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Proximity effect is the tendency for current to flow along one side of a conductor due to interaction of the magnetic fields of the current in the conductor considered and the currents in adjacent conductors.
The formulae are accurate providing $x_p$ does not exceed 2.8, and therefore applies to the majority of practical cases.
| $0$ | 1 single-core cable |
| $\frac{2.9{x_{p}}^4}{192+0.8{x_{p}}^4} \left(\frac{d_{c}}{s_{c}}\right)^2$ | two-core cables & 2 single-core cables |
| $\frac{2.9{x_{p}}^4}{192+0.8{x_{p}}^4} \left(\frac{d_{x}}{s_{c}}\right)^2$ | two-core cables, sector-shaped conductors |
| $\frac{{x_{p}}^4}{192+0.8{x_{p}}^4} \left(\frac{d_{c}}{s_{c}}\right)^2 \left(0.312\left(\frac{d_{c}}{s_{c}}\right)^2+\frac{1.18}{\frac{{x_{p}}^4}{192+0.8{x_{p}}^4}+0.27}\right)$ | three-core cables & 3 single-core cables |
| $\frac{\frac{2}{3} {x_{p}}^4}{192+0.8{x_{p}}^4} \left(\frac{d_{x}}{s_{c}}\right)^2 \left(0.312\left(\frac{d_{x}}{s_{c}}\right)^2+\frac{1.18}{\frac{{x_{p}}^4}{192+0.8{x_{p}}^4}+0.27}\right)$ | three-core cables, sector-shaped conductors |