Cable Ampacity Calculator

Method

The calculator follows the same standards and the same equations as CYMCAP, ETAP Underground Raceway Systems and SKM Cable Ampacity. Every quantity it uses is shown under Calculation steps, so a result can be checked by hand.

Steady-state rating

Standard

IEC 60287-1-1 (losses and the rating equation) and IEC 60287-2-1 (thermal resistances, conduits, duct banks and cables in air). IEEE Std 835 and the Neher-McGrath method give the same results for the same inputs.

Conductor

DC resistance at the operating temperature, then skin and proximity effects (ks = 1; kp = 0.8 for aluminum, 1.0 for copper). Resistance at 20 °C comes from the cable data, or from ICEA/ASTM (1.02 × ρ/A) when the data sheet value is not entered.

Shield losses

Circulating current for shields bonded at both ends (trefoil formula; the worse outer or centre cable for flat formation) plus eddy current in tape shields. Concentric wires and flat straps are taken by their copper area, with the lay length where the data sheet gives one. Single-point or cross-bonded shields carry no circulating current.

Dielectric loss

W_d = ωCU₀²tan δ, at the system voltage entered.

Cable thermal resistances

T1 for the insulation including both semiconducting screens (conductor screen 2.5 K·m/W, as CYMCAP); T2 for the filler of three-conductor cables; T3 for the jacket.

Conduits

Air space T4′ from the IEC 60287-2-1 U, V, Y constants, with the mean air temperature found by iteration; conduit wall T4″.

Earth

T4‴ for each conduit or cable group by the image method, plus the heating from every other conduit. A concrete envelope or thermal backfill of different resistivity adds (ρ_e − ρ_c)·G_b/2π. Optional two-zone soil drying, applied only when the soil next to the bank or cables would pass the critical temperature; otherwise the moist resistivity governs.

Duct banks

The temperature of every conduit is a linear function of the squared currents. The calculator solves it for equal current in all circuits (the hottest conduit reaches its limit) or with each circuit at its own limit. Existing circuits can be held at a known current.

Load profiles

Continuous

Constant current 24 hours a day.

Daily utility load curve

Neher-McGrath: losses beyond the distance Dx (8.3 in for a daily cycle) are reduced by the loss factor μ = 0.3 LF + 0.7 LF². The rating is the daily peak current. Use this for typical feeder load curves.

Block load

Full current for a set number of hours, then no load, repeated every day (battery charge or discharge, pumping). Solved with the IEC 60853-2 transient response repeated for 90 days; the rating brings the conductor to its normal limit at the end of the daily block. The daily-load-factor formula would overstate the rating for these profiles.

Emergency ratings

Emergency, no time limit

Steady-state rating at the emergency temperature of the cable.

Short-time emergency

IEC 60853-2. Each cable is a two-loop thermal network with van Wormer factors, with the conduit wall heat capacity included. The soil responds as a line source with its image (exponential integral). Every circuit carries the preload continuously, then all solved circuits step to the emergency current together. The rating brings the hottest conductor to the emergency temperature at the end of the duration. Resistance is corrected to the preload and emergency temperatures, so a very long duration returns the steady emergency rating.

Limits of use

• Cables in free air or in a riser are rated as one circuit. No short-time credit is calculated in air; use the steady emergency rating.
• Not modelled: magnetic losses in steel conduit and armour, pipe-type and paper-insulated cables, cables in trays or tunnels, and a non-isothermal earth surface.
• Generic cables are typical ICEA S-94-649 constructions. Confirm diameters, neutral and conductor resistance against the manufacturer data sheet before final design.
• Okonite, Southwire, LADWP and Pasadena Water and Power cables come from the MV cable library file in the LIBRARIES folder, which the page reads when it opens; the line under Library shows the revision in use. Opened from a disk, the page uses its built-in copy of the library, and Load library file adds a newer revision. Saved cases keep the data of their cables, and the Validation tab keeps its own copy of the benchmark cables. Data sheets list diameters over the insulation and the insulation screen; the conductor screen, the insulation screen and some jacket thicknesses are derived from them, and conductor diameters not on the sheet are ASTM nominal values. Cable details show the source page and every derived or adjusted value for the selected cable. Armored, aluminum-sheathed, paper-insulated, aerial and 69 kV and above cables are listed in the library but not offered here.
• Nonshielded 2.4 kV cables are rated with the same method, without shield losses. LSZH jackets use the PVC thermal resistivity (5.0 K·m/W).
• The cross-section is drawn to scale and can be zoomed to the individual neutral wires and conductor strands. Cables are drawn resting in the conduit; the rating itself uses the IEC equivalent group diameter. The temperature shading is an illustration from the same superposition, using the mean loss away from the conduits when a load factor applies. Rating values come from the conduit temperatures.
• Conduit fill is the total cable cross-section over the conduit inside area, compared with the NEC Chapter 9 percentages (53 % for one cable, 31 % for two, 40 % for three) as a guide. Utility duct banks fall under the NESC, so the check is information only.
• The 3D cable view is drawn to scale from the dimensions used in the rating. Strand counts follow ASTM class B; tape widths, lay lengths and fillers are typical where the library gives no value.

References

• IEC 60287-1-1 and IEC 60287-2-1, Electric cables - Calculation of the current rating.
• IEC 60853-2:1989, Calculation of the cyclic and emergency current rating of cables (cables above 18/30 kV).
• J.H. Neher and M.H. McGrath, “The calculation of the temperature rise and load capability of cable systems”, AIEE Transactions, 1957.
• IEEE Std 835-1994, IEEE Standard Power Cable Ampacity Tables.
• ICEA S-94-649, Concentric neutral cables rated 5 through 46 kV.
• Okonite Product Data, Section 2 (Medium Voltage Cables 2.4 kV and above), Sheets 1 to 68.
• Southwire medium voltage power cable data sheets (SPEC 46002 to 46805, 48204, 81242, 81264).
• LADWP purchase specifications (Contracts 678A, 402, 2688) and the TO13 CYME 35 kV cable library (Rev B, October 2025).
• Okonite technical data for Pasadena Water and Power, inquiry 61-40486 items 016 and 020 (May 2024).