Accurate control wire sizing for generators is critical to ensure safety, reliability, and compliance with standards. This calculation determines the appropriate wire gauge based on current, voltage drop, and environmental factors.
This article covers control wire sizing using NEC and IEC standards, including formulas, tables, and real-world examples. Learn how to select the correct wire size for generator control circuits efficiently and safely.
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- Calculate control wire size for a 120V generator control circuit, 50 feet length, 5A load.
- Determine wire gauge for 24V DC control wiring, 100 feet, 3A current, NEC compliant.
- Find IEC-compliant control wire size for 48V AC generator control, 75 meters, 7A load.
- Calculate voltage drop and wire size for 230V generator control circuit, 30 meters, 10A load.
Comprehensive Tables for Control Wire Sizing – NEC and IEC Standards
NEC Control Wire Ampacity and Recommended Wire Sizes
| Current (Amps) | Voltage (V) | Maximum Distance (ft) | Recommended Wire Gauge (AWG) | Voltage Drop (%) | Insulation Type | NEC Reference |
|---|---|---|---|---|---|---|
| 1 – 5 | 120 | 50 | 18 AWG | 3% | THHN/THWN | NEC 310.15(B)(16) |
| 5 – 10 | 120 | 100 | 16 AWG | 3% | THHN/THWN | NEC 310.15(B)(16) |
| 10 – 15 | 120 | 150 | 14 AWG | 3% | THHN/THWN | NEC 310.15(B)(16) |
| 15 – 20 | 120 | 200 | 12 AWG | 3% | THHN/THWN | NEC 310.15(B)(16) |
| 20 – 30 | 120 | 250 | 10 AWG | 3% | THHN/THWN | NEC 310.15(B)(16) |
| 1 – 5 | 24 DC | 50 | 16 AWG | 3% | MTW | NEC 725.144 |
| 5 – 10 | 24 DC | 100 | 14 AWG | 3% | MTW | NEC 725.144 |
| 10 – 15 | 24 DC | 150 | 12 AWG | 3% | MTW | NEC 725.144 |
IEC Control Wire Sizing – Metric Wire Sizes and Ampacity
| Current (Amps) | Voltage (V) | Maximum Distance (m) | Recommended Wire Size (mm²) | Voltage Drop (%) | Insulation Type | IEC Reference |
|---|---|---|---|---|---|---|
| 1 – 5 | 230 | 15 | 1.5 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 5 – 10 | 230 | 30 | 2.5 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 10 – 15 | 230 | 50 | 4 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 15 – 20 | 230 | 75 | 6 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 20 – 30 | 230 | 100 | 10 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 1 – 5 | 48 DC | 15 | 1.5 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 5 – 10 | 48 DC | 30 | 2.5 mm² | 3% | H07V-K | IEC 60364-5-52 |
| 10 – 15 | 48 DC | 50 | 4 mm² | 3% | H07V-K | IEC 60364-5-52 |
Essential Formulas for Control Wire Sizing – NEC and IEC
Voltage Drop Calculation
Voltage drop is a critical factor in control wire sizing to ensure proper operation of generator control circuits.
Voltage Drop (V) = (2 × L × I × R) / 1000
- Voltage Drop (V): Voltage lost over the length of the wire (Volts)
- L: One-way wire length (feet or meters)
- I: Load current (Amps)
- R: Resistance of the wire per 1000 feet or meters (Ohms)
The factor 2 accounts for the round trip (outgoing and return path) of the current.
Resistance of Wire
Resistance depends on wire gauge and material (usually copper or aluminum).
R = ρ × (L / A)
- R: Resistance (Ohms)
- ρ (rho): Resistivity of conductor material (Copper ≈ 1.68 × 10⁻⁸ Ω·m)
- L: Length of conductor (meters)
- A: Cross-sectional area of conductor (m²)
Minimum Wire Size Based on Ampacity
Wire size must be selected to safely carry the load current without overheating.
I ≤ Ampacity of Wire (A)
- I: Load current (Amps)
- Ampacity of Wire: Maximum current wire can safely carry (Amps), per NEC or IEC tables
Voltage Drop Percentage
Voltage drop percentage helps verify if the voltage drop is within acceptable limits (usually ≤ 3%).
Voltage Drop (%) = (Voltage Drop (V) / Supply Voltage (V)) × 100
Detailed Real-World Examples of Control Wire Sizing for Generators
Example 1: NEC-Compliant Control Wire Sizing for a 120V Generator Control Circuit
A 120V AC generator control circuit requires wiring for a control load of 8 Amps. The wire run length is 100 feet one-way. The maximum allowable voltage drop is 3%. Determine the minimum wire gauge according to NEC.
Step 1: Determine Maximum Voltage Drop Allowed
Maximum voltage drop = 3% of 120V = 0.03 × 120 = 3.6 V
Step 2: Select Wire Gauge Based on Ampacity
- Load current = 8 A
- From NEC 310.15(B)(16), 16 AWG wire has ampacity ~ 22 A (sufficient)
Step 3: Calculate Voltage Drop for 16 AWG Wire
- Resistance of 16 AWG copper wire ≈ 4.016 Ω per 1000 feet
- Voltage Drop = (2 × 100 ft × 8 A × 4.016 Ω) / 1000 = 6.43 V
Voltage drop (6.43 V) exceeds the allowable 3.6 V.
Step 4: Try 14 AWG Wire
- Resistance of 14 AWG copper wire ≈ 2.525 Ω per 1000 feet
- Voltage Drop = (2 × 100 × 8 × 2.525) / 1000 = 4.04 V
Still above 3.6 V, but closer.
Step 5: Try 12 AWG Wire
- Resistance of 12 AWG copper wire ≈ 1.588 Ω per 1000 feet
- Voltage Drop = (2 × 100 × 8 × 1.588) / 1000 = 2.54 V
Voltage drop is within the 3.6 V limit. Therefore, 12 AWG wire is the minimum acceptable size.
Example 2: IEC-Compliant Control Wire Sizing for a 48V DC Generator Control Circuit
A 48V DC generator control circuit carries 6 Amps over a 40-meter one-way distance. The maximum voltage drop allowed is 3%. Determine the minimum wire size according to IEC 60364-5-52.
Step 1: Calculate Maximum Voltage Drop Allowed
Maximum voltage drop = 3% of 48V = 1.44 V
Step 2: Select Wire Size Based on Ampacity
- Load current = 6 A
- From IEC tables, 1.5 mm² wire has ampacity ~ 14 A (sufficient)
Step 3: Calculate Voltage Drop for 1.5 mm² Wire
- Resistance of 1.5 mm² copper wire ≈ 12.1 mΩ/m (0.0121 Ω/m)
- Voltage Drop = 2 × 40 m × 6 A × 0.0121 Ω/m = 5.81 V
Voltage drop (5.81 V) exceeds 1.44 V limit.
Step 4: Try 2.5 mm² Wire
- Resistance of 2.5 mm² copper wire ≈ 7.41 mΩ/m (0.00741 Ω/m)
- Voltage Drop = 2 × 40 × 6 × 0.00741 = 3.56 V
Still above 1.44 V.
Step 5: Try 4 mm² Wire
- Resistance of 4 mm² copper wire ≈ 4.61 mΩ/m (0.00461 Ω/m)
- Voltage Drop = 2 × 40 × 6 × 0.00461 = 2.21 V
Still above 1.44 V.
Step 6: Try 6 mm² Wire
- Resistance of 6 mm² copper wire ≈ 3.08 mΩ/m (0.00308 Ω/m)
- Voltage Drop = 2 × 40 × 6 × 0.00308 = 1.48 V
Voltage drop is slightly above 1.44 V but close. For safety and compliance, 6 mm² wire is recommended.
Additional Technical Considerations for Control Wire Sizing
- Temperature Correction Factors: Both NEC and IEC require adjusting ampacity based on ambient temperature and conductor insulation ratings.
- Conduit Fill and Grouping: Multiple conductors in conduit may require derating ampacity per NEC 310.15(B)(3)(a) or IEC 60364-5-52.
- Short-Circuit Current Rating: Control wires must withstand potential short-circuit currents; select wire insulation and size accordingly.
- Voltage Level and Insulation: Control circuits at different voltages require appropriate insulation types (e.g., THHN, MTW, H07V-K).
- Environmental Factors: Moisture, chemical exposure, and mechanical stress influence wire selection and sizing.
- Grounding and Shielding: Proper grounding and shielding reduce electromagnetic interference in control wiring.
Authoritative References and Standards
- National Electrical Code (NEC) – NFPA 70
- IEC 60364-5-52: Electrical Installations of Buildings – Selection and Erection of Electrical Equipment
- Copper Resistivity and Conductivity – Copper Development Association
- Voltage Drop Calculations – Engineering Toolbox
Proper control wire sizing for generators ensures operational reliability, safety, and compliance with NEC and IEC standards. Using the provided tables, formulas, and examples, engineers can confidently select the correct wire gauge for any generator control application.