Screening check using after-diversity max demand against the main fuse. A single-phase 7.4kW charger adds ~32A. Where headroom is tight, load management (per BS 7671 Section 722 / 433.1) or a DNO supply upgrade is the fix — always confirm the existing demand by measurement. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This calculator is a quick screening check for whether a property's supply can take an EV charger without load management. Enter the main fuse rating, the installation's existing maximum demand and the charger size, and it shows the headroom left on the main fuse and a plain verdict.
It is the first question on every domestic EV charger job: will the existing 60, 80 or 100 amp supply cope, or does the charger need load management or a supply upgrade? Answering it before you quote saves an awkward conversation later and points you straight at whether the DNO needs involving.
The check adds the charger's current to the installation's after-diversity maximum demand and compares the total against the main fuse. A single-phase 7.4 kW charger draws about 32 A, which is a big chunk of a 60 or 80 A supply once the house's own demand is counted.
Headroom = main fuse rating − (existing max demand + charger current)
The charger's design current is added to the installation's after-diversity maximum demand. If the total is within the main fuse rating there is headroom and the charger fits as a fixed load; if it exceeds the fuse, the charger needs load management (which sheds charging current when the house load rises) or a supply upgrade from the DNO.
Choose the supplier's cut-out fuse size — typically 60, 80 or 100 A on a domestic supply. If you are not sure, this is the figure to confirm on site before committing to a design.
Enter the installation's after-diversity maximum demand in amps. Assess it by calculation using the diversity allowances, or better, by measurement over a representative period. The max demand calculator will give you a starting figure.
Pick the charger output. Most domestic units are 7.4 kW single-phase, which draws about 32 A; 3.6 kW units draw about 16 A. A 22 kW unit needs a three-phase supply.
The calculator shows the total demand with the charger added, the headroom remaining on the main fuse, and whether the charger fits or needs load management or an upgrade.
This is a screening tool, not a substitute for assessing the real maximum demand. The honest way to size the existing load is to measure it — a maximum-demand assessment over time is far more reliable than adding up rated currents, because diversity means a house never draws all its circuits at once. Feed a realistic figure in, not a worst-case paper total.
Where the headroom is tight or negative, load management is usually the answer rather than an expensive supply upgrade. A load-managing charger monitors the incoming current and reduces or pauses charging when the household demand rises, keeping the total within the main fuse — this is recognised in BS 7671 Section 722, and is how most borderline installs are made to work.
The result assumes a single-phase supply and a fixed charger load. A 22 kW unit needs three phases, which most homes do not have. Always confirm the main fuse rating, check whether the DNO needs notifying, and design the final circuit — cable, protection and earthing arrangement — to Section 722; this tool only tells you whether the load fits.
Screening check using after-diversity max demand against the main fuse. A single-phase 7.4kW charger adds ~32A. Where headroom is tight, load management (per BS 7671 Section 722 / 433.1) or a DNO supply upgrade is the fix — always confirm the existing demand by measurement. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
It depends on your main fuse and your existing demand. A 7.4 kW charger draws about 32 A. On a 100 A supply with a typical house demand of 40–50 A there is usually room; on a 60 A supply it is often tight, so the charger may need load management. Enter your figures above for a quick verdict.
It is a control feature that stops the charger overloading the supply. The charger monitors the incoming current and automatically reduces or pauses charging when the rest of the house is drawing heavily, then ramps back up when demand falls. It lets a charger be installed on a supply that could not take it as a fixed full-power load.
Either assess it by calculation, applying the standard diversity allowances to the circuits, or — more accurately — measure it with a clamp meter or logger over a representative period. Because of diversity the real demand is far below the sum of every circuit's rating, so a measured figure usually frees up headroom a paper calculation hides.
Often not. Where the load does not fit as a fixed charger, a load-managing charger is usually cheaper and quicker than a DNO supply upgrade, and satisfies BS 7671. An upgrade is only really needed where even managed charging plus the household demand cannot be accommodated, or where the supply is already at its limit.
Domestic EV charge point installation end to end — supply assessment, protection and earthing.
What BS 7671 Section 722 requires for EV charging installations, including load control.
Applying diversity to work out the after-diversity maximum demand of an installation.
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