Technical insight from XINHONG ELECTRICAL — your professional partner in transformer manufacturing and electrical solutions
When purchasing oil-immersed transformers, many customers ask a crucial question: Since copper and aluminum have different resistivities, how can transformers with different winding materials achieve the same load losses?
This is a valid concern. After all, copper and aluminum are fundamentally different metals with distinct physical properties. Yet under national and international standards (GB 20052-2024, IEC 60076), both copper-wound and aluminum-wound transformers of the same model must meet identical load loss requirements to pass type tests and enter the market.
This article explains the engineering principles behind this apparent paradox, brought to you by XINHONG ELECTRICAL — a trusted manufacturer of oil-immersed transformers, dry-type transformers, prefabricated substations, and LV/HV switchgear based in Xuzhou, Jiangsu.
Load losses (also known as copper losses) are the heat generated when current flows through the transformer windings. According to Joule's Law, load loss follows the formula:
P = I² × R
Where P is load loss, I is load current, and R is winding resistance.
For a given transformer design, the load current is fixed by the rated capacity. Therefore, the only variable the manufacturer can control to meet loss standards is resistance. And resistance depends on three factors:
Material resistivity (ρ)
Conductor length (L)
Conductor cross-sectional area (A)
R = ρ × (L / A)
This is the key equation that makes equivalent losses possible.
To understand how aluminum can match copper's losses, we must first examine their natural properties:
| Property | Copper | Aluminum |
|---|---|---|
| Resistivity at 20°C | 1.68 × 10⁻⁸ Ω·m | 2.82 × 10⁻⁸ Ω·m |
| Conductivity (IACS) | ~100% | ~61% |
| Density | 8.96 g/cm³ | 2.70 g/cm³ |
| Melting point | 1085°C | 660°C |
| Tensile strength | ~220 MPa | ~90 MPa |
Copper has significantly lower resistivity than aluminum — approximately 60% of aluminum's value. This means that for identical cross-sectional area, a copper conductor will have about 40% lower resistance than an aluminum conductor, resulting in substantially lower load losses.
So how can aluminum ever match copper's performance?
The short answer: aluminum windings are not simply "copper replacements" — they are systematically redesigned from the ground up.
When properly engineered, aluminum-wound transformers can achieve the same certified load loss values as copper-wound transformers. The key lies in three strategic design compensations:
Since aluminum has higher resistivity, manufacturers increase the cross-sectional area of the aluminum conductor to reduce its resistance. To achieve the same resistance as copper, the aluminum conductor's cross-sectional area needs to be approximately 1.6 to 1.66 times larger.
For example, if a copper conductor requires a certain cross-section, an equivalent aluminum conductor must be about 60% larger. This larger cross-section increases manufacturing costs but successfully lowers the resistance to match copper's electrical performance.
Beyond simple cross-section enlargement, professional manufacturers employ advanced design techniques:
Stranded and transposed conductors to reduce eddy current losses
Improved insulation processes to minimize additional stray losses
Optimized winding arrangements to control current density and heat distribution
These measures ensure that even with aluminum's higher resistivity, the total load losses remain within standard limits.
Because aluminum windings have larger conductor cross-sections, the overall winding volume increases. While this might seem disadvantageous, it actually creates greater heat dissipation surface area. More cooling surface means better heat transfer to the oil, which can offset some of the thermal challenges associated with aluminum's lower thermal conductivity.
The answer is YES — but only when both are properly designed and manufactured to the same standard.
Take the S20 energy efficiency standard as an example. Both copper-wound and aluminum-wound S20 transformers of the same capacity must meet identical loss limits to pass type tests and obtain certification. Whether it is an S20-M-630kVA copper transformer or an S20-M-630kVA aluminum transformer, as long as both have passed national certification, their loss values are equivalently low, both qualifying as Tier 2 energy efficiency products.
However, this equivalence applies only to:
Same capacity rating
Same energy efficiency level
Same design standard (not a simple "copper-to-aluminum" substitution)
If someone simply takes a copper-wound transformer design and directly replaces copper conductors with aluminum conductors of the same cross-sectional area, the load losses would increase dramatically — by approximately 60% or more. This would fail to meet any standard and would produce an unqualified product.
Legitimate manufacturers like XINHONG ELECTRICAL never take such shortcuts. Every aluminum-wound transformer we produce undergoes systematic redesign and rigorous type testing to ensure compliance with national and international standards.
This is where many buyers get confused. While load losses can be made equivalent through design compensation, copper and aluminum windings differ significantly in other aspects of performance:
| Comparison Aspect | Copper Windings | Aluminum Windings |
|---|---|---|
| Load losses (certified) | Meet standard | Meet standard ✓ |
| Winding volume | Compact | ~60% larger |
| Transformer weight | Heavier | Lighter (aluminum density 30% of copper) |
| Mechanical strength | Excellent (220 MPa) | Lower (90 MPa) |
| Short-circuit withstand | Superior | Acceptable with proper design |
| Oxidation resistance | Good (Cu₂O conductive) | Poor (Al₂O₃ insulating) |
| Connection stability | Stable long-term | Requires special terminals, prone to loosening |
| Maintenance requirement | Minimal | Requires periodic inspection |
Mechanical Strength: Copper's tensile strength (approximately 220 MPa) is about 2.4 times that of aluminum (approximately 90 MPa). This gives copper windings superior ability to withstand the electromagnetic forces generated during short-circuit events.
Oxidation and Connections: Aluminum readily forms an insulating aluminum oxide layer (Al₂O₃) on its surface. This oxide layer has extremely high resistivity and creates connection instability over time. Copper's oxidation products are conductive and less problematic.
Long-term Reliability: While laboratory tests show both materials can meet initial standards, actual field performance differs. Aluminum connections tend to develop increasing contact resistance over time, leading to localized heating and accelerated insulation aging. Copper connections remain more stable over the transformer's service life.
Copper windings are the preferred choice for:
Industrial facilities and data centers — where reliability is paramount
Hospitals and critical infrastructure — where downtime is unacceptable
Urban power distribution networks — where space is limited
Applications requiring high short-circuit withstand capability
Projects with a 20-30 year design life where long-term reliability justifies higher upfront cost
According to industry studies, in continuous duty applications such as data centers and industrial plants, the efficiency advantage of copper windings can offset the higher initial investment over a 20-year lifespan.
Aluminum windings can be considered for:
Budget-sensitive projects with up to 30-50% lower upfront cost
Rural power networks and temporary construction power
Applications where weight reduction is beneficial (e.g., transportation-constrained sites)
Shorter-term installations where 15-20 year service life is acceptable
Both copper and aluminum are permitted materials for transformer windings under global standards including IEC 60076 and GB 20052. No standard prohibits the use of aluminum coils in oil-immersed transformers. However, both materials must be used according to proper manufacturing practices and design guidelines.
At XINHONG ELECTRICAL, we manufacture:
Oil-immersed transformers — S11, S13, S20 series, copper or aluminum windings as specified
Dry-type transformers — SCB series for indoor applications
Prefabricated substations — integrated compact power solutions
LV and HV switchgear — complete electrical distribution systems
Every transformer we produce undergoes:
Material verification (high-purity oxygen-free copper or A00-grade aluminum)
Rigorous type testing according to national and IEC standards
Certified test reports accompanying each unit
We do not practice material substitution without design adjustment. Whether you choose copper-wound or aluminum-wound transformers, you receive products that meet the required loss standards and safety requirements.
To directly answer the original question: Yes, oil-immersed transformers with copper and aluminum winding materials can achieve the same certified load losses.
This is accomplished through systematic design compensation — primarily increasing the aluminum conductor's cross-sectional area to approximately 1.6 times that of copper to match resistance values, combined with optimized winding structures and enhanced cooling designs.
However, load loss equivalence does not mean the two materials are identical in all aspects. Copper offers superior mechanical strength, better long-term connection stability, and higher reliability in demanding applications. Aluminum offers lower initial cost and lighter weight.
The right choice depends on your specific application requirements, budget, and expected service life.
For professional guidance on transformer selection for your project, contact XINHONG ELECTRICAL. Our engineering team will help you evaluate the trade-offs and select the optimal solution for your power distribution needs.
XINHONG ELECTRICAL — Jiangsu Xinhong Electrical Equipment Co., Ltd.
Professional manufacturer of oil-immersed transformers, dry-type transformers, prefabricated substations, and LV/HV switchgear
Serving customers across China and international markets
