What it is, and why the number has to be big enough to worry about

When to use this: selecting a consumer unit or a distribution board; justifying a 6 kA board on a supply declared at 16 kA; sizing switchgear from a transformer nameplate; recording a PFC reading on a certificate; or feeding the I²t figure into a conductor check.

Prospective fault current is the current that would flow if a fault of negligible impedance happened at a given point — a bolted fault, with nothing limiting it but the impedance of the supply and the conductors leading to it.

I = U ÷ Z

That is the whole calculation. Ohm’s law applied to the fault loop. The difficulty is never the arithmetic; it is knowing which impedance and which voltage belong to the fault you are actually worried about.

It matters because every protective device has to be able to interrupt it. A device asked to break more current than it is rated for does not politely fail to trip. It can fail destructively — welded contacts, an arc it cannot extinguish, a fault that stays live behind a device that has already given up.

Three names for two things

• PSCC — prospective short-circuit current. Line to neutral, or line to line.
• PEFC — prospective earth fault current. Line to earth, through the protective conductor.
• PFC — whichever of those two is greater, because that is the one the device has to survive.

On a TN-C-S supply the two are usually close together. On TN-S the earth path is longer and the short-circuit figure normally wins. Measure both; record the higher.

From a transformer nameplate

The supply-side calculation, and the one that produces the alarming numbers.

Isc = Irated ÷ (Z% ÷ 100)

The impedance voltage on a transformer’s plate — typically 4% to 6% on a distribution unit — is defined as the percentage of rated voltage you need to apply to drive rated current through a short-circuited secondary. Invert it and you have, directly, how many times rated current a bolted fault at the terminals produces.

A 500 kVA 400 V three-phase transformer is rated 722 A. At 5% impedance a fault at its terminals draws 14.4 kA — twenty times rated current. At 4% it draws 18 kA. A stiffer supply means a bigger fault, which is worth sitting with for a moment: the better the supply, the harder the switchgear has to work.

The three-phase doubling rule — and why it is exact

Take the highest line-to-neutral reading and double it. This gets taught as a rule of thumb and it is not one; it falls out of counting conductors.

A line-to-neutral fault travels out along the line conductor and back along the neutral. It sees two conductors in series:

ILN = U0 ÷ (ZL + ZN) = U0 ÷ 2Z

A three-phase symmetrical fault has no return through the neutral at all — the three phase currents are 120° apart and sum to zero at the fault — so each phase sees only its own impedance:

I3ph = U0 ÷ Z

Divide one by the other and the factor is exactly 2, whenever the neutral has the same impedance as the line. Where the neutral is reduced — a half-size neutral on a sub-main — the line-neutral loop is 3Z rather than 2Z, the true factor is 3, and doubling understates the fault. Measure line-to-line directly if that applies.

The contradiction everyone runs into

A UK distributor declares up to 16 kA at the origin of a domestic supply. The consumer unit you fit is rated 6 kA. Every house in the country is wired this way, and on the face of it every one of them is non-compliant.

They are not, and the reason is Regulation 434.5.1: a device may have a breaking capacity below the prospective fault current where an upstream device provides backup protection.

The distributor’s cutout fuse — normally a 100 A BS 88-3 — is current-limiting. On a severe fault it clears within the first few milliseconds, long before the current reaches its prospective peak, so the energy that actually arrives at the breaker behind it is a fraction of what an unrestricted 16 kA fault would deliver. The pair together has a conditional short-circuit rating well above the breaker’s own rating.

Fault current falls with distance

Every conductor between the source and the fault adds impedance, so the further into the installation you go, the smaller the fault:

Same house, a third of the current. Using the origin figure everywhere is conservative — it oversizes breaking capacity but never undersizes it — which is why it is the normal design practice and why nobody calculates PFC circuit by circuit.

The trade-off nobody mentions

Low loop impedance is what makes a circuit disconnect quickly. Low loop impedance is also what produces a large fault current. The two requirements pull in opposite directions:

• Loop impedance wants Z low, so the device operates in time.
• Breaking capacity wants the fault current low, which means Z high.

A large prospective fault current is therefore usually a sign of a good supply, not a bad design. It is the reason both calculations exist and the reason neither can be done without the other.

What this does not settle

Breaking capacity is about whether the device can interrupt the fault. It says nothing about whether the conductor survives it for however long that takes. That is the adiabatic check, S = √(I²t) ÷ k, and the I²t figure this page reports is exactly what it takes as input.

Two requirements, two failure modes, one fault current feeding both. A circuit can have entirely adequate switchgear and a protective conductor that will not survive the fault the switchgear is about to clear.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 4 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 25 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.