry 1,026 A at 100 °C. In 2 ft/s of wind, the air takes away 25.01 W/ft of heat.
Basis · IEEE 738-2023, linerate 5.0.0 (Statnett), pypacity 0.1.3 · 2023 · Cross-checkedCross-checked · Higher wind raises the rating, 1,000 A at 230 kV is 398.4 MVA, and US and SI inputs agree within 0.01%. 50 cases spanning conductor size, surface, temperature limits, weather, elevation, latitude, bearing, date, and hour match two open IEEE 738 implementations, linerate 5.0.0 and pypacity 0.1.3, within 1% (the largest difference is under 0.3%). What this means
Screening estimate: a conductor thermal limit only, not a facility rating; terminal equipment, emergency ratings, and clearances also limit a line. · Example values · Review pending Report a problem
Your conductor
Not listed? Enter its diameter and its AC resistance at two temperatures from the manufacturer's data sheet. The sizes listed come from USDA RUS Bulletin 1724E-200 (public domain)
Ambient-adjusted rating table
The conductor's rating at each air temperature, day and night, under the wind above. It is the conductor's thermal limit only: terminal equipment and emergency ratings, which an Order 881 facility rating must also consider, are not included.
Widen the range to cover your site's historical extremes.
How we got this
Method
At the maximum conductor temperature, the steady-state current is √((convective cooling + radiated heat loss − solar heat gain) ÷ AC resistance), following the IEEE 738-2023 heat balance: forced convection by the larger of its low- and high-wind correlations with the wind-angle factor, or natural convection if larger; radiation from emissivity and the temperatures; clear-sky solar gain from the sun's position, elevation, and line bearing; and resistance interpolated between the two values entered.
Steps
- Resistance at 100 °C, interpolated from the two values entered: 0.15090 Ω/mile.
- Convective cooling: 25.012 W/ft (forced 25.012, natural 12.929; the larger is used), IEEE 738 clause on convective heat loss.
- Radiated heat loss: 11.921 W/ft; solar heat gain: 6.847 W/ft on 2025-06-10 at solar hour 11.
- Ampacity: √((cooling + radiation − solar) ÷ resistance) = 1,026 A.
- Three-phase rating at 230 kV: √3 × 230 kV × 1.026 kA = 408.7 MVA.
- This is the conductor's thermal limit only; terminal equipment, emergency ratings, and clearances also limit a line.
Assumptions
- Weather is steady at the values entered; the rating is for that weather only.
- A clear atmosphere; no ground albedo is added to the solar gain.
- Temperatures are in °C in both unit systems, as IEEE 738 states them.
Sources
- IEEE 738-2023, Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors, 2023, Steady-state heat balance: convective, radiated, and solar heat terms and conductor resistance (cited, not reproduced)
- linerate 5.0.0 (Statnett), an open implementation of IEEE 738 and CIGRE 601, 5.0.0, IEEE738 thermal model, used only to generate the stored reference cases
- pypacity 0.1.3, an open implementation of IEEE 738, 0.1.3, Steady-state rating mode, used only to generate the stored reference cases
Known limits
- Steady state only: no transient or emergency rating.
- Resistance must be entered from the manufacturer's data; no public-domain resistance table exists.
- Clear-sky solar gain; haze, industrial atmosphere, and ground reflection are not modeled.