Motor Control Center Design (480V)
Design a 480V motor control center — feeder sizing, individual motor branch circuits, SCGF coordination, and fault current verification.
Scenario Overview
Design a motor control center (MCC) serving five motors in an industrial facility. Size the feeder, branch circuits, overloads, and SCGF protection per NEC 430.
Given Information
- •5 motors on 480V, 3-phase service
- •Motor 1: 50 HP (FLC = 65A)
- •Motor 2: 25 HP (FLC = 34A)
- •Motor 3: 15 HP (FLC = 21A)
- •Motor 4: 10 HP (FLC = 14A)
- •Motor 5: 7.5 HP (FLC = 11A)
- •Available fault current at MCC: 25 kAIC
- •All motors: SF = 1.15, inverse-time breakers
Calculation Steps
Size Individual Motor Branch Conductors
NEC 430.22: conductor ≥ 125% of FLC.
M1: 65 × 1.25 = 81.3A → 3 AWG copper (100A) or 1 AWG aluminum
M2: 34 × 1.25 = 42.5A → 8 AWG copper (50A)
M3: 21 × 1.25 = 26.3A → 10 AWG copper (35A)
M4: 14 × 1.25 = 17.5A → 14 AWG copper (20A)
M5: 11 × 1.25 = 13.8A → 14 AWG copper (20A)
Result: Individual conductors sized: 3, 8, 10, 14, 14 AWG Cu
Size Branch Circuit SCGF (Breakers)
NEC 430.52: inverse-time breaker max = 250% of FLC.
M1: 65 × 2.50 = 162.5A → 150A (next lower standard)
M2: 34 × 2.50 = 85A → 80A
M3: 21 × 2.50 = 52.5A → 50A
M4: 14 × 2.50 = 35A → 35A
M5: 11 × 2.50 = 27.5A → 25A
Result: Breakers: 150, 80, 50, 35, 25A
Size Overload Protection
NEC 430.32: SF ≥ 1.15 → overload ≤ 125% of FLA.
Using FLC as proxy for FLA (nameplate values would be used in practice):
M1: 65 × 1.25 = 81.3A
M2: 34 × 1.25 = 42.5A
M3: 21 × 1.25 = 26.3A
M4: 14 × 1.25 = 17.5A
M5: 11 × 1.25 = 13.8A
Result: Overload heaters sized per motor FLA
Size MCC Feeder
NEC 430.24: 125% of largest motor + 100% of all others.
125% of 65A = 81.3A.
Others: 34 + 21 + 14 + 11 = 80A.
Total feeder: 81.3 + 80 = 161.3A.
Conductor: 2/0 AWG copper (175A at 75°C) or 4/0 aluminum.
Result: 161.3A feeder → 2/0 AWG copper
Size Feeder SCGF
NEC 430.62: feeder SCGF = largest branch breaker + sum of other FLCs.
Largest branch breaker: 150A.
Other FLCs: 34 + 21 + 14 + 11 = 80A.
Max feeder breaker: 150 + 80 = 230A → 225A standard.
Result: 225A feeder breaker
Verify AIC Ratings
Available fault current at MCC: 25 kAIC.
All breakers must have interrupting rating ≥ 25 kAIC.
Standard molded case breakers: typically 22 kAIC at 480V. NOT adequate.
Need high-interrupting rated breakers: 35 kAIC or 65 kAIC series.
NEC 110.9: equipment interrupting rating must exceed available fault current.
Result: All breakers must be rated ≥ 25 kAIC — select 35 kAIC series
Final Answer
MCC fed by 225A breaker, 2/0 AWG copper feeder (161.3A demand). Five motor branches: 150A/3AWG (50HP), 80A/8AWG (25HP), 50A/10AWG (15HP), 35A/14AWG (10HP), 25A/14AWG (7.5HP). All breakers rated 35 kAIC minimum for 25 kA available fault current.
Key Takeaways
- •MCC feeder uses 125% of largest motor + sum of all other FLCs (NEC 430.24)
- •Feeder SCGF = largest branch SCGF + sum of other motor FLCs (NEC 430.62)
- •Always verify AIC ratings against available fault current — standard 22 kAIC breakers are inadequate for many industrial locations
- •Overload protection uses nameplate FLA, but conductor and SCGF sizing uses NEC table FLC
Calculators Used
NEC References
- NEC 430.22 — Motor Branch Conductor
- NEC 430.24 — Motor Feeder
- NEC 430.52 — Motor SCGF
- NEC 430.62 — Feeder SCGF
- NEC 110.9 — Interrupting Rating