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.

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

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.

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%. 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.