PART II Distribution
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Feeder Design

NEC 430.24 · voltage drop · raceway · derating

A feeder carries power between two pieces of distribution equipment — service to switchgear, switchgear to panel, panel to PDU. Sized like a branch circuit but with one big difference: the largest motor on the feeder gets a 25% bonus.

Branch vs Feeder vs Service — NEC Definitions

Three terms, three different sets of rules. NEC Article 100 defines them precisely.

Conductor type Definition (NEC 100) NEC article OCPD multiplier Atlas DC1 examples
Branch circuitConductors between the final OCPD and the outlet/equipment210, 430 (motors)125% × cont (NEC 210.20). Motors: per Table 430.52RPP breaker → rack PDU; chiller branch CB → motor
FeederConductors between service equipment / source and the final branch-circuit OCPD215, 430.24 (motor feeders)125% × cont (NEC 215.3). Motors: 125% largest + 100% rest (430.24)SWGR-A → UPS-A1 input; PDU-A1 → RPP-A1-1; SWGR → MCC
Service conductorsConductors from utility supply to service equipment230Calculated per Article 220 demandUtility 12.47kV → MV switchgear primary
Tap conductorSmaller conductor tapped from a larger feeder, with restrictive rules on length and termination240.21(B)Special — 10ft / 25ft / 100ft tap rulesDisconnect taps in switchgear sections

NEC 430.24 — The Motor Feeder Rule

This is the most-tested motor sizing rule in the PE exam, and one of the most-used in real practice. Whenever a feeder serves multiple motors (an MCC, a mech-room sub-panel, a chilled-water plant), the largest motor gets the 125% bonus and all others contribute their FLC at face value.

NEC 430.24 — multi-motor feeder ampacity
Ifeeder ≥ 1.25 × FLClargest + Σ FLCother motors + other loads
"Other loads" = lighting + receptacles + non-motor equipment, each per their respective demand rules.
Feeder serving 4 motors of different sizes — 430.24 application SWGR-A 480V bus FEEDER all motor currents flow through this MCC-MR1 M CH-1 480 A ⭐ M CH-2 480 A M CWP-1 96 A M CWP-2 96 A M FAN 37 A FEEDER ≥ 1.25 × 480 + 480 + 96 + 96 + 37 = 600 + 709 = 1,309 A ⭐ = largest motor gets 125% bonus
Only the largest motor's FLC gets multiplied by 1.25. Every other motor contributes its FLC at face value. All currents must be at the same voltage — if motors are at different voltages downstream of a transformer, convert all currents to the feeder's voltage first.
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Why only the largest motor?
The 125% accommodates the worst case: the largest motor starting last, drawing inrush current, while every other motor is already running at full FLC. Smaller motors starting don't change the picture meaningfully because their inrush is small. NEC 430.24 captures this in a clean formula instead of dynamic load-flow analysis.

Voltage Drop on Feeders

NEC 215.2(A)(1) Informational Note recommends ≤ 3% on feeder, ≤ 5% combined feeder + branch. This is a recommendation, not a requirement, but most AHJs and engineering specs treat it as mandatory. Long feeder runs frequently dictate wire size more than ampacity does.

Single-phase voltage drop (NEC Ch 9 Table 8 — DC method)
VD = (2 × I × R × L) / 1000
L in feet. R = ohms/1000 ft from NEC Ch 9 Table 8 (DC) or Table 9 (AC). Factor of 2 = round trip.
Three-phase voltage drop
VD = (√3 × I × R × L) / 1000
No factor of 2 — neutral current is 0 in balanced 3φ. R from NEC Ch 9 Table 9 if at typical 75°C.
Convert to %VD
%VD = VD / Vnom × 100
Use system voltage (480 for 3φ, 240 or 120 for 1φ).
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Quick rule of thumb (copper)
For any conductor: VD ≈ I × L / (Cu factor). The Cu factor for typical 75°C copper is about 22 × CM × 1000. So VD% on 480V 3φ ≈ (1.732 × I × L × 12.9) / (CM × 480), where CM = circular mils of conductor.

NEC Ch 9 Table 9 — AC resistance (75°C, copper, in steel conduit)

Conductor R (Ω/1000 ft) X (Ω/1000 ft) Effective Z @ 0.85 PF
#12 AWG2.00.0541.74
#10 AWG1.20.0501.05
#8 AWG0.780.0520.69
#6 AWG0.490.0510.44
#4 AWG0.310.0480.29
#2 AWG0.200.0450.19
1/0 AWG0.120.0440.13
3/0 AWG0.0790.0420.094
250 kcmil0.0540.0410.073
500 kcmil0.0290.0390.054
750 kcmil0.0210.0380.048

Worked Example 1 — Atlas DC1 Feeder from UPS-A1 to PDU-A1

The feeder between UPS-A1 (480V output) and PDU-A1 (480V input) — 250 ft of cable. Sized for the full UPS output rating, not just current load.

Example 01 · Atlas DC1 spine UPS-A1 (1250 kVA, 480V 3φ) → PDU-A1 (480V input) · 250 ft route through cable tray

Given

UPS rating
1250 kVA at 480V 3φ
UPS full-load current
I = 1,250,000 / (√3 × 480) = 1,504 A
Run length
250 ft (cable tray + conduit)
PDU input
500 kVA = 602 A actual; UPS sized at 1250 kVA = 1,504 A possible
Continuous?
Yes (IT load, 24/7)

Step-by-step

  1. Decide the feeder ampacity basis. Must serve worst-case downstream load. Since UPS could feed multiple PDUs in real designs, size feeder to UPS full-output current (1,504 A), not just one PDU.
    Feeder Ibasis = 1,504 A (UPS rating). Apply 125% continuous → 1,880 A.
  2. Conductor selection. 1,880 A is well above any single conductor. Need parallel runs.
    Try 4 parallel sets of 600 kcmil (each rated 350A at 75°C) = 4 × 350 = 1,400 A — fails.
    Try 4 sets of 750 kcmil = 4 × 400 = 1,600 A — fails.
    Try 5 sets of 600 kcmil = 5 × 350 = 1,750 A — fails.
    Try 5 sets of 750 kcmil = 5 × 400 = 2,000 A. ✓ (Or 6 sets of 500 kcmil = 1,920 A.)
  3. Voltage drop check. R for 750 kcmil = 0.021 Ω/1000 ft (Table 9).
    VD = (√3 × 1,504 × 0.021 × 250) / 1000 / 5 (parallel paths divide R) = (1.732 × 1504 × 0.021 × 250) / 5000 = 13,684 / 5000 = 2.74 V.
    %VD = 2.74 / 480 × 100 = 0.57% — excellent. Well below 3%.
  4. Feeder OCPD at UPS-A1 output.
    125% × 1,504 = 1,880 A. Round up to standard: 2,000 A molded-case CB. Or use a 1,600 A breaker if downstream coordination allows.
  5. Final spec.
    2,000 A CB at UPS · 5 sets of 750 kcmil THWN-2 Cu in 5 separate 4" EMT/cable tray runs · 350 kcmil EGC per NEC 250.122 (Table — for 2000A breaker, EGC = 4/0 Cu)
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Cable tray cuts conductor cost dramatically
For 5 parallel sets of 750 kcmil at 250 ft, the conductor cost alone is > $50,000. Cable tray (NEC 392) lets you use Type TC or MC cable instead of conduit-wired THWN, with substantial savings on installation labor. We explore this in §08.

Worked Example 2 — Apartment Building Service Feeder

The 50-unit apartment building from §03 had a calculated demand of 980 A at 208V 3φ. Now we size the actual service feeder from the utility transformer.

Example 02 · Alternate scale 50 units · 980 A demand at 208Y/120V 3φ-4W · 80 ft underground service from pad-mount

Given

Demand load
980 A at 208Y/120V 3φ-4W (per §03 calc)
Run length
80 ft underground (PVC duct under the building)
Voltage drop allowed
≤ 3% feeder

Step-by-step

  1. Service entrance breaker (or fuses) + bus. Round 980 A up to standard size: 1,200 A.
  2. Conductor sizing. Need 1,200 A in parallel.
    3 parallel sets of 500 kcmil (each 320A) = 960 A — fails for 1,200A breaker.
    4 sets of 500 kcmil = 1,280 A. ✓
    Or 3 sets of 750 kcmil = 1,200 A — exactly meets.
  3. Neutral conductor. 3φ-4W with 120V residential loads = significant unbalanced neutral current. Often sized 100% of phase conductors for residential, even though NEC allows reduction.
  4. Voltage drop check (3 sets of 750 kcmil, R = 0.021 Ω/1000 ft):
    VD = (√3 × 980 × 0.021 × 80) / 1000 / 3 = 2,852 / 3000 = 0.95 V. %VD = 0.95 / 208 = 0.46%. ✓
  5. Final spec.
    1,200 A main service breaker (or 1,200 A fuse) · 3 sets of 750 kcmil Cu THWN-2 in 4" PVC underground · neutral 750 kcmil (full size) · 4/0 Cu EGC. (Or aluminum at lower cost — see §07.)

Drill — Quick Self-Check

Work each problem mentally; reveal to check. Goal: reflex, not deliberation.

Drill 1 · Branch vs feeder

A 200 A breaker feeds a sub-panel which has its own branches. The 200 A circuit is a:

Drill 2 · Voltage drop — 3φ

350 ft of 4/0 Cu (R = 0.062 Ω/kft) carrying 180 A at 480V 3φ. %VD?

Drill 3 · Tap rule — 10 ft

Can you tap a 400 A feeder with #6 AWG (75 A) for 8 feet without sizing the wire to 400 A?

Drill 4 · Nonlinear neutral

A 4-wire feeder serves 100% nonlinear server load. Neutral sizing?

Drill 5 · Atlas UPS feeder

Atlas DC1 UPS-A1 = 1500 A. Continuous. Min feeder ampacity?

If You See THIS, Think THAT

If you see…Think / use…
"Feeder" between switchgear and panelNEC Article 215. Sized: 125% × continuous + 100% × non-continuous, with NEC 220 demand factors applied.
"Multiple motors on a feeder"NEC 430.24: 125% × largest motor FLC + 100% × all other motor FLCs + other loads.
"Mixed motor + non-motor on feeder"Above formula PLUS lighting + receps with their NEC 220 demand factors. Sum.
Long feeder run (> 100 ft, large I)Voltage drop check. ≤ 3% target. May need to upsize beyond ampacity-only sizing.
Feeder current > 400 AAlmost always parallel runs. Watch terminations and cable management.
"Tap conductor" (NEC 240.21)Special rules: 10ft tap (no termination protection), 25ft tap (with restrictions), 100ft tap (industrial only). All have specific size minima.
"Service entrance"NEC Article 230. Different rules from feeder — service has no upstream OCPD inside the building. Sized for full demand load.
"Heavy unbalanced 3φ-4W"Neutral can carry phase current or more (with harmonics). Don't undersize neutral.
"Harmonic loads on the feeder" (servers, VFDs, LEDs)Neutral becomes a current-carrying conductor for derating purposes. NEC 310.15(C). Often size neutral 200% in pure nonlinear feeders.
Underground feeder in PVCNEC 310.60 different ampacity table for direct burial / duct bank. Soil thermal resistivity matters.