Skin Effect Is Real at 50Hz — Here's When It Actually Affects Your Cable Sizing

Skin effect forces AC current toward the outer surface of a conductor, reducing the effective cross-sectional area and increasing resistance. At 50Hz, the skin depth in copper is approximately 9.3mm. For small cables, the conductor radius is well within one skin depth and the effect is negligible. For large cables, it is not.

AS/NZS 3008.1.1:2017, Table 35 provides skin and proximity effect multipliers. IEC 60287-1-1, Clause 2.1.2 gives the calculation method.

Consider a 500m run of single-core 630mm² copper cable supplying a 400A load at 400V three-phase.

• Using DC resistance (0.0283 Ω/km): voltage drop = √3 × 400 × 0.0283 × 0.5 × cosφ ≈ 2.8%
• Using AC resistance (0.0334 Ω/km, after skin + proximity effect): voltage drop ≈ 3.3%

That 0.5% difference pushes the circuit from comfortably within the 3% BS 7671 lighting limit to marginally over it. For an engineer who used DC resistance from a quick table lookup, the cable passes the check. In reality, it fails.

Skin effect’s lesser-known companion is proximity effect: adjacent conductors carrying current distort each other’s current distribution, further increasing effective resistance. For trefoil-laid single-core cables at 630mm², proximity effect adds another 2–5% on top of skin effect. IEC 60287-1-1, Clause 2.1.4 provides the calculation. Standard tables like AS/NZS 3008 Table 35 include both effects in their published correction factors.