NEC 695.7 / NFPA 20 §9.4 — Starting & Running Voltage Drop
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Starting — controller line terminals (15% max)
—%
—
Running — controller load terminals (5% max)
—%
—
Motor & Load Data
Basis for both starting and running checks below
Interpolated by HP & voltage — preliminary only, verify against the actual motor/pump submittal nameplate FLA.
Added to starting current for worst-case controller line-terminal check.
Preferred — use the value stamped on the motor nameplate or pump submittal.
Across-the-line uses the full locked-rotor current above for every starting-condition calculation below (starting voltage drop, disconnect/OCPD sizing, generator starting kVA). Soft start scales that same locked-rotor current down by a percentage reduction (Advanced Settings). Solid-state/VFD instead starts from near-zero frequency and voltage, so its starting current is modeled as a percentage of FLA (also set in Advanced Settings) rather than a reduced fraction of locked-rotor current.
Typical induction motor LRC PF ≈ 0.3–0.5 lagging.
Typical NEMA fire pump motor full-load PF ≈ 0.85–0.90.
Feeder Diagram
Source → feeder segment(s) → fire pump controller · click a value to edit it
Feeder — Utility Source to Controller
One physical feeder, evaluated at both starting and running current · NEC 695.7(A)/(B)
Same voltage basis used for both checks below — also editable directly on the diagram above, under the source symbol.
Segment 1
Above 600V, this segment is automatically fixed to #2 AWG medium-voltage cable (see rating on the diagram) rather than user-selected — length and sets are still editable below.
Above 600V, this segment is automatically fixed to #2 AWG medium-voltage cable (see rating on the diagram) rather than user-selected — length and sets are still editable below.
The disconnect/OCPD rating at this junction is calculated automatically from the fire pump's locked-rotor current (NEC 695.4(B)(1)) — see the diagram above and the results below. Remove this junction with the "−" button on the diagram, or the Remove button above.
The installed transformer's kVA and nameplate %Z; blank uses the minimum kVA (125% of FLA) and a typical %Z (shown). The transformer counts toward the starting and running voltage drop, and often dominates it.
Secondary/motor-side conductors, sized from 125% of the fire pump FLA at the transformer's secondary voltage — unlike Segments 1 and 2, not reduced for the source-side current, since this segment carries the actual motor-side current. Removed automatically when the transformer is removed.
Results
Both checks below apply to the same feeder, summed across all active segments
Starting voltage drop
—%
Within limit
⏚ SourceController ▷
15%
0%
Current used: starting current per the selected starting method (Motor & Load Data) + jockey. Exception: the 15% limit does not apply to emergency-run mechanical starting (NEC 695.7(A) Exception).
Running voltage drop
—%
Within limit
⏚ SourceController ▷
5%
0%
Current used: 115% × motor FLA (NEC 695.7(B)).
NEC 695.6(B)(1) minimum conductor ampacity
Required ≥ 125% × (fire pump FLA + jockey FLA), checked independently against each segment's selected conductor.
Minimum conductor size finder
Holds every other segment fixed and steps up through the table (same material/method as the selected segment) to find the smallest size for that segment that brings starting VD ≤ 15%, running VD ≤ 5%, and 695.6(B)(1) ampacity all within limits at once.
NEC 695.4(B)(1) / 430.52 disconnecting means sizing
Minimum rating ≥ starting current per the selected starting method (+ jockey if energized during start); maximum standard breaker size per 250% of the fire pump motor's FLA (NEC 430.52), for the disconnect/OCPD ahead of the controller.
Step-down transformer
Feeder — Generator to Transfer Switch
Emergency power path, evaluated the same way as the normal path · NEC 695.7(A)/(B)
The generator main breaker's rating is calculated automatically at 250% of the fire pump FLA (NEC 430.52), rounded up to the next standard size — unless an intermediate OCPD is added below, in which case that OCPD carries this rating instead. Remove the generator entirely with the "−" button on the diagram.
This junction's OCPD carries the 250%-of-FLA rating (NEC 430.52) that would otherwise apply to the generator main breaker. Remove this junction with the "−" button on the diagram, or the Remove button above.
Emergency Power Feeder Results
Same starting/running checks, evaluated through the generator feeder instead of the normal feeder
Starting voltage drop
—%
Within limit
Running voltage drop
—%
Within limit
NEC 695.6(B)(1) minimum conductor ampacity (emergency path)
NEC 695.4(B)(1) / 430.52 intermediate OCPD sizing
Generator starting analysis (informational)
Apparent power the source must supply to start the fire pump motor across the line with no other load energized (kVA = √3 × V × LRA for three-phase, V × LRA for single-phase).
Enter both to estimate predicted starting voltage dip (dip % ≈ starting kVA ÷ generator kVA × X"d). Not a substitute for the generator manufacturer's own skewed-load starting/voltage-dip rating — coordinate with their submittal, especially for large motors or a strict project voltage-dip requirement (NFPA 20 §9.2–9.3).
When on (default), every active segment's conductor size — and parallel set count, once a single conductor's ampacity is no longer enough — is auto-selected to the smallest standard size whose NEC Table 310.16/310.17 ampacity meets or exceeds 125% of the motor FLA. Turn off to size conductors manually as before.
Percentage below full locked-rotor current assumed when "Soft start" is selected as the starting method — typical soft starters limit inrush to roughly 300–400% FLA vs. 500–700% FLA across-the-line. Adjust to match the actual starter's published current-limit setting.
A VFD ramps frequency and voltage together from near-zero rather than limiting a fraction of locked-rotor inrush, so its starting current is expressed as a percentage of motor FLA instead of a reduction off LRA — commonly ~100–150% FLA. Adjust to match the drive's actual current-limit setting.