Key takeaways
- Copper conducts roughly three and a half times better than common brass, so the same cross-section runs much cooler.
- Brass is stronger and machines faster, which suits small terminals, studs and earth parts rather than power conductors.
- A brass lug can be acceptable at low current or as a mechanical clamp, but the current path and the heat rise must be checked.
- Look at the alloy, not the colour. Zinc content also decides how the part behaves in wet and warm service.
Two materials, two jobs
Copper and brass are both copper alloys, but they behave differently. Pure copper, or a high conductivity copper such as C101 or C110, is chosen for current path. Brass is a copper zinc alloy, chosen for strength, machinability and cost.
A lug has to do both jobs at once: carry current through the barrel and palm, and clamp the conductor without deforming. That is why some designs use copper for the current path and brass for a screw, a nut or a clamp body. The mistake is to treat the whole lug as one material decision.
The colour is not a guide. A tin plated brass lug and a tin plated copper lug look the same. The drawing, the material certificate or a conductivity check is the only reliable way to tell them apart.
The alloy names matter as well. A high copper brass with a low zinc content behaves differently from a free-machining grade with lead added for chip control. The first is closer to copper in conductivity and corrosion resistance. The second is easier to machine but weaker in wet service and is subject to restrictions on lead in some markets.
Conductivity: the number that decides temperature
High conductivity copper is usually specified at around 100 per cent IACS, which corresponds to about 58 MS/m. Common free-machining brass is far lower, often around 25 to 30 per cent IACS depending on the alloy and temper. The exact figure belongs on the material certificate.
That difference matters because heat in a joint comes from resistance, and resistance is inversely proportional to conductivity for a given shape. A brass part with the same dimensions as a copper part will dissipate more heat at the same current. The temperature rise is not linear, because the resistance also rises with temperature and the heat has to leave through the same surface area.
This is why a brass lug that looks substantial can run hotter than a slimmer copper lug. Wall thickness and surface area help, but they cannot fully compensate for a conductivity that is a third of copper.
A brass lug is not a copper lug with a different label
If a drawing calls for a copper lug for a power cable, a brass substitute is a design change. It needs a current rating check and a temperature rise test, not just a dimensional comparison.
Strength and machinability
Brass is stronger and harder than annealed copper, and free-machining grades machine quickly with good surface finish. That makes brass attractive for threaded parts, small terminals, studs, spacers and earth blocks where the part is loaded mechanically rather than electrically.
Copper is softer and galls more easily, so threads cut in copper wear faster and a copper screw is a poor choice. A brass screw in a copper lug is a sensible combination, provided the screw is not the main current path.
The strength advantage has a limit. Some brasses are brittle in certain tempers, and a thin brass palm can crack when it is bent to fit a terminal. If the palm has to be formed after plating, check the bend radius and the temper, not just the tensile strength.
Strength is also not the same as stiffness. A brass lug can be strong in tension and still deflect under a bending load if the section is thin. On a cable termination the palm sees a bending moment from the cable weight, so the section modulus matters as much as the tensile figure on the certificate.
The comparison in practice
The table sets out the properties that change a selection. Values are typical ranges, not a specification for a particular alloy. Always use the certificate for the grade you are buying.
| Property | High conductivity copper | Common brass |
|---|---|---|
| Conductivity | About 100 per cent IACS | About 25 to 30 per cent IACS |
| Tensile strength | Lower in the annealed condition | Higher, depends on temper |
| Machinability | Poor, gummy | Good in free-machining grades |
| Threads | Wear and gall easily | Hold threads well |
| Corrosion | Stable, tarnishes slowly | Can dezincify in wet service |
| Cost | Higher metal cost | Lower metal and machining cost |
| Best use | Barrel and palm of power lugs | Screws, clamps, small terminals |
Typical ranges only. Alloy, temper and plating change the values, so ask for the material certificate.
Where brass is acceptable
Brass is common in earth terminals, small soldering lugs, test points, cable glands and mechanical connector bodies where the current is low and the mechanical duty is high. It is also used for the clamp body of a mechanical lug, with a copper or tinned copper current path inside.
It can be acceptable on a power joint if the design has been rated for it. That means a current rating, a temperature rise test and a corrosion assessment, not an assumption that a thicker wall solves the problem. Some brass terminal blocks are rated for hundreds of amps because the mass and surface area are large, but the rating comes from test data.
Ask how the rating was reached. A block tested as part of a switchgear assembly may carry a rating that does not transfer to a free standing joint with a different cable and a different enclosure. The rating belongs to the assembly and the test setup, not to the alloy in isolation.
The environment matters. Brasses with a high zinc content can dezincify in warm, wet or chloride bearing service, leaving a porous copper sponge that loses strength. If the joint is outdoors, near the coast or washed down, a copper alloy with better resistance, or a plated surface, is the safer choice.
Corrosion, plating and the zinc question
Copper forms a thin oxide that is stable and protective. Brass forms a mixed oxide with zinc in it, and in some environments the zinc is selectively removed. The result is a weaker part with a rougher surface, and a joint that can lose clamp load.
Tin plating protects both materials and gives a consistent contact surface. On brass, the plating also limits zinc exposure at the surface. The plating thickness and adhesion matter more than the substrate colour, so specify the standard and the thickness.
A brass lug on an aluminium conductor adds a third metal to the galvanic couple. That is usually a reason to use a bimetallic lug or a copper alloy with a suitable transition, not a reason to rely on plating alone.
Plating porosity is the weak point. A coating with pinholes lets the electrolyte reach the substrate, and the corrosion starts at the pore. Thicker coatings and good process control reduce porosity, but a bend or a crimp can open a crack in the coating that was not there at inspection.
How to specify the material
Name the alloy and the temper, not just copper or brass. For copper, state the grade and the conductivity requirement. For brass, state the alloy and whether free-machining or dezincification resistant grade is required. Add the plating standard and thickness, and the current rating the part must meet.
Ask for the material certificate with the shipment and, for a new part, a temperature rise test at the rated current. If the part is a substitute for an approved copper lug, treat it as a new design and run the same checks.
Put the material callout in the title block as well as on the face of the drawing. A note that says brass near a dimension is easy to miss when a part is quoted from a PDF, and a substituted alloy is hard to see after tin plating. The title block is the one place every reader looks.
If the part carries a thread, state the thread standard and the engagement length. A brass thread holds better than a copper one, but a short engagement in a soft substrate still strips under torque. The material and the thread call belong together, because one decides the other.
- Alloy designation and temper
- Conductivity requirement for copper parts
- Dezincification resistance for brass in wet service
- Plating standard, thickness and adhesion
- Current rating and temperature rise evidence
- Thread specification and engagement length
- RoHS or lead content declaration for machined brass
Frequently asked questions
Are brass lugs as good as copper lugs?
For current carrying power joints, usually not. Brass conducts about a third as well as high conductivity copper, so the same size runs hotter. Brass is useful for screws, clamps and small low current terminals where strength and machinability matter more.
How can I tell a brass lug from a copper lug?
Colour is unreliable, especially after plating. Check the material certificate, the drawing callout or the conductivity. A simple conductivity meter can separate copper from brass on a bare area, but it will not identify the alloy.
Does tin plating make a brass lug suitable for power?
No. Plating protects the surface and improves contact, but it does not change the bulk conductivity of the brass. The current rating still depends on the substrate, the cross-section and the temperature rise.
What is dezincification?
It is the selective removal of zinc from brass in certain wet or chloride bearing environments, leaving a porous copper rich layer. The part loses strength and can leak or crack. Use a dezincification resistant alloy or a different material where the risk is present.
Sources and further reading
- EN 13601, Copper and copper alloys, copper rod, bar and wire for general electrical purposes
- ASTM B152, Copper sheet, strip, plate and rolled bar
- IEC 61238-1, Compression and mechanical connectors for power cables
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Reviewing a material substitution on a lug?
Send the original drawing, the proposed alloy and the rated current. The engineering desk will check whether the substitution needs a new temperature rise test.
Send a drawing