FREE TOOL · ELECTRICIANS
Calculate PSCC in kA and verify your protective devices have adequate breaking capacity. Works for single-phase and three-phase supplies — enter Ze and get an instant result.
Measure at the distribution board origin. Typical TN-C-S: 0.15–0.40Ω.
Enter 0 for measurement at the board origin.
Prospective Fault Current
0.657 kA
657 A
Min Breaking Capacity
3kA
required for MCBs / fuses
Standard 6kA MCBs are adequate
PFC of 0.66kA is within the 6kA MCB breaking capacity rating for this location.
Next step
Turn these figures into a professional PDF quote or invoice — free, no account needed to start.
Prospective fault current is a fundamental electrical safety calculation required by BS 7671 Regulation 536.4.1. The method follows standard UK practice for domestic and commercial installations.
For single-phase installations: PFC = 230V ÷ Zs, where Zs is the total loop impedance at the point of measurement. At the board origin (Zs = Ze), this gives the maximum PFC for the whole installation. For circuits, add the cable resistance R1+R2 to Ze. The result in amps is divided by 1,000 to express in kA for comparison with device breaking capacity ratings.
For three-phase installations the bolted line-to-line fault current is 400V ÷ Zs. This is the worst-case fault at the point of measurement. Line-to-neutral faults use 230V ÷ Zs. Three-phase installations near the distribution transformer typically have lower Zs and therefore higher PFC than single-phase supplies served over the same cable run — this is why industrial switchgear often has higher breaking capacity ratings.
BS 7671 Regulation 536.4.1.1 requires the rated short-circuit capacity of every protective device (Icc or Ics) to be not less than the prospective fault current at the point of installation. Standard domestic MCBs are rated 6kA. Industrial MCBs are typically 10kA. Where PFC exceeds 6kA, 10kA-rated MCBs or current-limiting HRC fuses (BS 88-2) must be used. Where PFC exceeds 10kA, consult the manufacturer's data for higher-capacity switchgear.
Adding cable resistance (R1+R2) reduces the PFC at the far end of a circuit compared to the board origin. This is why PFC is highest at the main distribution board and decreases along the circuit. For sub-main cables and distribution circuits, always calculate PFC at each board or sub-board to verify that all devices at that level have adequate breaking capacity.
WORKED EXAMPLE
An electrician is installing a sub-distribution board in a commercial unit fed by a 25mm² three-phase sub-main, 15m from the main LV board. Ze at the main board is 0.08Ω. What breaking capacity is required for MCBs in the sub-board?
Ze at main board
0.08 Ω
Sub-main R1+R2 (25mm², 15m)
20.7 mΩ
Zs at sub-board
0.101 Ω
PFC (400V ÷ Zs)
3.96 kA
Standard 6kA MCBs
Adequate
Min. breaking capacity
6kA
Result
3.96 kA — 6kA MCBs OK
Sub-main resistance reduces PFC enough that standard 6kA MCBs are compliant
Always record the measured PSCC on the distribution board schedule and EIC. Where Ze is lower than 0.04Ω (near a large transformer), check PFC at the main board origin as well.
Verify BS 7671 Zs compliance for any MCB or fuse type.
Size your earth conductor using BS 7671 Table 54.7 or the adiabatic equation.
Apply BS 7671 derating factors for temperature, grouping and insulation.
Calculate household electricity consumption and consumer unit upgrade needs.
Calculate income tax, NI and monthly set-aside for 2024/25.
Log business miles and expenses to maximise your tax deduction.
Find the right hourly rate for electricians by region.
See exactly how much you make on any job after all costs.
Calculate total labour cost including NI and travel.
Set your sell price and see the true margin on materials.
STOP UNDERCHARGING
You've verified your fault current. Now turn it into a professional quote in minutes. Sleepless Tradesman quotes, invoices and plans every job — for free, no card required.
No card · No commitment · Cancel anytime
Common questions about prospective fault current and breaking capacity.
Prospective fault current (PFC), also called prospective short-circuit current (PSCC), is the maximum current that can flow through a circuit under fault conditions — either a short circuit between live conductors or an earth fault. It is calculated by dividing the supply voltage by the total circuit impedance (Zs). PFC determines the minimum breaking capacity that protective devices (MCBs, fuses) must have. If the fault current exceeds the breaking capacity, the device may fail catastrophically rather than safely interrupting the fault.
PFC (prospective fault current) and Icc (rated short-circuit making capacity) refer to the same fundamental concept — the available fault current at a point in the installation. PFC is the quantity you calculate and measure on site; Icc (or Ics) is the rated value stamped on the protective device that must equal or exceed the measured PFC. BS 7671 Regulation 536.4.1 requires that every protective device has a rated short-circuit capacity not less than the prospective fault current at the point of installation.
The 6kA rating covers the vast majority of domestic and commercial final circuits where the total loop impedance is high enough to limit fault current below 6,000A. Most UK domestic TN-C-S installations have Ze around 0.2–0.4Ω, giving PFC of 575–1,150A at the board — far below 6kA. However, at the origin of the installation (the main board), with very low Ze (0.05–0.1Ω), PFC can approach or exceed 2,300A and may reach 6kA+ at the DNO cut-out. Industrial premises near the local transformer can have PFC exceeding 10kA, requiring 10kA or higher rated devices.
Specify 10kA MCBs when the measured or calculated PFC at the distribution board exceeds 6,000A. This typically occurs: in commercial or industrial premises close to the local distribution transformer; in large residential blocks with a short incoming supply cable and low external impedance; at the main distribution board of large installations. A Ze measurement at the origin below 0.04Ω (230V ÷ 6,000A) suggests 10kA MCBs may be required. When in doubt, the 10kA type can always replace a 6kA — it provides extra protection at minimal additional cost.
Prospective short-circuit current is measured using a loop impedance tester set to measure at the line-to-line or line-to-neutral terminals (not line-to-earth). The tester measures the loop impedance and calculates the prospective current. For a single-phase supply, PFC = 230V ÷ Zs; for three-phase, the line-to-line PFC = 400V ÷ Zs. The measured PFC must not exceed the breaking capacity of the installed protective devices. This measurement should be taken at the incoming terminals of each distribution board and recorded on the installation certificate (EIC) or condition report (EICR).
A ring final circuit has a lower loop impedance than a radial circuit because current can flow both ways around the ring. The mid-point of the ring (the point furthest from the board) has the lowest fault current, but the board end has the highest. For ring circuits, the prospective fault current at the board end is typically dominated by the Ze and the incoming supply impedance rather than the cable. The PSCC at the socket-outlet with the lowest impedance (closest to the board) should be checked against the MCB's breaking capacity.