How is the conductor selected?

The tool steps through standard MV copper sizes and picks the smallest whose ampacity for the chosen installation method meets or exceeds the load current. Ampacities follow representative NEC Table 310.60 and IEEE Std 835 values for shielded medium-voltage cable, which differ by direct-buried, conduit, and open-tray conditions.

Why does the installation method change the size?

Heat dissipation depends on the surroundings. A directly buried cable in soil cools differently than one in an underground duct or open tray, so each method has its own ampacity. Conduit installations generally carry the least current for a given size, which is why the method selector matters.

How is voltage drop calculated?

For a three-phase circuit the drop is the square root of 3 times the current times the run length times the cable impedance, using the per-1000-foot resistance and reactance with the load power factor. The result is shown in volts and as a percentage of the line-to-line voltage, with a 3% target.

Do I still need to apply correction factors?

Yes. The listed ampacities are single-circuit baseline values. You must apply corrections for ambient earth or air temperature, grouping of multiple circuits, burial depth, and soil thermal resistivity. In hot or congested installations the corrected ampacity can be significantly lower, requiring a larger conductor.

Does this replace an engineered cable study?

No. Medium-voltage systems require attention to short-circuit withstand, shield grounding and sizing, terminations, and coordination that this tool does not address. Use it for preliminary sizing and verify the final design against IEEE Std 835, the NEC, and the cable manufacturer's published ampacity tables.

What is the Medium-Voltage Cable Sizing Calculator (5–35 kV)?

Selects the minimum copper conductor for medium-voltage shielded cable (5/8/15/25/35 kV class) from NEC 310.60 and IEEE Std 835 ampacity values for direct-buried, conduit, and tray installations, then checks voltage drop at the maximum load current. It runs free in your browser on Gera Tools, with nothing uploaded.

Medium-Voltage Cable Sizing Calculator (5–35 kV)

Name: Medium-Voltage Cable Sizing Calculator (5–35 kV)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Sizing cable for primary distribution

Medium-voltage feeders — the 5 kV to 35 kV cables that move power between substations, switchgear, and large loads — must carry the load current without overheating and deliver acceptable voltage at the far end. Both checks depend on the conductor size and how the cable is installed. This calculator sizes a copper MV conductor for ampacity, then verifies voltage drop.

How it works

Sizing happens in two steps. First, ampacity: the tool selects the smallest standard copper size whose rated current for the chosen installation method — direct buried, in conduit, or in cable tray — meets or exceeds the load. These ampacities follow representative NEC Table 310.60 and IEEE Std 835 values for shielded MV cable.

Second, voltage drop. For a three-phase circuit:

Vdrop = sqrt(3) × I × L_kft × (R·cosθ + X·sinθ)

where R and X are the cable resistance and reactance per 1000 feet and the angle comes from the power factor. The drop is reported in volts and as a percent of the line-to-line voltage, checked against a 3% target. If the smallest ampacity-adequate conductor exceeds 3% drop, step up a size.

Why installation method matters so much for MV cable

Heat is the limiting factor in cable ampacity. A conductor carrying current generates heat in proportion to its resistance and the square of the current (I²R losses). How quickly that heat dissipates into the surroundings determines how much current the cable can safely carry continuously.

Direct buried. Soil acts as the thermal medium. Sandy dry soil dissipates heat poorly; moist dense soil conducts better. The thermal resistivity of the soil significantly affects the final number, which is why NEC and IEEE 835 corrections for soil thermal resistivity are important on long rural runs.

In conduit (underground duct). The air gap between cable and duct wall, plus the duct wall itself, adds thermal resistance compared to direct burial in good soil. This is why conduit installation generally yields the lowest ampacity for a given conductor size — the cable has to be derated relative to direct burial.

In cable tray. Open cable tray allows air convection around the cable, which can provide better cooling than underground duct installations. Tray ampacities are typically closer to direct-buried values for well-ventilated horizontal trays.

Voltage class and insulation level

The cable voltage class (5, 8, 15, 25, or 35 kV) affects insulation thickness, which in turn affects the cable’s overall diameter and weight but has only a secondary effect on conductor resistance. Ampacity is driven more by conductor cross-section and installation method than by voltage class. However, selecting the correct voltage class is critical for insulation integrity — undersizing the class creates overstress on the insulation that leads to premature failure. Always size the voltage class to meet or exceed the circuit’s maximum line-to-ground voltage plus any relevant overvoltage margin.

Correction factors that must be applied separately

The bare ampacities from NEC 310.60 and IEEE 835 are baseline values for defined reference conditions. Real installations require corrections for:

Ambient earth or air temperature — reference conditions assume a specific earth temperature (typically 20°C) and air temperature. Higher ambient temperatures reduce ampacity.
More than one circuit in a duct bank — mutual heating between adjacent circuits is substantial in underground duct banks and significantly reduces each circuit’s allowable current.
Burial depth — deeper burial generally means poorer heat dissipation in most soil conditions.
Soil thermal resistivity — this varies widely with soil type and moisture content and is the most significant correction for long direct-buried runs.

Failure to apply these corrections can result in chronic thermal stress, insulation degradation, and eventual cable failure.

Worked example and notes

A 260 A load in conduit on a 15 kV system over 500 ft at 0.9 power factor selects a conductor sized for that ampacity and shows a voltage drop well under 3%. If the run were extended to 2,000 ft at the same current, voltage drop would become the governing constraint and a larger conductor would be selected to keep the drop within the 3% target.

Always apply ambient, grouping, burial-depth, and soil thermal-resistivity corrections to the listed ampacities, and confirm short-circuit withstand, shield sizing, and grounding separately. This is a preliminary sizing aid, not a substitute for an engineered cable study per IEEE Std 835 and the NEC.