Can an AC Copper Busbar Use a DC Rating? Skin Depth and Proximity Effects

At a glance

Calculate copper skin depth at 50 Hz, 400 Hz and 20 kHz, then identify the geometry, current spectrum and thermal checks needed before using a DC busbar rating.

Direct answer Equal copper cross-sectional area does not justify transferring a DC ampacity to AC. Establish the current spectrum in the actual bar, evaluate AC resistance for its geometry and return path, and verify temperature rise in the installed assembly. The switching frequency of a device is not the frequency of all current in its busbar.

Calculate the scale before choosing a model

For nonmagnetic copper, estimate skin depth with δ=√[ρ/(πfμ)]. Use resistivity ρ in Ω·m, frequency f in Hz and permeability μ in H/m; δ is in metres. The following worked values use ρ=1.7×10⁻⁸ Ω·m near 20 °C and μ≈μ₀=4π×10⁻⁷ H/m, excluding plating and temperature changes.

Frequency	Approximate skin depth	Scope
50 Hz	9.28 mm	Power-frequency dimensional screening
400 Hz	3.28 mm	Reassess AC loss for the same bar
20 kHz	0.46 mm	Applies to that frequency component

These calculations are not thickness limits. Skin depth does not mean that the interior carries no current. COMSOL's copper example gives about 9 mm at 50 Hz, a useful order-of-magnitude check with different material inputs.

Include neighbouring conductors

Copper for Busbars treats skin and proximity effects separately. Relative current direction, phase and spacing affect current distribution. Include outgoing and return conductors, parallel branches, joint geometry and nearby magnetic structures. An AC resistance factor for an isolated bar cannot automatically be reused for a different arrangement.

Report S=R_ac/R_dc at equal temperature and length. Where the materials and system can be treated as linear, a useful conductor-loss estimate is P≈I_dc²R_dc+ΣI_h,rms²R_ac(f_h). Do not assign the highest-frequency resistance to the entire RMS current.

Prepare useful simulation and test inputs

Provide a dimensioned layout, current waveform or harmonic RMS values, material and finish, operating temperature estimate, orientation and cooling boundaries. CDA tables explicitly distinguish emissivity and multi-bar configurations. During assembly tests, record ambient temperature, joint condition and measurement locations. Acceptable conductor loss does not rule out a hot connection.

Questions from design reviews

Are multiple thin bars always better? Recheck proximity loss, current sharing, joints, insulation and assembly constraints. Twice as many bars does not automatically mean twice the ampacity.

Must a DC bar with small ripple be sized entirely for high frequency? Measure ripple magnitude and spectrum, then evaluate its loss contribution before simplifying the model.

Sources

Sources checked on 6 October 2026. Worked examples explain methods, not HC-SP product ratings or test results. Cover is an AI-generated engineering illustration, not an actual product photograph.

Apply this to your component selection

Start with the product catalog and model guide. For a specific project, send the part number or drawing, quantity and key operating conditions. This article will be included in your enquiry for context.

Articles explain selection considerations and do not replace the confirmed drawing, test conditions or supply documents for a specific part. Check any cited source and its applicable edition for standards and parameters.