Earthing & Bonding Calculator
BS 7671 Sizing Engine up to 2500A (Chapter 54)
Connecting the main earthing terminal to the supplier's earth facility or local earth electrode. (Table 54.7 / Adiabatic Equivalent).
Equipotential bonding to extraneous-conductive-parts (gas, water, steelwork) based on Table 54.8 requirements.
Earthing and Protective Bonding Guidelines (BS 7671 Chapter 54)
Ensuring an electrical installation has correctly sized earthing and bonding conductors is the cornerstone of shock protection. While they look similar—typically consisting of green and yellow insulated copper—they serve two entirely different safety functions.
The Main Earthing Conductor
The Main Earthing Conductor connects the Main Earthing Terminal (MET) of the consumer unit or distribution board directly to the means of earthing (the supplier's lead sheath, PME block, or a driven earth rod). Its primary job is to carry massive fault currents safely back to the source during a short circuit, forcing the fuse or circuit breaker to trip instantly.
Under Table 54.7, the size of this conductor is determined by the size of the incoming line (phase) conductor. For large industrial supplies up to 2500A, where line conductors exceed 35 mm², the earthing conductor must be at least half the cross-sectional area (CSA) of the line conductor.
Main Protective Equipotential Bonding
Main Protective Bonding connects extraneous-conductive-parts (such as incoming metallic water pipes, gas pipes, and structural steelwork) to the MET. Its job is not to carry fault current back to the supplier, but to ensure that all metallic parts inside the building rise to the exact same voltage during a fault. If everything is at the same voltage, no current can flow through a person touching them.
Why TN-C-S (PME) Requires Larger Bonding (Table 54.8)
If you select a TN-S or TT system in the calculator above, you will notice the bonding requirement is relatively small (never required to exceed 25 mm² in copper). However, if you select a TN-C-S (PME) system, the bonding sizes increase dramatically.
This is because, in a PME system, the supplier's earth and neutral share the same conductor in the street. If the street neutral breaks (an open-PEN fault), the entire street's neutral current will try to find a path to earth through your property's gas and water pipes. Table 54.8 dictates that PME bonding conductors must be sized against the incoming Neutral conductor to safely handle these continuous diverted network currents without melting and causing a fire.