Why is the conductor sized at 125% of motor FLA?

NEC 430.22(A) requires branch-circuit conductors for a single continuous-duty motor to be rated at no less than 125% of the motor full-load current. This margin covers continuous heating and the slight overload the motor can run at indefinitely.

Do VFD output cables really need a special shielded cable?

Yes for most installations. NEMA 250 and IEEE 1100 recommend a listed VFD or Type WTTC cable with a continuous tape or braid shield to contain common-mode noise and reflected-wave voltage. Standard THHN in conduit lets high-frequency energy radiate and can damage motor bearings.

Why size the ground larger than the NEC 250.122 table value?

The VFD ground carries high-frequency leakage current, not just fault current. A symmetrical drain matched to the phase conductor size, or three symmetrical grounds, keeps the high-frequency return impedance low and reduces shaft voltage and EMI.

How do ambient and bundling factors change the answer?

High ambient temperature and many bundled conductors both reduce a conductor's safe ampacity. The tool divides the 125% requirement by these factors so the conductor you choose, after derating, still clears the required current.

Does this size the overload and short-circuit protection too?

No. This tool sizes the branch-circuit conductor and recommends cable type and grounding only. Motor overload relays, short-circuit/ground-fault protection, and disconnect sizing follow separate NEC 430 Part III and IV rules.

VFD Motor Cable Derating & Shielded Cable Calculator

Conductors feeding a variable-frequency drive carry more than ordinary motor current — they also carry high-frequency switching energy that ordinary cable in conduit handles badly. This calculator sizes the branch-circuit conductor to NEC 430.22(A), applies the ambient and bundling derates, and then recommends a shielded VFD cable and a high-frequency drain ground so the installation is both code-compliant and electrically clean.

How it works

The required conductor ampacity is built up in three steps and then matched to a real conductor:

required      = motor FLA × 1.25            (NEC 430.22(A))
derated req.  = required / (ambient factor × bundling factor)
conductor     = smallest 75°C Cu size whose table ampacity ≥ derated req.

The ambient correction factors come from NEC 310.15(B)(1) for 75°C insulation, and the bundling adjustment factors from NEC 310.15(C)(1) for the count of current-carrying conductors. Dividing the 125% requirement by these factors is the correct way to make sure the installed conductor still clears the required load after it is derated for its real-world conditions.

Shielded cable and grounding notes

For the cable type, NEMA 250 and IEEE 1100 recommended practice is a listed VFD or Type WTTC cable with a continuous shield of at least 85% coverage and a symmetrical drain. The drive ground should be sized to carry high-frequency leakage current — match it to the phase-conductor gauge or use three symmetrical grounds — rather than the smaller equipment-grounding conductor from NEC Table 250.122. A low-impedance HF return path is what prevents motor bearing damage and nuisance EMI on nearby control wiring.

Example and tips

A 21 A motor at 30 °C with three current-carrying conductors needs 21 × 1.25 = 26.25 A, which 10 AWG copper (35 A) covers comfortably. Raise the ambient to 40 °C and the same load needs 26.25 / 0.88 = 29.8 A, still met by 10 AWG. Always keep the VFD cable run as short and direct as practical, bond the shield at both ends, and verify the result against the adopted NEC edition and the drive manufacturer’s installation manual.