Prospective Fault Current: The Two Voltage Factors BS 7671 Gives You

The fault level is the current that would flow in a bolted short circuit at a given point. It is an unglamorous number that quietly governs two safety-critical things: whether your switchgear can break the fault, and whether your cables survive it.

It is also a number most people quote as if there were one of it. There are at least two, and BS 7671 tells you both — along with a new appendix, added in Amendment 4, that most guidance has not caught up with yet.

1. Two voltage factors, one point

A prospective fault current calculated from an impedance needs a voltage to divide by, and the supply voltage is not a fixed number. BS 7671 handles that with a voltage factor, and it publishes two of them.

One distribution board feeding two panels. On the left, Cmax = 1.1 from the BS 7671 Part 2 symbols table, taken at the origin where the fault is highest, answering whether the device can break it and driving breaking capacity and withstand. On the right, Cmin = 0.95 from Table 41.3 NOTE 1 and Regulation 411.4.4, taken at the electrically furthest point where the fault is lowest, answering whether the device will still operate and driving disconnection times and every maximum earth fault loop impedance in Chapter 41. — Cmin is familiar because every maximum Zs in Chapter 41 is built on it. Cmax sits in the Part 2 symbols table and is almost never quoted — yet the two are 16 % apart at the same point, before impedance is considered at all.

The pair, and where each is written down
Factor	Value	Source	Question	Where it is taken
C_max	1.1	Part 2 symbols	Can the device break it?	At the origin, where the fault is highest. Drives breaking capacity and withstand.
C_min	0.95	Table 41.3 NOTE 1 · Reg 411.4.4	Will the device still operate?	At the electrically furthest point. Drives disconnection times and every maximum Zs in Chapter 41.

Same wording, same standard, 16 % apart — and only one of them is ever quoted.

C_max is easy to miss for a reason: the symbols table points it at Item A722.2, which Amendment 2 deleted. It is genuinely used in A722.3, the electric-vehicle PEN-fault electrode formula — but its definition and value sit in the symbols table itself, in wording general rather than EV-specific: “the maximum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations. For a low voltage supply given in accordance with the ESQCR, C_max is given the value 1.1.” That 1.1 is the statutory tolerance: 230 V + 10 % = 253 V.

So the maximum and the minimum prospective fault at the same point differ by 16 % before you have considered a single ohm of impedance. Take the wrong one and you either fit switchgear that cannot break the real fault, or condemn a circuit whose protection would in fact have operated.

2. When the factor does not apply

This is the qualifier that makes the rest defensible, and it is easy to get wrong. Regulation 434.1 says the prospective fault current shall be determined at every relevant point “by calculation, measurement or enquiry”. A voltage factor belongs to the first of those only.

Calculated from a transformer impedance and the nominal voltage — apply the factor. The real voltage could genuinely be 10 % higher.
Declared by the distributor, or measured — do not. That figure already accounts for whatever the supply was doing. Scaling it inflates an answer; it does not make it safe.
A UPS or inverter — do not. Its fault contribution is an electronic current limit, delivered regardless of what the supply voltage does.
A standby generator — do not, at least not with this number. BS 7671 scopes C_max to “a low voltage supply given in accordance with the ESQCR”, and a private set is not that. Its fault is governed by X″_d and AVR action, which is different physics.

We got this wrong first time. The initial change applied C_max everywhere, and a round-trip test — “a declared 16 kA supply must resolve back to 16 kA” — caught it. Without that test every declared supply on the site would have been silently inflated by 10 %.

3. It is a per-board number

A distribution run from an 800 kVA transformer through a main switchboard, a sub-board and a final distribution board, with the prospective fault falling from about 25 kA at the transformer to about 4 kA at the final board, and the breaking capacity each device needs falling with it. Regulation 434.1 requires the fault current to be determined at every relevant point — every device that may have to disconnect a fault. — A device that is perfectly adequate at the final board would be destroyed at the transformer. The minimum fault matters at the far end for the opposite reason: too little current and the protection never operates at all.

Fault level is highest at the origin and falls along every cable run, because the cable adds impedance. Regulation 434.1’s “every relevant point” is doing real work here: Appendix 14 defines relevant points as the switchgear and protective devices that may have to operate and disconnect a fault current.

One subtlety that catches people out: impedances in series add as vectors, not as magnitudes. Adding |Z₁| + |Z₂| always over-states the total and so under-states the fault — the unsafe direction for a breaking-capacity decision. The error runs to 4–8 % on a sub-main, which is exactly where breaking capacity gets checked, because that is where a resistive cable meets a reactive source.

4. Root 3, or 2? Appendix 14 gives both

Appendix 14 is new in Amendment 4:2026. The previous content, on earth fault loop impedance, moved into Appendix 3, and Appendix 14 became Determination of Prospective Fault Current. It contains a detail that is routinely conflated.

They are not alternatives and they are not interchangeable. Root 3 gives the line-to-line fault; 2 gives the three-phase fault.

Measure line-to-neutral, then:

× √3 approximates the fault between two line conductors.
× 2 approximates the simultaneous fault between all three line conductors — the highest, and the one that sizes switchgear.

They are not alternatives. Both fall out of one assumption — that the neutral impedance is close to the line impedance, so a line-to-neutral loop is about twice a single phase impedance Z:

I_LN = U₀ / 2Z · I_LL = √3·U₀ / 2Z = √3 × I_LN · I_3ph = U₀ / Z = 2 × I_LN

Which also tells you when the shortcut weakens: where the neutral is reduced under Regulation 524.2, the line-to-neutral loop is higher than twice a phase, and both approximations under-state the fault.

5. The regulation asks for the earth fault too

Two fault currents feeding a single result. The short-circuit current, line to line or across all three lines, and the earth-fault current, line to Earth through an earth-fault loop of Ze plus R1 plus R2. Appendix 14 takes the greater of the two as the prospective fault current Ipf. On a TN-C-S system the earth fault can exceed the short-circuit fault; on TT it can be only a few amperes. — A tool that reports only the short-circuit current answers half the regulation — which is exactly what this site's own fault calculator did until this article was written.

Regulation 643.7.3.201 is explicit, and the word that matters is “and”: “the prospective short-circuit current and prospective earth fault current shall be measured, calculated or determined by another method, at the origin and at other relevant points in the installation.”

Appendix 14 completes the thought: in a single-phase system the prospective fault current is the greater of the line-to-neutral fault or the line-to-Earth fault. That is the figure a certificate records — I_pf = max(PSCC, PEFC).

It matters because the two can be in either order. On a TN-C-S supply with a low Z_e, the earth-fault loop can be comparable to the short-circuit path and the earth fault can be the larger. On TT it can be a few amperes — which is precisely why TT needs an RCD rather than relying on overcurrent protection.

6. The fault types, and the one real ratio

Which fault, and what it governs
Fault	Of the three-phase	Governs	Note
Three-phase symmetrical	100 %	Equipment breaking capacity and withstand	Appendix 14: the highest prospective fault current in a three-phase installation.
Line to line	86.6 %	An intermediate check	Exactly √3/2 of the three-phase value — arithmetic, not an estimate.
Line to Earth	varies	Earth-fault protection and automatic disconnection	Set by the earthing system and Z_s, so it has no fixed ratio. On TT a few amperes; on TN-C-S it can exceed the short-circuit fault.

86.6 % is exactly √3/2 — arithmetic, not a rule of thumb.

There is also a distinction the RMS figure hides. In the first half-cycle the current is asymmetric because of a decaying DC component, and the peak can be roughly 2.2–2.5× the RMS symmetrical value depending on the X/R ratio — i_p = κ√2 I_k, with κ bounded between 1 and 2. That peak is what making capacity and the mechanical forces on busbars and cleats are judged against. The calculator now reports it alongside the RMS value, and cable cleats & short-circuit forces works through what it does to a cleated run — where the force goes as the square of the peak, so using the RMS understates it roughly five times.

7. What the number is actually for

Sizing the device and protecting the cable are different questions asked of the same number, and only one of them can be answered by the current alone.

Breaking capacity. The device’s rating must equal or exceed the prospective fault at its point of installation — unless combined short-circuit protection to Regulation 536.4.2.1 is used, which is a real and legitimate technique but needs the manufacturer’s tested data. The symbols people quote here — I_cu, I_cs, I_cn — come from BS EN 60947-2 and BS EN 60898-1, and appear nowhere in BS 7671.

Cable withstand. Regulation 434.5.2: the energy the device lets through must not exceed what the conductor can take, I²t ≤ k²S². Note that this one cannot be answered by the fault current alone — it needs the disconnection time as well, and a larger fault clears faster, so the answer is not monotonic.

8. How to determine it, and how to measure it safely

Regulation 434.1 gives three routes: calculation, measurement or enquiry. Appendix 14 adds that other methods are not precluded, and covers two practical cases worth knowing.

The domestic exemption. In domestic premises, where a consumer unit to BS EN 61439-3 is used and the maximum prospective fault current declared by the distributor is 16 kA, it is not necessary to measure or calculate the prospective fault current at the origin. That rests on the consumer unit’s conditional rating, which in turn rests on the distributor’s fuse ahead of it — so it is an exemption with conditions, not a default assumption.

Multiple sources. Where there is more than one source, Appendix 14 asks for the fault current to be determined for all combinations of supply arrangement, so that any contribution from privately controlled embedded generation or an uninterruptible supply is included. The worst case may be a combination nobody normally runs.

Measuring it safely. Appendix 14 is direct about this. Measure on the output terminals of a suitably rated protective device; if one is not present, fit a temporary one. Never measure where there is no overcurrent protection between the point of connection and the supply transformer — and fused test leads alone do not meet that requirement.

Frequently asked questions

What is prospective fault current?

The current that would flow in a bolted (zero-impedance) fault at a given point. It is set by the impedance of everything between the source and that point, so it is highest at the transformer and falls along the cables — which is why Regulation 434.1 requires it to be determined at every relevant point, not once for the site. Relevant points are the switchgear and protective devices that may have to operate and disconnect a fault.

Does BS 7671 give a voltage factor for fault calculations?

It gives two, and the second is easy to miss. Cmin = 0.95 is well known because Table 41.3 states it in NOTE 1 and every maximum Zs in Chapter 41 is derived with it. Cmax = 1.1 is defined in the Part 2 symbols table in almost identical wording — "the maximum voltage factor to take account of voltage variations depending on time and place, changing of transformer taps and other considerations. For a low voltage supply given in accordance with the ESQCR, Cmax is given the value 1.1". That is the ESQCR statutory tolerance: 230 V + 10 % = 253 V. The two are 16 % apart at the same point.

When does the voltage factor NOT apply?

When the fault current was not calculated. Regulation 434.1 allows determination by calculation, measurement or enquiry. A figure declared by the distributor, or one you measured, already accounts for whatever the supply was doing — scaling it by 1.1 inflates an answer rather than making it safe. The factor belongs to a fault derived from an impedance and a nominal voltage. The same reasoning excludes a UPS or inverter, which is electronically current-limited and delivers its limit regardless of supply voltage.

Should I multiply a line-to-neutral measurement by root 3 or by 2?

It depends which fault you want, and Appendix 14 gives both. Multiplied by root 3 you get the fault between two line conductors. Multiplied by 2 you get the simultaneous fault between all three line conductors, which is the higher figure and the one that sizes switchgear. Both follow from the same assumption — that the neutral impedance is close to the line impedance, so a line-to-neutral loop is roughly twice a single phase impedance. Where the neutral is reduced under Regulation 524.2 that assumption weakens and both approximations under-state the fault.

Do I need the earth-fault current as well as the short-circuit current?

Yes. Regulation 643.7.3.201 requires the prospective short-circuit current AND the prospective earth fault current to be measured, calculated or determined at the origin and at other relevant points. Appendix 14 adds that in a single-phase system the prospective fault current is the greater of the line-to-neutral fault or the line-to-Earth fault. On a TN-C-S supply with a low Ze the earth fault can exceed the short-circuit fault, so taking only the short-circuit figure can understate what the equipment has to survive.

Is 16 kA always assumed for a domestic supply?

Not assumed — exempted, and only under conditions. Appendix 14 says that in domestic premises, where a consumer unit to BS EN 61439-3 is used and the maximum prospective fault current declared by the distributor is 16 kA, it is not necessary to measure or calculate the prospective fault current at the origin. That relies on the consumer unit having a conditional short-circuit rating, which in turn depends on the distributor's fuse ahead of it. Change any of those conditions and the exemption does not apply.

What are Icu, Ics and Icn?

Product-standard symbols. Icu is the ultimate short-circuit breaking capacity and Ics the service breaking capacity, after which the device is still fit for continued use — both from BS EN 60947-2. Icn is the rated short-circuit capacity of a BS EN 60898-1 circuit-breaker. None of them appears anywhere in BS 7671, which writes the capacity out in words; Part 2 has no definition of "breaking capacity" at all. Worth knowing when a specification quotes one and the regulations quote none.

What about the peak current, not just the RMS value?

The RMS symmetrical current is only part of it. In the first half-cycle the current is asymmetric because of the DC component, and the peak can be roughly 2.2 to 2.5 times the RMS symmetrical value depending on the X/R ratio: i-peak = kappa x root-2 x Ik, with kappa between 1 and 2. That peak is what making capacity and the mechanical forces on busbars and cleats are judged against, and because force goes as current squared the RMS figure understates it by about five times. The calculator reports the peak alongside the RMS value.

What if there is more than one source?

Appendix 14 is explicit: where there are multiple sources of supply, measures should be taken to determine the prospective fault current for all combinations of supply arrangement, so that any contribution from privately controlled embedded generation or uninterruptible supplies is included. A standby generator running in parallel, or a UPS that can feed a fault, changes the answer — and the worst case may be a combination nobody normally runs.

The figures here come from a single-source impedance method, not a full IEC 60909 study: there is no motor contribution and no peak or asymmetric factor. Transformer impedance presets are typical values — use the actual nameplate u_k %, and confirm the X/R ratio and Z_e with the distributor before relying on a breaking-capacity margin.

Where this fits

Fault level is the input to almost every protection decision downstream of it. It sets the breaking capacity in discrimination and selectivity, it drives the adiabatic check in cable sizing and derating, and the earth-fault half of it is decided by the earthing system. The fault level calculator runs the same engine as the widget above and now reports both factors and the earth-fault current.

Getting this right across a whole installation is exactly what outsourced design is for. Get in touch, or see ProDesign cable calculations for how it fits into a full design package.