LibraryReferenceEarthing

Conductor sizing

The conductor of an earth mat carries the fault current for the duration of the fault. The design sizes it at the design fault level of the HV level, not at the grid current. It must do so without its temperature passing a limit that its joints withstand. IEEE Std 80-2013 gives the cross-section that keeps the conductor within that limit (§11.3.1).

1The heating of the conductor

The temperature the conductor reaches depends on the current, the time, the cross-section and the metal's properties. The minimum cross-section keeps the final temperature at the limit $T_{m}$ .

A = \frac{I}{\frac{TCAP \cdot 1 0 ^{- 4}}{t _{c} α _{r} ρ _{r}} ln \frac{K _{0} + T _{m}}{K _{0} + T _{a}}}

(1)

IEEE Std 80-2013 §11.3.1, Eq. 37

where
$A$	minimum cross-section of the conductor	mm²
$I$	rms fault current	kA
$t_{c}$	duration of the current	s
$TCAP$	thermal capacity per unit volume	J/(cm³·°C)
$α_{r}$	thermal coefficient of resistivity at 20 °C	1/°C
$ρ_{r}$	resistivity of the conductor at 20 °C	µΩ·cm
$K_{0}$	$1/ α_{0}$ , the inverse of the coefficient at 0 °C	°C
$T_{m}$	maximum allowable temperature	°C
$T_{a}$	ambient temperature	°C

The IEC + CEA / CBIP set's mat is mild steel rod, taken with the constants of steel 1020 from IEEE Std 80-2013 Table 1. Its limit $T_{m}$ is the design's own, 500 °C, below the fusing temperature of welded steel joints, and is not a figure of IEEE 80. The ANSI / IEEE set's mat is bare annealed copper with exothermic connections, with the copper constants as RUS Bulletin 1724E-300 Table 9-2 gives them.

2Corrosion and the standard size

A steel rod buried for the life of the station loses metal from its surface. The design adds 0.12 mm a year per surface over 25 years to the diameter from Eq. (1), and takes the next size of the standard mild steel rods, 16, 20, 25, 32, 40 and 50 mm. The copper conductors of the ANSI / IEEE set take the next standard size from 4/0 AWG, with no corrosion allowance.

Example 1Conductor of the default 220 kV mat

Given the design fault level of 50 kA for 1.0 s, steel 1020 with TCAP = 3.28 J/(cm³·°C), $α_{r}$ = 0.00160 1/°C, $ρ_{r}$ = 15.90 µΩ·cm and $K_{0}$ = 605 °C, a limit of 500 °C and an ambient temperature of 50 °C.

Cross-section by Eq. (1):

A = \frac{50}{\frac{3.28 \cdot 1 0 ^{- 4}}{1.0 \times 0.00160 \times 15.90} ln \frac{605 + 500}{605 + 50}} = 608.9 m m^{2}

Diameter and corrosion: the area gives a diameter of 27.8 mm. The allowance of 0.12 mm a year per surface over 25 years adds 6 mm.

Result The mat takes the next standard rod, 40 mm in diameter.

Sources

[1]IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding. §11.3.1, Eq. 37.

CitationSubstationLayout Reference, Conductor sizing, revision 1, 8 Nov 2026, https://substation.solarlayout.app/library/reference/conductor-sizing

Conductor sizing

1The heating of the conductorCopy link to The heating of the conductor

2Corrosion and the standard sizeCopy link to Corrosion and the standard size

1The heating of the conductor

2Corrosion and the standard size