Selecting the correct fuse for electrical circuits ensures safety, equipment protection, and compliance with strict NEC guidelines.
This guide details NEC fuse selection with tables, formulas, and real examples for engineers and electricians.
NEC Fuse Selection Calculator
Extensive Tables for Fuse Selection Based on NEC Guidelines
To streamline fuse selection, the NEC (especially Articles 240 and 310) specifies fuse ampacity relative to conductor sizes, load types, and voltage ratings. Below are detailed tables illustrating common fuse ratings matched to conductor sizes and typical circuit applications.
Table 1: Fuse Size vs. Copper Conductor AWG (NEC 240.6)
| Conductor Size (AWG) | Typical Insulation | Ampacity (A) | Maximum Fuse Size (A) per NEC 240.6(A) | Common Fuse Ratings (A) |
|---|---|---|---|---|
| 14 | THHN, TW | 15 | 15 | 15 |
| 12 | THHN, TW | 20 | 20 | 15, 20 |
| 10 | THHN, TW | 30 | 30 | 20, 25, 30 |
| 8 | THHN, TW | 40 | 40 | 30, 35, 40 |
| 6 | THHN, TW | 55 | 60 | 40, 50, 60 |
| 4 | THHN, TW | 70 | 80 | 60, 70, 80 |
| 3 | THHN, TW | 85 | 90 | 70, 80, 90 |
| 2 | THHN, TW | 95 | 100 | 90, 100 |
| 1 | THHN, TW | 110 | 125 | 100, 110, 125 |
| 1/0 | THHN, TW | 125 | 150 | 125, 135, 150 |
| 2/0 | THHN, TW | 145 | 175 | 150, 175 |
| 3/0 | THHN, TW | 165 | 200 | 175, 200 |
| 4/0 | THHN, TW | 195 | 225 | 200, 225 |
Note: The “Maximum Fuse Size” column represents NEC-allowed maximum fuse ratings, often rounded to standard fuse sizes.
Table 2: Fuse Selection Based on Load Type and NEC Article 240 Guidance
| Load Type | Typical Continuous Load (%) | Recommended Fuse Sizing Approach | Reference NEC Article |
|---|---|---|---|
| General Purpose Branch | ≤ 80% Load | Fuse rating = 125% of continuous load | 240.6, 210.19(A)(1) |
| Motor Loads | 115% to 175% of FLA | Use Motor Circuit Protectors (MCP) or fuses | 430.52, 430.32 |
| Transformer Primary | 125% of full load | Based on transformer kVA and primary current | 450.3(B) |
| Capacitor Banks | 135% to 200% of rated current | Special capacitor fuses | 460.7 |
| Emergency Circuits | As per critical load | Select fuse to avoid nuisance trips | 700.27 |
Table 3: Common Standard Fuse Ratings (Ampere) Used in Industrial and Commercial Applications
| Fuse Type | Voltage Rating (V) | Standard Ampere Ratings (A) |
|---|---|---|
| Time-Delay Fuses | 250, 600 | 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300 |
| Fast-Acting Fuses | 250, 600 | 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 20, 25, 30, 35, 40 |
Essential Formulas for Fuse Selection According to NEC
Correct fuse sizing demands understanding the electrical load characteristics, conductor ampacity, and NEC mandates. Below are critical formulas to calculate fuse ratings, with explanations of each variable and typical value ranges.
1. General Fuse Sizing for Continuous Loads
Where:
Typical values:
- Continuous load currents range from a few amperes (residential circuits) to thousands of amperes (industrial machinery).
2. Fuse Sizing for Motor Circuits (Per NEC Article 430)
Motors have starting currents that may be 6 to 8 times the full load current (FLA). Fuse sizing considers this to prevent nuisance trips:
Where:
Common multipliers:
- For time-delay fuses: K=1.25 to 1.75
- For fast-acting fuses: usually lower values or not recommended.
3. Fuse Sizing for Transformer Primary Protection
Where:
This formula is derived from NEC Article 450.3(B).
4. Minimum Fuse Rating Based on Conductor Ampacity (NEC 240.4(B))
Fuses must never be smaller than the conductor ampacity to prevent conductor damage:
Where:
5. Calculating Full Load Current (FLA)
When only load power and voltage are known, FLA can be estimated:
- Single-phase AC:
- Three-phase AC:
Where:
- I = Full load current (Amps)
- P= Load power (Watts)
- V= Voltage (Volts)
- PF= Power factor (typically 0.8 to 1.0)
- η= Efficiency (typically 0.9 to 1.0)
Detailed Explanation of Variables and Typical Values
Real-World Examples of Fuse Selection According to NEC
Example 1: Selecting a Fuse for a Residential Branch Circuit
Scenario:
A 120V, 15A general lighting circuit with copper conductors AWG 14 is installed in a residence.
Steps:
1.Determine conductor ampacity:
From NEC Table 310.16, AWG 14 copper conductor has ampacity of 15A.
2.Calculate fuse rating:
For general branch circuits, NEC 210.19(A)(1) requires fuse rating ≥ 125% of continuous load. If full load is 15A, fuse rating is:
3.Select nearest standard fuse size:
The nearest standard fuse rating is 20A.
4.Check conductor protection:
NEC 240.4(D) allows 15A fuse on 14 AWG conductor.
Result:
Select a 15A fuse (as 20A is too high for 14 AWG conductor). Common practice uses 15A fuse protecting 14 AWG copper.
Example 2: Fuse Selection for a 10 HP Motor on a 480V Supply
Scenario:
A 10 HP, 480V, 3-phase motor requires fuse protection.
Steps:
1.Find motor full load current (FLA):
From NEC Table 430.250, a 10 HP motor at 480V typically has an FLA of about 14A.
2.Calculate fuse rating using multiplier K=1.75
3.Select nearest standard fuse:
Choose a 25A time-delay fuse to handle starting current surges.
4.Check conductor size:
From NEC Table 310.16, conductor ampacity must be ≥ motor FLA (14A). Typically, AWG 12 copper (20A) or AWG 10 copper (30A) is selected. For 25A fuse, AWG 10 copper conductor is appropriate.
Result:
Use a 25A time-delay fuse with AWG 10 copper conductor for motor protection.
Further Insights and Detailed Considerations for Fuse Selection
Time-Delay vs. Fast-Acting Fuses
- Time-Delay Fuses: Suitable for motors and inductive loads with high inrush current. They tolerate temporary surges without blowing, preventing nuisance trips.
- Fast-Acting Fuses: Ideal for sensitive electronics and resistive loads with minimal surge currents, providing quicker protection.
Ambient Temperature and Correction Factors
NEC Article 240.4(B) and 310.15(B)(2)(a) require adjusting fuse size or conductor ampacity for ambient temperature deviations. High ambient temperature reduces conductor ampacity; fuse selection must accommodate this to avoid premature fuse blowing or conductor damage.
Coordination and Selectivity
Fuse coordination is essential for selective tripping in multi-level electrical systems. Proper fuse selection avoids unnecessary downstream trips and minimizes system downtime.
External Authority References
- National Fire Protection Association (NFPA): NFPA 70 – National Electrical Code
- IEEE: IEEE Guide for Electrical Fuse Coordination
- Underwriters Laboratories (UL): Fuse safety and testing standards (UL 248 Fuse Standards)
- Electrical Safety Foundation International (ESFI): Fuse and circuit protection overview (ESFI Fuse Info)
