Motor Starting: The 75kW Pump — Why Fuse Selection Changes Between Standards
A 75kW DOL motor start across four standards. Same motor, different fuse sizes, different protection coordination — because fuse characteristics are not universal.
Motor circuits are among the most complex to design because they must handle two contradictory requirements: carry high starting currents without nuisance tripping, while still protecting against overload and short circuit. Each standard takes a different approach to balancing these demands — and the fuse or breaker that results can differ by two or three sizes.
The Scenario
A water pump in an industrial plant:
- Motor: 75kW, 400V, three-phase, DOL (Direct On Line) start
- Full load current (FLC): 140A (from motor nameplate)
- Starting current: 6× FLC = 840A for approximately 10 seconds
- Starting method: DOL (most demanding scenario)
- Cable run: 30m to MCC
- Required protection: Short circuit + overload
Side-by-Side Results
Scenario
75kW DOL motor, 400V, 140A FLC, 6× starting current, 30m cable
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Full load current source | 140ANameplate or AS/NZS 3000 TableAS/NZS 3000, Table E2 | 140ANameplate per BS 7671BS 7671, Appendix B | 140ANameplate per IECIEC 60364-5-52 | 124ANEC Table (not nameplate)NEC Table 430.250 |
Overload protection setting | ≤ 1.15 × FLC≤ 161AAS/NZS 3000, Cl 4.6 | ≤ 1.05 × FLC≤ 147ABS 7671, 435.3.2 | ≤ 1.05 × FLC≤ 147AIEC 60364-4-43 | ≤ 1.15 × FLC≤ 161A (143A NEC FLC)NEC 430.32 |
Short circuit protection type | HRC fuseTo AS 1048 / IEC 60269AS/NZS 3000, Cl 4.6 | BS 88 fusegG or gM typeBS 7671, 435.3 | IEC 60269 fusegG or aM typeIEC 60364-4-43 | Dual-element fuseOr inverse-time breakerNEC Table 430.52 |
Fuse/breaker size selected | 250A HRCMust ride through 840A × 10s | 250A BS 88 gGOr 200A gM motor fuse | 200A gMMotor-rated fuse preferred | 350A dual-element124A × 2.5 = 310 → 350ATable 430.52 |
Cable size (for protection coordination) | 50mm²250A fuse, derated for 140A | 50mm²250A fuse coordination | 35mm²200A gM fuse, 140A load | 70mm² (#2/0 AWG)350A fuse coordinationNEC 430.22 |
Why Fuse Sizes Differ So Much
The Starting Current Problem
All four standards face the same challenge: a DOL motor draws 6-8× full load current for 5-15 seconds during starting. The short circuit protection device must:
- Not trip during starting — allow 840A for 10 seconds
- Trip quickly on fault — clear a 10-20kA fault within the cable's I²t rating
- Coordinate with the overload relay — let the OL relay handle overloads, the fuse handle faults
These requirements pull in opposite directions. A fuse big enough to ride through starting may be too slow to protect the cable on a fault.
Fuse Characteristics Are Not Universal
This is the key insight: a "250A fuse" under BS 88 does not have the same time-current characteristic as a "250A fuse" under NEC/UL standards.
BS 88 gG fuse (general purpose):
- Tightly defined pre-arcing I²t
- Narrower tolerance band
- Faster clearing at high fault currents
NEC dual-element fuse:
- Time-delay element allows motor inrush
- Wider tolerance band
- Higher let-through at moderate overloads
IEC 60269 gM fuse (motor-rated):
- Specifically designed for motor circuits
- Rated with two values: e.g., 200M315 (200A motor rating, 315A carrier)
- Optimised balance between starting ride-through and fault clearing
The gM Fuse Advantage
IEC 60269-4 defines the gM (motor) fuse type specifically for motor circuits. A 200A gM fuse can ride through the same starting current that requires a 250A gG fuse — because its time-delay characteristic is designed for the motor starting profile. This is why IEC 60364 achieves the most compact solution.
NEC's Multiplier Approach
NEC Table 430.52 provides simple multipliers:
| Protection Type | Multiplier × FLC |
|---|---|
| Non-time delay fuse | 3.0× |
| Dual-element (time-delay) fuse | 1.75× |
| Inverse-time breaker | 2.5× |
| Instantaneous trip breaker | 8.0× |
For our 75kW motor (NEC FLC = 124A):
- Dual-element fuse: 124 × 2.5 = 310A → next standard size = 350A
- Non-time delay fuse: 124 × 3.0 = 372A → next standard size = 400A
- Instantaneous trip breaker: 124 × 8.0 = 992A → 1000A!
NEC Motor Branch Circuit Protective Device
Fuse rating = FLC (Table 430.250) × multiplier (Table 430.52)
The NEC result is larger because the multiplier includes generous margins for American fuse characteristics and motor starting conditions.
The Cable Size Cascade
Larger fuses require larger cables — not for current carrying capacity, but for fault protection coordination. The cable must be able to withstand the energy let-through of the larger fuse during a fault.
This creates a cascade effect under NEC:
- 350A fuse → cable must withstand 350A fuse I²t
- This typically requires 70mm² (2/0 AWG) cable for a 140A motor
- Under IEC with a 200A gM fuse, 35mm² is adequate
The NEC installation uses twice the copper as the IEC installation — for the same motor, same voltage, same starting conditions.
Cost Impact
For a single 75kW motor, the NEC approach costs approximately $2,000 more in copper than IEC. In a large plant with 50 motors, this difference is $100,000+ in cable alone — not counting larger cable trays, bigger terminations, and more labour.
Practical Guidance
Choosing the Right Protection Scheme
- For new IEC/BS installations: Use gM (motor-rated) fuses where available — they give the most efficient solution
- For NEC installations: Consider Type 2 coordination with contactor + overload relay + fuse — this allows the smallest fuse size
- For all standards: VFD (Variable Frequency Drive) starting eliminates the inrush problem entirely — starting current drops from 6× to 1.5× FLC
When Standards Cross
If an Australian engineer designs a motor circuit to AS/NZS 3000 and ships it to a US site:
- The 250A HRC fuse must be replaced with a UL-listed equivalent
- The cable size must be verified against NEC 430.22 (often requires an increase)
- The overload relay setting changes (NEC uses Table 430.250 FLC, not nameplate)
Key Takeaways
- NEC requires the largest protection (350A) where IEC needs only 200A gM — nearly half
- Fuse types are not interchangeable — BS 88 gG ≠ NEC dual-element ≠ IEC gM
- Cable size is driven by fuse size, not motor current — NEC requires 70mm² vs IEC's 35mm²
- gM motor fuses (IEC 60269-4) give the most efficient motor protection
- NEC uses table FLC (124A) while others use nameplate FLC (140A) — the starting point differs
Related Resources
- King's Cross Fire: Motor Circuit Cable Sizing — Why motor starting current must be checked during cable selection
- Motor Voltage Drop: Startup vs Running — The voltage drop spike that catches engineers off guard
- Cable Sizing: The 50m Office Feeder — Standard cable sizing differences before motor loads complicate matters
- Voltage Drop: The 100m Workshop Cable — Motor feeders on long runs amplify voltage drop differences
- View all worked examples →
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Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.