LV Cable Ampacity Calculator

Method

Every cable gets two ratings when Neher-McGrath is switched on: the NEC table value (310.15) and the calculated value under engineering supervision (310.14(B)). Every quantity is shown under the calculation sections of the Calculator, so a result can be checked by hand.

NEC table method

Base value

Table 310.16 for conductors in raceway, cable or earth, and Table 310.17 for single conductors in free air with at least one diameter of clearance, in the ampacity column chosen (60, 75 or 90 °C), for 30 °C ambient. Conductors entered as spaced but closer than that are rated as flat and touching.

Ambient

Table 310.15(B)(1)(1), or the equation of 310.15(B)(1) for the exact ambient. Underground installations use the earth temperature. Raceways and cables on a rooftop, in sun and less than 3/4 in. above the roof, add 33 °C, 310.15(B)(2), except Type XHHW-2.

Adjustment

Table 310.15(C)(1) for more than three current-carrying conductors. The count is every conductor in the raceway, or in a multiconductor cable. A neutral counts when it carries the unbalanced current of a 2-phase-plus-neutral circuit or of a mostly nonlinear load, 310.15(E). Equipment grounding conductors do not count, 310.15(F). Direct-buried circuits count by circuit unless they are marked as laid together without spacing. A smaller neutral that carries current is rated with the same factors and governs when lower.

Terminations

Correction and adjustment apply to the column chosen; the result is then limited to the Table 310.16 value in the termination column, 110.14(C) and 310.15(A). Table 310.16 is used for this limit also when the conductor is rated from Table 310.17, because equipment terminals are listed with Table 310.16 conductor sizes.

Not included

Annex B ampacity tables, the 310.15(C)(1) exception for nipples under 24 in., cable trays (Article 392), and 240.4(D) protection limits for small conductors, which are shown as a note only.

Neher-McGrath, NEC 310.14(B)

Rating equation

I = √[(T_c − (T_a + ΔT_d)) / (R_dc(1 + Y_c) R_ca)], with ΔT_d = 0 below 2 kV. The calculator writes the same balance in the IEC 60287 form so that several circuits heat each other: each conductor temperature is a linear function of the squared currents of every circuit.

Conductor

DC resistance from NEC Chapter 9 Table 8 at 75 °C, corrected to the operating temperature. Skin and proximity effects from IEC 60287-1-1 (k_s = 1; k_p = 1 copper, 0.8 aluminum). In steel conduit the eddy factors are increased by 1.5 and the conduit loss is added with the IEC pipe-type formulas. A loaded neutral carries the phase current.

Cable

T1 from the insulation wall: THHN as PVC (5.0 K·m/W) plus the nylon jacket (4.0), XHHW-2 and RHW-2/USE-2 as XLPE (3.5). Multiconductor TC-ER cables use the IEC geometric factor for the cores and fillers and the UL 1277 jacket thickness. Diameters come from NEC Chapter 9 Table 5 and 5A, and from manufacturer data for 2 kV and PV wire.

Conduit

Air space T4′ = U / [1 + 0.1(V + Yθ_m)D_e], with the IEC constants for plastic and metal conduit and the mean air temperature found by iteration. Two or more cables in one conduit use the Neher-McGrath equivalent diameter (1.65, 2.15, 2.5 times the cable diameter for two to four cables; the enclosing circle beyond four). Conduit wall T4″ from its resistivity and dimensions.

Earth

Kennelly line source with its image for each conduit or cable group. Direct-buried circuits use the IEC formulas for trefoil, flat touching and spaced cables. A concrete envelope adds (ρ_e − ρ_c)G_b/2π from the IEC geometric factor of the bank. Every conduit and every bank heats the others by the image method.

Load factor

Losses beyond D_x (8.3 in. for a daily cycle) are reduced by μ = 0.3 LF + 0.7 LF². For spaced cables each cable has its own D_x zone. The rating is then the daily peak current.

Air

Surface resistance from the IEC 60287-2-1 heat transfer coefficient (Z, E, g for one cable, two touching, trefoil and flat), solved directly for the surface temperature rise. Optional solar heating, with absorptivity of the jacket or conduit; a rooftop run is always taken to be in sun. Each conduit or circuit in air is rated alone.

Rating modes

“Each cable at its temperature limit” solves all circuits together, each reaching its own limit; a circuit that cannot reach it because of its neighbors is reported as having no capacity. “Temperatures at the entered loads” returns the conductor temperatures for the loads in Step 1.

Limits of use

• Sizes 14 AWG to 1000 kcmil. Cable trays, thermal backfill, soil drying and emergency ratings are not modeled.
• In air, the IEC heat-transfer coefficients assume a dark surface (emissivity about 0.9). For large conductors the calculated ratings in air are 15 to 20 % above Tables 310.16 and 310.17; the termination column usually governs.
• Groups in air (risers, racks) are not heated by each other. Use the NEC adjustment for bundled raceways and cables.
• More than four cables in one conduit use the enclosing circle of the cables, which is conservative for loose bundles.
• Steel conduit losses use the IEC pipe-type formulas, written for larger pipes. They are small for LV circuits but should be confirmed for large single conductors in steel conduit.
• Thermal resistivity of nylon and PVC are typical values. Confirm cable diameters against the manufacturer data sheet before final design.
• Validation tab: the calculation is compared live with NEC Annex B Table B.2(7) (Figure B.2(2), detail 1 to 3) and with published commercial program results.

References

• NFPA 70, National Electrical Code, 2023: 110.14(C), 240.4(D), 310.14, 310.15, Tables 310.16 and 310.17, Chapter 9 Tables 5, 5A and 8, Informative Annex B.
• J.H. Neher and M.H. McGrath, “The calculation of the temperature rise and load capability of cable systems”, AIEE Transactions, 1957.
• IEC 60287-1-1 and IEC 60287-2-1, Electric cables - Calculation of the current rating.
• IEEE Std 835-1994, IEEE Standard Power Cable Ampacity Tables.
• UL 1277, Electrical Power and Control Tray Cables; UL 44 and UL 83, thermoset- and thermoplastic-insulated wires.