Electric Motor Rewinding Calculation: Formulas, SWG Chart & Wire Weight

Rewinding an electric motor is a precise engineering task. Even a tiny error in calculating the number of turns or selecting the wrong wire gauge can lead to excessive heating, a significant drop in efficiency, or premature insulation failure. Whether you are servicing a 3-phase industrial induction motor or a single-phase household unit, mastering the electric motor rewinding calculation is absolutely crucial. In this technical guide, we break down the standard formulas step-by-step, provide a clear Standard Wire Gauge (SWG) reference chart, and explain exactly how to calculate the copper wire weight required for your stator winding.

Important Note: Always follow lockout/tagout procedures and manufacturer safety guidelines when handling electric motors. Incorrect calculations can cause motor burnout or severe electrical hazards.

Why Accurate Rewinding Calculations Matter

Before diving into the mathematics, it is important to understand why these calculations are so critical. The lifespan of a rewound motor depends entirely on how closely the new winding matches the original design specifications. If you use a wire that is too thin, the current density will exceed safe limits, causing rapid overheating. Conversely, if you use a wire that is too thick, it may not physically fit into the stator slots, leading to a poor fill factor and mechanical damage. Furthermore, incorrect turns per coil will alter the motor’s magnetic flux, resulting in reduced torque or dangerous overcurrent conditions.

Key Step 1: Determining Turns per Coil (The Master Formula)

The number of turns per coil directly controls the magnetic flux and the induced voltage in the motor. For a standard stator winding, the fundamental electromotive force (EMF) equation is:

E = 4.44 × f × Φ × N × Kw

Where:

• E: Supply Phase Voltage (Volts)
• f: Supply Power Frequency (50 Hz or 60 Hz)
• Φ (Phi): Magnetic Flux per Pole (Webers)
• N: Number of Turns per Phase
• Kw: Winding Factor (typically between 0.92 and 0.96)

However, in practical rewinding scenarios—especially when you are changing the motor’s operating parameters (such as voltage, frequency, or speed)—you must use the EASA Master Formula to calculate the new turns per coil (T₂) based on the original data (T₁). This formula incorporates all necessary changes simultaneously:

T₂ = T₁ × √(hp₁/hp₂) × √(F₁/F₂) × √(P₂/P₁) × (E₂/E₁) × (CF₁/CF₂) × (N₂/N₁) × CC

In this equation, hp represents horsepower, F is frequency, P is the number of poles, E is voltage, CF is the chord factor, N is the number of circuits, and CC is the connection change factor (1.73 for star-to-delta, 0.58 for delta-to-star). If you are simply duplicating the original winding, you do not need this formula; however, if you are modifying the motor, this master equation is your most reliable tool.

Key Step 2: Wire Gauge Selection (SWG vs. AWG)

Selecting the correct wire gauge ensures that the current density remains safe—typically between 4 to 6 Amperes per square millimeter. Using a thinner wire increases internal resistance, leading to higher I²R losses (heat) and thermal overload risks. Below is a reference table comparing Standard Wire Gauge (SWG) sizes, which are commonly used in motor rewinding, with their American Wire Gauge (AWG) equivalents.

Standard Wire Gauge (SWG) Reference Chart

SWG Size	AWG Equivalent	Diameter (mm)	Cross-Section (mm²)	Copper Weight (kg/km)	Max Current Rating (A)
16 SWG	14 AWG	1.63 mm	2.08 mm²	18.5 kg/km	10.0 A
18 SWG	16 AWG	1.22 mm	1.17 mm²	10.4 kg/km	5.8 A
20 SWG	19 AWG	0.91 mm	0.65 mm²	5.8 kg/km	3.3 A
22 SWG	21 AWG	0.71 mm	0.40 mm²	3.5 kg/km	2.0 A
24 SWG	23 AWG	0.56 mm	0.25 mm²	2.2 kg/km	1.2 A
26 SWG	25 AWG	0.46 mm	0.16 mm²	1.4 kg/km	0.75 A

When selecting a wire gauge, always account for the fill factor. Copper wire with heavy synthetic film insulation typically has a working fill factor of around 0.60 to 0.65. This means that the total cross-sectional area of the bare copper in a slot should not exceed 65% of the available slot area. If the calculated wire does not fit, you may need to use two parallel strands of a thinner wire to achieve the equivalent cross-sectional area.

Key Step 3: Copper Wire Weight Calculation

Calculating the copper wire weight is essential for costing and material procurement. The weight of the copper in a given winding can be calculated using the Mean Length of Turn (MLT). The fundamental formula is:

W_t = (MLT) × (N) × (Weight per unit length)

Where:

• W_t: Total weight of copper (grams or kg)
• MLT: Mean Length of Turn (the average length of one complete loop of wire around the stator tooth)
• N: Total number of turns in the winding
• Weight per unit length: Derived from the SWG chart (e.g., grams per meter or kg per km)

For a more fundamental calculation based on physical dimensions, the weight can be expressed as W = δ × l × a, where δ is the density of copper (approximately 8.96 g/cm³), l is the total length of the conductor, and a is the cross-sectional area. Because the ratio of weight to resistance is proportional to the fourth power of the wire diameter (d⁴), even a tiny error in measuring the wire diameter can lead to significant errors in weight and resistance calculations.

Example: Calculating Copper Weight for a Stator

Suppose you have a stator with 36 slots, 4 poles, and 15 turns per coil. The Mean Length of Turn (MLT) is measured as 0.45 meters. If you are using 20 SWG wire (which weighs approximately 5.8 kg per kilometer, or 0.0058 kg per meter), the total number of turns is 36 slots × 15 turns = 540 turns. The total copper weight would be:

W_t = 0.45 m × 540 × 0.0058 kg/m ≈ 1.41 kg

This means you would need approximately 1.41 kg of bare copper wire for the entire winding, excluding insulation and waste allowance. Always add a 5–10% margin for waste and end connections.

Key Step 4: Insulation Material and Slot Fit

Modern rewinding relies heavily on quality slot insulation. Using Class H or Class F Nomex/Mylar paper provides long-lasting dielectric protection against high heat build-up. Always preserve the original coil pitch pattern during teardown to avoid altering the speed and torque characteristics. If you are changing the wire gauge or the number of turns, verify that the new coil bundle will physically fit into the slot without damaging the insulation. A tight fit is acceptable, but forcing the wire can cause insulation breakdown and eventual ground faults.

Frequently Asked Questions (FAQs)

What is the difference between SWG and AWG?

SWG (Standard Wire Gauge) is a British imperial standard, while AWG (American Wire Gauge) is primarily used in North America. They use different numbering systems and diameter steps, so always verify which standard your wire supplier is using to avoid mismatches. For example, 18 SWG is approximately equivalent to 16 AWG.

Can I use a wire gauge that is one size thicker than the original?

It is generally not recommended. While a thicker wire reduces resistance and heating, it may not physically fit into the stator slots, leading to a poor fill factor and difficulty in winding. If the original gauge is unavailable, it is safer to use two parallel strands of a thinner wire that together equal the original cross-sectional area.

How do I calculate the number of turns if I do not have the original data?

If the nameplate data is missing, you must rely on the stator core dimensions. Use the EMF equation E = 4.44 × f × Φ × N × Kw, and estimate the magnetic flux (Φ) based on the core area (Length × Diameter of stator core). This requires careful measurement and some experience, as incorrect flux estimation will lead to poor motor performance.

Why is the Mean Length of Turn (MLT) important?

The MLT determines both the total resistance of the winding and the amount of copper required. An inaccurate MLT measurement will throw off your weight calculation and may result in either excessive resistance (if too long) or a winding that does not reach the terminals (if too short). Always measure the MLT of the original winding before stripping it out.

What happens if I use too few turns per coil?

Using too few turns will increase the magnetic flux density in the core, leading to magnetic saturation, excessive iron losses, and overheating. It will also increase the starting current and may cause the motor to draw more current than its rated value under load.

Conclusion

Mastering electric motor rewinding calculations requires a solid understanding of the EMF equation, the EASA master formula, wire gauge standards, and copper weight estimation. By following the formulas and the SWG chart provided in this guide, you can ensure that your rewound motor delivers the same efficiency, torque, and reliability as the original design. Always double-check your measurements, use high-quality insulation materials, and never skip the final continuity and insulation resistance tests before putting the motor back into service.

For more technical guides, check out our detailed article on How to Test Single Phase Motor Winding with a Multimeter.

Usman Motors