Engineering Blasphemy: Smaller is not Always Better

USING ALUMINUM FOR BUS BARS

Design engineers strive to make equipment smaller and more efficient.  The hidden assumption being that smaller is always better.

Copper bus bars require less cross-sectional area than aluminum and can fit inside a tighter footprint.  Thus, the vast majority of electrical equipment is designed with copper bus bars.

However, aluminum has a higher conductivity by weight than copper and often costs less to produce the same level of current.  Electrical grade Aluminum offers a unique combination of both electrical and mechanical uses.

Then why don’t we see more aluminum bus bars designed into equipment?

In reality, most large manufacturers of electrical equipment have aluminum bus integrated into their products.  Busway, transformers, and circuit breakers all utilize aluminum in low voltage applications.  These industries have also proved that equipment can be integrated with both copper and aluminum and remain reliable for decades of service.

Even with many examples of successful integration of aluminum, some engineers doubt its efficacy in certain industries.  Designers are typically hesitant to use aluminum unless it is familiar to them.  The doubts center around four main points:

1. Conductivity
2. Strength
3. Electroplating
4. Connections and Terminations

1. Conductivity

There is no getting around the fact that copper is more conductive than aluminum given the same cross-sectional area.  Aluminum 6101 T-61 has around 56% of the ampacity of copper.  In some circles it is blasphemy to consider using a material that is larger than necessary, even if it does cost less. However, there are good reasons to go further.

Solution:  Size the aluminum properly for the needed current.  The aluminum cross over tables can help you determine what size you need to choose.  In some instances, you can simply rate the aluminum version differently.

Busway is a good illustration of how the industry defines different current ratings using aluminum.  UL 857, which is the busway standard, allows for the use of either conductor so long as it meets temperature rise specifications.  For example, the 1200-amp copper bus system will be rated at 800 amps aluminum.   Both versions have the same .25” x 2.5” bus bars but have different ratings.

2. Strength

Electrical grade Aluminum alloy 6101 has a tensile strength around 28,000 psi.  By comparison copper has a tensile of 32,000 psi.  The mechanical properties of aluminum are important and need to be considered with respect to loading (e.g. elastic moduli, hardness, ductility).  All materials experience physical changes while undergoing mechanical loads which can lead to plastic deformation and even failure.  Although aluminum has a lower Young’s and Shear modulus than copper, aluminum bussing is always sized to mitigate the differences.  When designing for yield strength and tensile strength, true stress and strain must be considered to account for the effects of elongation and the instantaneous shrinkage in the cross-sectional area of interest.

Copper sized for a given amperage will approach plastic deformation and failure sooner as the cross-sectional area shrinks when compared to an appropriately sized aluminum bus bar.  Moreover, both copper and aluminum have a Poisson’s ratio of .33 revealing a similar compression resistance and no major advantages of one over the other on creep at bolted/jointed connections.

Solution:  Size the aluminum bus bar properly and the mechanical properties are very similar to copper.

3. Electroplating

Both copper and aluminum bus bars oxidize when exposed to the air.  The key difference is the conductivity of the oxidation layer on the conductor. Oxides on aluminum are much less conductive than oxides on copper.  At connection points the oxide could result in loss of current resulting in unwanted heat or even failure.

Solution:   Electroplating is vital to the integrity of aluminum bus bars.  Plating the aluminum with either tin or silver gives more corrosion resistance and eliminates the possible issues that arise with aluminum oxidation.

4. Connections and Terminations

Aluminum and copper should never be connected without proper hardware and plating.  The thermal expansion of aluminum is 42% greater than copper.  To maintain a low resistance joint during thermal cycling using the proper components will distribute the clamping force without creating a permanent deformation.  Belleville washers and split lock washers allow for a mechanical flex during cycling.  The steel washer has less thermal expansion and increases the clamping force.  Additionally, cable connections should include lugs that are ALCU rated or tin-plated mechanical set screws.

Solution:  Use washers that are either Belleville or split lock washers to ensure you have a stable joint that will respond well during expansion and contraction.

The payoff:  Sample quote

After sizing the bar properly and selecting the right plating finish the bus bar should react the same as a copper version.  The sample quote below is for a simple flat copper bus bar and the aluminum equivalent.  The best candidates are the ones that are relatively simple.  The quote below is for a copper bar that is .25” x 3.5” x 12.75” and contains 6 holes.  The aluminum version is .50” x 5” x 12.75” long.  Ampacities for both would be 1150 and 1148 respectively at 30° rise.