Why Do Transformers Use Copper Instead of Brass?

"Brass is also a type of copper, and it's cheaper - so why must transformer windings use oxygen-free copper?" This is one of the most common questions GNEE's sales team receives every day.

 

As an ISO9001:2015 certified manufacturer with over 18 years of experience and a state-of-the-art production facility, we owe our customers a responsible answer. Why do transformers use copper instead of brass? The answer is simple: conductivity is nearly four times lower, and heat generation is much higher. Using brass for windings turns a transformer into an "electric furnace." This article uses data and physics to explain once and for all the fundamental differences between copper and brass inside a transformer.

 

GNEE engineer winding oxygen-free copper coils

 

 

GNEE is not a trading company. We are a direct manufacturer with an annual capacity of 50,000 tons and over 200 in-house engineers. We insist on using oxygen-free copper for our windings - never brass or aluminum to cut costs. Every transformer we produce complies with IEC 60076 and GB 1094 standards.

 

When you choose GNEE, you choose uncompromised conductivity, lower temperature rise, and a service life of 20+ years.

 

 

To answer why transformers use copper instead of brass, we must first look at the key metric: electrical conductivity (% IACS - International Annealed Copper Standard). Conductivity determines how well a material carries current.

 

Oxygen-Free Copper: A Nearly Perfect Conductor

• Conductivity: ≥ 99.9% IACS
• Impurity content: Extremely low (oxygen content ≤ 0.002%)
• Resistivity: Approximately 0.01724 Ω·mm²/m (at 20°C)

 

Brass (Copper-Zinc Alloy): Only One-Quarter the Conductivity

• Conductivity: Only 25%–30% IACS
• Composition: Approx. 60-70% copper, 30-40% zinc
• Resistivity: Approximately 0.07–0.08 Ω·mm²/m (at 20°C)
• Direct comparison: Brass has 3–4 times higher resistance than copper. For the same current flowing through the same cross-sectional area, a brass winding generates 3–4 times more heat than a copper winding.

 

In short: Copper lets current flow "smoothly." Brass makes current flow "with difficulty."

 

 Conductivity comparison chart – copper vs. brass

 

 

 

Once you understand the conductivity gap, the heat gap becomes obvious. And heat is the number one killer of transformer life.

 

 Joule's Law: The Math of I²R Heat

 

Heat generated = Current² × Resistance (P = I²R)

 

Assume a 1500kVA transformer operating at full load current I:

• Copper winding resistance: R
• Brass winding resistance: Approximately 3.5R
• Copper heat generation: I² × R
• Brass heat generation: I² × 3.5R = 3.5 times copper's heat

A transformer with brass windings essentially has an internal electric heater 3.5 times more powerful.

 

Every 8°C Rise Cuts Insulation Life in Half

The insulation paper and oil inside a transformer follow a well-known thermal aging curve (Montaltinger's Rule):

• Rated temperature rise of 65K: Service life of 20–30 years
• Every additional 8°C rise: Insulation life halves
• Temperature rise caused by brass windings: Often 20–30°C above rated limits
• Real-world consequence: A transformer using brass windings may suffer insulation aging, turn-to-turn short circuits, and complete burnout within only 3–5 years. In contrast, GNEE's oxygen-free copper windings strictly maintain a temperature rise below 65K, delivering 20+ years of stable operation.

 

The Chain Reaction of Excessive Heat

Beyond destroying insulation, severe heat also causes:

• Oil degradation: High temperatures accelerate sludge formation, blocking cooling ducts.
• Loose connections: Thermal expansion and contraction loosen bolts, creating arcing.
• Plummeting efficiency: Heat itself represents energy loss, directly reducing transformer efficiency.

 

 

Some customers ask: "Brass is stronger - isn't that better?" For transformer windings, the answer is no.

 

Copper's Softness: Naturally Suited for Winding

• Elongation: Annealed oxygen-free copper can exceed 30%
• Bendability: Can be tightly wound into circular or oval coils
• Solderability: Forms strong, low-resistance joints with copper busbars and leads

 

Brass's Hardness: Difficult to Wind, Prone to Cracking

• Elongation: Typically only 10–15%, with significant work hardening
• Bendability: Springs back when bent, difficult to form
• Welding issues: Zinc vaporizes at high temperatures, creating porosity and cracks in welds
• Manufacturing conclusion: Using brass to wind transformer coils not only severely reduces production efficiency but also leaves high internal mechanical stress. Over long-term operation, brass windings can loosen or crack. This is not "saving money" - it is "planting a time bomb."

 

 

GNEE does not reject brass entirely. As an engineering material, brass has its proper place. Why do transformers use copper instead of brass? Because copper and brass have different jobs.

 

Brass's Three Advantages (In the Right Places)

• Higher strength: Tensile strength 1.5–2 times that of copper
• Better corrosion resistance: Especially in atmospheric and mildly acidic environments
• Good machinability: Easy to cast, turn, and drill

 

Correct Uses of Brass in a Transformer

Component	Material Choice	Reason
Conductors (HV/LV bushings)	Brass	Needs mechanical strength; current density is low
Terminals	Brass	Corrosion-resistant, withstands repeated tightening
Nuts, washers, bolts	Brass or stainless steel	Rust-proof, non-magnetic
Tap changer contacts (partial)	Brass	Wear-resistant, good spring property

 

Where Brass Must NEVER Be Used

❌ HV windings

❌ LV windings

❌ Tap windings

❌ Any continuous conductor carrying main current

In short: Copper handles efficient conduction. Brass handles mechanical connections. Each in its proper role - never mixed.

 

 

In a competitive environment where some manufacturers cut corners using "copper-clad aluminum" or even brass, GNEE maintains transparent material standards.

 

Our Oxygen-Free Copper Winding Standard

Parameter	GNEE Standard
Copper grade	TU1 or TU2 oxygen-free copper
Conductivity	≥ 99.9% IACS
Oxygen content	≤ 0.002%
Resistivity	≤ 0.01724 Ω·mm²/m
Winding type	Copper foil/copper wire, layer or disc type

 

How Can Customers Verify?

We welcome customers to:

• Inspect on site: Visit GNEE's factory to witness the winding process.
• Use third-party testing: Authorize SGS, BV, or other agencies to randomly sample and test copper purity.
• Request material certificates: Each batch of copper comes with an original mill certificate.

 

 

Why do transformers use copper instead of brass?

Because copper offers high conductivity, low heat generation, long life, and reliable workability. Brass offers only one-quarter the conductivity, generates 3–4 times more heat, and shortens transformer insulation life from 20 years to just 3–5 years. Copper handles efficient conduction. Brass handles mechanical connections. Each in its proper role - never mixed.

 

GNEE transformers insist on oxygen-free copper windings. We do not cut costs. We do not lower standards. We do this so you can operate reliably for decades.

 

Are you sourcing a transformer or need technical selection advice? Don't be misled by low prices. The downtime cost of a burned-out transformer due to poor materials far exceeds any upfront savings.

 

Contact GNEE today for our "all-copper winding" technical datasheet and factory-direct pricing. Welcome to visit our factory - seeing is believing!

Request A Quote

 

FAQ

 

How much more expensive is a copper-wound transformer compared to a brass-wound one?
Copper material cost is indeed higher than brass, but it represents a small fraction of total transformer cost. Saving a few hundred dollars by using brass would shorten the transformer's life from 20 years to 5 years - a terrible trade-off.

 

Do any manufacturers actually use brass for windings?
Very few low-quality manufacturers or repair shops might cut corners. GNEE recommends that customers request material test reports or witness factory production.

 

How do aluminum windings compare to copper?
 Aluminum has about 62% of copper's conductivity, plus larger volume and lower strength. Aluminum-wound transformers are bulkier and have shorter lifespans. GNEE recommends copper windings for long-term industrial investments.

 

Will copper windings oxidize?
The inside of a transformer is filled with insulating oil, which contains no oxygen and includes antioxidant additives. Copper windings will not oxidize or corrode inside the oil and can operate stably for decades.