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Sizing the 32 A earth
For a 32 A circuit on copper, the minimum protective (earth) conductor under IEC 60364-5-54 is 2.5 mm², because the live conductors a 32 A breaker protects are almost always 16 mm² or less. In North American sizes, 14 AWG at 2.08 mm² falls short and 12 AWG at 3.31 mm² clears it, so 12 AWG is the answer. Two things can push the size upward: the installation-method column of the standard, and the loop-impedance test that proves the fault actually disconnects in time.
For copper wiring on a 32 A breaker, the minimum earth is 2.5 mm² under IEC 60364-5-54 — and 12 AWG (3.31 mm²) is the smallest North American size that clears it.
The number printed on the breaker does not set the earth size. IEC 60364-5-54 keys the protective conductor to the cross-section of the live conductors, and a 32 A circuit is normally wired with live conductors far below 16 mm². That places it in the first tier of the table: live conductors up to 16 mm² require a copper protective conductor of at least 2.5 mm².
Translated into AWG, the answer is 12 AWG. The nearest smaller size, 14 AWG, is 2.08 mm² — about 17% below the 2.5 mm² floor, a real shortfall rather than a rounding error. Note the asymmetry on the live side: under NEC 310.16 with 75 °C terminations, 10 AWG is capped at 30 A, so a 32 A breaker in AWG country actually pairs with 8 AWG live conductors at 8.37 mm² and 40 A.
Two mechanisms can raise the answer above the table minimum. The installation-method column of the standard imposes larger sizes for certain runs, and the automatic-disconnection test can demand a bigger earth wherever the measured loop impedance is too high for the required disconnection time.
IEC 60364-5-54 sets the copper protective-conductor minimum in three tiers, and the installation-method column can raise every one of them.
The standard divides installations by live conductor cross-section. Up to 16 mm² of live conductor, the copper earth minimum is 2.5 mm². Between 16 and 35 mm² it steps to 6 mm², and between 35 and 150 mm² it steps again to 16 mm². Each tier is a floor, not a target.
The installation-method column is the part most often skipped. How the cable is run — buried, in conduit, clipped to a surface — can impose a larger protective conductor than the headline tier, so the table value is the starting point of the decision, never the end of it.
The tiers exist because the earth's job is abnormal duty. In a fault it must carry current only until the protective device clears: overloads are handled by a thermal element that must release by 1.45 × In within an hour, but earth faults are high-current events where IEC 60364-4-41 demands disconnection within 3 s for circuits up to 32 A and within 0.3 s above 32 A.
No AWG size equals a round metric number, so the working rule is to pick the first AWG size whose cross-section meets or exceeds the metric requirement.
The four sizes that dominate small installations convert as follows: 14 AWG is 2.08 mm², 12 AWG is 3.31 mm², 10 AWG is 5.26 mm² and 8 AWG is 8.37 mm². The steps between them are uneven, which is why a metric minimum often lands between two AWG numbers.
Permitted current under NEC 310.16 at 75 °C terminations climbs with the same steps: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG and 40 A for 8 AWG. This is where the 32 A question bites twice — 10 AWG stops at 30 A, so the live side of a 32 A circuit needs 8 AWG even though the earth side only needs to clear 2.5 mm².
The mismatch matters most for protective conductors. A 2.5 mm² metric earth minimum falls between 14 and 12 AWG, and rounding down to 14 AWG leaves the installation 0.42 mm² short — a compliance failure produced by arithmetic, not by workmanship.
The table gives a minimum; the ohmmeter gives the verdict. A protective conductor is sized on paper and proven by measurement.
A 30 mA device protects people, a 300 mA device protects buildings — and only one of them belongs on a socket-outlet.
| Conductor designation | Cross-section | Permitted current (75 °C terminations) | Role around a 32 A circuit |
|---|---|---|---|
| 14 AWG | 2.08 mm² | 15 A | Too small even for the earth: below the 2.5 mm² minimum |
| 12 AWG | 3.31 mm² | 20 A | Smallest AWG that clears the 2.5 mm² earth minimum |
| 10 AWG | 5.26 mm² | 30 A | Under the breaker's rating — cannot be the live conductor |
| 8 AWG | 8.37 mm² | 40 A | Live conductor that properly pairs with a 32 A breaker |
| 2.5 mm² metric | 2.5 mm² | set by installation method | Minimum copper earth for live conductors up to 16 mm² |
| 6 mm² metric | 6 mm² | set by installation method | Minimum copper earth for live conductors of 16–35 mm² |
| 16 mm² metric | 16 mm² | set by installation method | Minimum copper earth for live conductors of 35–150 mm² |
Reading the table takes minutes; proving the circuit disconnects within its required time is a five-step measurement job.
The table gives a minimum; the automatic-disconnection test decides whether that minimum is enough. In TT systems IEC 60364-4-41 allows 3 s for circuits up to 32 A but tightens to 0.3 s above 32 A, and the loop impedance is what determines whether those times are met.
Three named failures account for most circuits that look correct on paper and fail under test: the corroded joint, the hot measurement and the borderline overload. Each is identified in the steps below before any upsizing is specified.
A protective device does not respond to its rating plate; it responds to how many ohms stand between the fault and the supply.
The ohm is the whole story in one unit: a potential difference of 1 V across a current of 1 A. A 2 kW heater drawing 8.7 A on a 230 V supply implies an element resistance of about 26.4 Ω — the same arithmetic that turns a measured loop impedance into a predicted fault current.
A healthy 5 m loop of 1.5 mm² twin-and-earth measures roughly 0.5–1.2 Ω end to end. One corroded joint can add more than 1 Ω on its own — more resistance than the cable itself — and push the loop past the tripping threshold even though the RCD is rated correctly.
Temperature moves the goalposts. Copper resistivity is 1.68 × 10⁻⁸ Ω·m at 20 °C and rises roughly 0.4% per degree, so a run measured at 60 °C reads about 16% higher than its 20 °C value; a marginal loop can pass on a cool morning and fail at working temperature.
The standards and physical constants behind each figure are named in the text; no commercial source is cited anywhere on this page.