Copper Busbar Bend Allowance: Worked Examples

Copper busbar bend allowance controls whether a formed conductor reaches its terminals without forcing the assembly. A flat pattern that is only one millimeter wrong can shift a hole, reduce electrical clearance or preload an insulator after several bends.

This article works through practical 90-degree C110 copper examples. It complements our broader K-factor for copper guide, which covers temper, minimum radius, springback and surface protection in more detail.

Bend allowance formula

Bend allowance is the developed arc length through the neutral axis:

BA = A x (R + K x T)

Where:

• BA = bend allowance in millimeters.
• A = bend angle in radians.
• R = inside bend radius.
• K = K-factor, the neutral-axis position as a fraction of thickness.
• T = material thickness.

For a 90-degree bend, A = 1.5708 radians.

Starting K-factors for preliminary copper CAD are commonly around:

Copper condition	Preliminary K-factor
Annealed/soft	0.35
Quarter-hard to half-hard	0.40
Hard	0.45

These are not certified material constants. Actual bend data depends on tooling, radius, thickness, rolling direction and material lot.

Example 1: 2 mm soft C110 copper

Assume:

• Thickness T = 2.0 mm.
• Inside radius R = 2.0 mm.
• Bend angle A = 90 degrees = 1.5708 rad.
• Preliminary K = 0.35.

BA = 1.5708 x (2.0 + 0.35 x 2.0)
BA = 1.5708 x 2.70
BA = 4.24 mm

If both outside flange dimensions are 30 mm, first calculate the outside setback for a 90-degree bend:

OSSB = (R + T) x tan(45 degrees)
OSSB = (2.0 + 2.0) x 1
OSSB = 4.00 mm

Then bend deduction is:

BD = 2 x OSSB - BA
BD = 8.00 - 4.24
BD = 3.76 mm

The initial flat length is therefore:

Flat length = 30 + 30 - 3.76 = 56.24 mm

This is a calculation baseline, not a substitute for the fabricator’s validated bend table.

Example 2: 3 mm half-hard C110 busbar

Assume a larger power busbar:

• T = 3.0 mm.
• R = 3.0 mm.
• A = 90 degrees.
• Preliminary K = 0.40.

BA = 1.5708 x (3.0 + 0.40 x 3.0)
BA = 1.5708 x 4.20
BA = 6.60 mm

For two 50 mm outside flanges:

OSSB = (3.0 + 3.0) x 1 = 6.00 mm
BD = 12.00 - 6.60 = 5.40 mm
Flat length = 50 + 50 - 5.40 = 94.60 mm

Half-hard copper normally shows more springback than annealed material. The flat-pattern length may be close while the released angle still needs CNC overbend correction.

Example 3: 1.5 mm hard copper with a larger radius

Assume:

• T = 1.5 mm.
• R = 3.0 mm or 2T.
• A = 90 degrees.
• Preliminary K = 0.45.

BA = 1.5708 x (3.0 + 0.45 x 1.5)
BA = 1.5708 x 3.675
BA = 5.77 mm

The larger radius protects the harder temper from cracking. Do not reduce the radius only to shorten the developed blank; electrical reliability and fatigue life are more important than a small material saving.

How grain direction changes the result

Where possible, orient the bend line perpendicular to the copper rolling direction. Bending parallel to the grain raises crack risk, especially in harder tempers. It can also change springback enough to invalidate a bend table developed in the other orientation.

Show grain direction on the drawing when:

• Temper is half-hard or hard.
• Radius is near the material’s minimum.
• The busbar sees vibration or thermal cycling.
• Multiple parts must share a controlled electrical layout.
• The same flat pattern is nested in different orientations.

See our sheet metal grain direction guide for drawing conventions.

Multi-bend busbars

For a busbar with multiple bends, calculate each bend using the intended radius and angle, then validate the total formed chain. Small errors accumulate.

Control these dimensions from functional datums:

• Hole center to formed terminal face.
• Distance between terminal planes.
• Offset between parallel legs.
• Electrical clearance to enclosure walls.
• Coplanarity at bolted contact pads.

Avoid locating every feature from one end of a long flat strip if the installed part is controlled by two formed contact faces. Dimension the functional interfaces in the formed state.

Contact surfaces and plating

Bend calculations are only one part of busbar manufacturing. The drawing should also define:

• Contact-pad flatness and surface condition.
• Tin, nickel, silver or bare copper areas.
• Masking at threaded and contact interfaces.
• Burr direction and edge radius.
• Hole finish and allowable distortion after bending.
• Conductivity or material certificate requirements.

Plating after bending normally protects the finished geometry, while bending plated stock can crack or mark the coating. Confirm the sequence with the supplier.

Where calculation stops and shop data begins

The calculation gets the first blank close enough to run a meaningful first article. It should not be presented as a universal production answer. Once the actual lot and tooling are available, measured flange and angle results become the better source of truth.

1. Lock alloy, temper and thickness.
2. Confirm grain orientation and inside radius.
3. Select the intended production tooling.
4. Calculate the initial flat with a preliminary K-factor.
5. Bend a coupon or first article from the ordered material lot.
6. Measure released angle, flange dimensions and surface condition.
7. Update bend deduction and CNC compensation.
8. Freeze the validated recipe for repeat orders.

The drawing should control the formed result. The supplier’s released flat and machine recipe should control how that result is achieved.

Busbar RFQ checklist

• C110/C101 alloy and temper.
• Thickness and material specification.
• Grain direction and bend orientation.
• Critical formed dimensions and datums.
• Inside radius and acceptable surface marking.
• Contact-pad and plating requirements.
• Current, temperature-rise or joint-test requirement.
• Prototype quantity and annual demand.

Related customer project: EV Battery Busbars: Resistance and Insulation Control. Related resources: custom copper busbars · CNC bending · copper fabrication capabilities · send the formed model and current requirements.