Can you record 16 kA for prospective fault current without measuring it?
Sometimes. Appendix 14 of BS 7671 says that in domestic (household) premises with a consumer unit to BS EN 61439-3, where the distributor has declared a maximum prospective fault current of 16 kA, it is not necessary to measure or calculate it at the origin. Everywhere else, Regulation 434.1 still requires it to be determined.
What Regulation 434.1 requires
Section 434 of BS 7671 covers protection against fault current, and its first regulation is short:
The prospective fault current shall be determined at every relevant point of the installation. This shall be done by calculation, measurement or enquiry.
Two things in that sentence matter for everything that follows. The first is every relevant point, not just the origin. The second is that enquiry sits on the list alongside calculation and measurement, so a figure obtained from the distributor is a legitimate way of determining it. Nothing in 434.1 says the figure has to come off a meter.
Section 434 also opens by limiting its own scope: it “only considers the case of a fault between conductors belonging to the same circuit.”
What the testing regulation adds
Regulation 643.7.3.201, headed Prospective fault current, covers the same ground from the inspection and testing side:
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.
It names both currents, short-circuit and earth fault, and again names the origin and other relevant points. Its note points to Appendix 14 for further information on how to determine them.
What Appendix 14 is, and what it says a relevant point is
Appendix 14, Determination of Prospective Fault Current, is marked Informative. It explains how to meet the requirement. The requirements themselves are 434.1 and 643.7.3.201.
It defines the term the regulations lean on: “Relevant points are switchgear and protective devices that may have to operate and possibly disconnect a fault current. The devices have to be able to withstand the fault currents safely and protect downstream equipment from damage in the event of a fault.”
That is the reason the figure matters. It is the current the devices at that point have to be able to withstand.
The 16 kA shortcut: three conditions, one location
This is the sentence that gets quoted on site, usually shortened to “it’s domestic, so 16 kA”:
In domestic (household) 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 prospective fault current at the origin of the supply.
Read it as a checklist. All of these have to be true:
- Domestic (household) premises. A shop, a surgery, an office or a plant room is not.
- A consumer unit to BS EN 61439-3. The shortcut is tied to the assembly that is actually installed.
- A maximum of 16 kA declared by the distributor. A declared figure, not an assumed one.
And it applies at one location: the origin of the supply. It says nothing about any other relevant point in the installation, and 434.1 still applies to those.
Where one of the conditions fails, the shortcut is gone but the requirement is not. 434.1 is back to calculation, measurement or enquiry. Enquiry is still available, so a declared figure can still be the answer. What changes is that the appendix no longer excuses you from establishing it.
Single-phase: the greater of two figures
Appendix 14 says: “In a single-phase system the prospective fault current is the greater of either the fault current between the line conductor and neutral or the fault current between line conductor and Earth.”
So the figure recorded is whichever of the two is higher, not whichever one was measured first.
Three-phase: the multiplier that gets skipped
Fault currents can occur between live conductors and between line conductors and Earth. On a three-phase installation Appendix 14 is specific about which one is worst: “the highest prospective fault current occurs with a simultaneous fault between all line conductors.”
It then gives two approximations, both starting from a line to neutral measurement:
- Between line conductors: the line to neutral measurement multiplied by √3, roughly 1.73.
- A simultaneous fault between all line conductors: the line to neutral measurement multiplied by 2.
A worked illustration, with arithmetic only: a line to neutral reading of 5 kA gives roughly 8.7 kA between line conductors and roughly 10 kA for a fault across all three. The 10 kA figure is the one the devices have to withstand. Recording the 5 kA reading on its own, with no multiplier applied, records the least onerous of the three.
The appendix adds that “other methods of determining prospective fault current are not precluded”, so the multipliers are an approximation offered, not the only route.
More than one source of supply
Where an installation has multiple sources of supply, Appendix 14 says measures should be taken to determine prospective fault current for all combinations of supply arrangements, so that the contribution made by any privately controlled embedded generation or uninterrupted supply arrangements is included.
Measuring it safely: fused test leads are not enough
Appendix 14 reminds anyone testing of the danger of connecting instruments to live parts, and points to the Electricity at Work Regulations 1989 and to the HSE publication HSR25 for guidance. Then it sets out where the measurement is taken:
The measurement should always be made on the output terminals of a suitably rated protective device. If such a device is not present then a temporary one should be fitted. Measurement should never be made where overcurrent protection is not present between the point of connection and the supply transformer. Fused test leads alone do not meet this requirement.
The last sentence is the one that gets read past. The fuses in the leads do not stand in for overcurrent protection between the test point and the transformer. If that protection is not there, the appendix says the measurement should not be made at that point at all.
Where the figure goes on the certificate
The model Electrical Installation Certificate and the model Electrical Installation Condition Report in BS 7671 both carry a Prospective fault current box, in kA, among the supply characteristics, and a breaking capacity box, also in kA, in the same section. Two figures in the same units, a few centimetres apart, and the second has to be able to deal with the first.
Common questions
Can I write 16 kA on a domestic certificate without measuring?
At the origin, where all three conditions in Appendix 14 hold: domestic (household) premises, a consumer unit to BS EN 61439-3, and a maximum of 16 kA declared by the distributor. The appendix says it is then not necessary to measure or calculate prospective fault current at the origin of the supply.
Does the 16 kA shortcut apply to commercial premises?
No. The sentence is limited to domestic (household) premises. On anything else, 434.1 applies as written: calculation, measurement or enquiry, at every relevant point.
Is a figure from the distributor acceptable?
Yes. Regulation 434.1 lists enquiry alongside calculation and measurement, and 643.7.3.201 allows the figure to be determined by another method as well as measured or calculated.
How do I get the three-phase figure from a single reading?
Appendix 14 gives the prospective fault current between line conductors as approximately the line to neutral measurement multiplied by √3, and a simultaneous fault between all line conductors as approximately the line to neutral measurement multiplied by 2. The second is the highest.
On single-phase, do I record line to neutral or line to earth?
The greater of the two. Appendix 14 defines the single-phase prospective fault current as whichever is higher.
Are fused test leads enough to make a PFC test safe?
Not on their own. Appendix 14 says the measurement should be made on the output terminals of a suitably rated protective device, never where overcurrent protection is absent between the point of connection and the supply transformer, and that fused test leads alone do not meet this requirement.
Is Appendix 14 a requirement?
It is marked informative. The requirements are Regulation 434.1 and Regulation 643.7.3.201. Appendix 14 explains how to meet them and where the shortcut applies.
Prospective fault current is one box on the EIC and the EICR, and on a three-phase job the reading on its own is not the figure. iCertifi: Electrical Certs produces both certificates on an iPhone or iPad. iCertifi on the App Store.
Regulation 434.1, Regulation 643.7.3.201 and Appendix 14 were read from BS 7671:2018+A4:2026 at printed pages 105, 252 and 582. The certificate fields were read from the model forms at printed pages 538 and 548.