EV Charger Diversity and Load Calculator

Summary

The word "diversity" causes real confusion in EV charger load calculations, because it means something different here than it does everywhere else in domestic electrical design. For nearly every other domestic load — lighting, sockets, cookers, showers — BS 7671 Appendix 1 gives you a percentage to knock off the full connected load, on the statistical basis that not everything runs at once. For a single EV charger, that discount doesn't exist: the table entry for electric vehicle chargers is 100%, full stop, because a charger left plugged in overnight is treated as continuous, sustained demand, not intermittent use.

This calculator is the practical companion to electrical load, which covers the full domestic ADMD calculation across all load types. Here the focus narrows to the EV-specific questions that come up on almost every quote: how much does adding a charger actually add to the property's demand, when does that push an existing supply over its limit, and what changes when dynamic load management or multiple charge points are involved. None of this replaces a proper site assessment — see ev charger and part s ev charging for the installation and notification requirements — but it gives you the numbers to work through before you're on site.

A note on citation: the diversity guidance referenced throughout this article draws on BS 7671:2018+A4:2026 Appendix 1 and Section 722, and the IET Code of Practice for Electric Vehicle Charging Equipment Installation. The Code of Practice has been through several editions since it was first published; always check the current edition with the IET before relying on a specific clause number **** — the diversity principles below are stable across editions, but clause references may shift.

Key Facts

• BS 7671 Appendix 1 diversity for a single EV charger — 100% (no diversity applied); this is the single most important fact in this article and the most commonly misapplied
• Standard domestic charger rating — 7.4kW at 32A single-phase is the most common UK domestic install; 3.6kW (16A) is the fallback where supply capacity is limited
• Continuous load classification — an EV charger is treated as a continuous load for cable sizing and thermal purposes because it can run for 8+ hours unbroken, unlike a shower (minutes) or a cooker (intermittent cycling)
• Dynamic Load Management (DLM) — a device (often built into the charger, e.g. via a current transformer/CT clamp on the incoming supply) that reduces or pauses charger output when household demand elsewhere is high, keeping total demand within the supply's rated capacity
• DLM is not a BS 7671 diversity factor — it's a real-time load management control strategy; BS 7671 Appendix 1 gives no percentage discount for "the charger has DLM fitted", but a correctly specified DLM system is the recognised way to install a charger on a supply that couldn't otherwise take the full 32A continuously
• Multiple charge points on one supply (workplace car park, HMO with shared parking, apartment block) — diversity across multiple charge points is assessed via the DNO connection agreement and a load management system, not a fixed BS 7671 table value; consult Engineering Recommendation G100 and the DNO for connection capacity
• DNO notification threshold — single-phase EV chargers above 3.68kW (16A) require DNO notification under Engineering Recommendation G100 before installation
• Heat pump diversity — 100% of rated input when running (no diversity), similarly to EV chargers — relevant where a property is adding both an ASHP and an EV charger, since neither gets a discount
• Standard domestic supply capacity — 60A, 80A or 100A single-phase at 230V, giving roughly 13.8kW, 18.4kW or 23kW of continuous capacity respectively
• Meter tails and cable sizing — must be sized to carry the full ADMD including the charger's 100% contribution, not a diversified figure; undersizing tails because "the charger won't run at full load all the time" is a design error
• Open-PEN / PME earthing — a separate safety requirement to diversity, but it interacts with load calculations because some open-PEN protection devices also incorporate DLM functionality; check the specific device's specification rather than assuming
• Three-phase alternative — where ADMD including the charger exceeds single-phase supply capacity, a DNO three-phase upgrade or a DLM system are the two practical options; three-phase upgrades typically cost £2,000–£8,000 and take 8–16 weeks

Quick Reference Table — Adding a Single EV Charger to Existing ADMD

These figures assume no diversity discount on the charger itself, consistent with BS 7671 Appendix 1. "Within supply" is judged against continuous rated capacity, not a momentary peak.

Detailed Guidance

Step 1 — Establish the existing property ADMD

Before adding the charger, calculate the property's current ADMD using the full domestic diversity table in electrical load — lighting at 66%, cooker per the BS 7671 formula, shower(s) at 100% of the largest plus a reduced allowance for additional units, ring finals at 100% first ring + 40% additional rings, and so on. This gives you the baseline figure the charger will be added to.

Worked example — 3-bed semi, existing ADMD calculation:

• Lighting: 2,000W × 66% = 1,320W = 5.7A
• Ring finals (2 rings): 30A + (30A × 40%) = 42A
• 10kW electric shower: 43.5A at 100%
• Immersion heater (3kW): 13A at 100%

Existing ADMD = 5.7 + 42 + 43.5 + 13 = 104.2A

This property already exceeds an 80A supply on ADMD alone, before any EV charger is considered — a common finding on older properties with an electric shower and no gas heating. Confirm the actual supply fuse rating before proceeding.

Step 2 — Add the EV charger at 100%, with no diversity

Add the charger's full rated current — 32A for a 7.4kW single-phase unit — directly to the existing ADMD figure. Do not apply any percentage reduction at this stage, regardless of how "unlikely" it seems that the shower, the charger and the immersion heater will all run simultaneously; BS 7671 Appendix 1 does not offer a diversity allowance for EV chargers, precisely because overnight charging is a sustained multi-hour load that can coincide with any other overnight or morning load.

Continuing the example: 104.2A (existing) + 32A (charger) = 136.2A

This is well beyond even a 100A supply. The property needs either a DLM-managed charger, a reduced-rate 16A (3.6kW) charger, or a DNO discussion about supply capacity — a straightforward 32A charger install is not viable here without one of those interventions.

Step 3 — Decide between DLM, a lower-rated charger, or a supply upgrade

Where the raw arithmetic (Step 2) exceeds the supply's continuous rating, three options exist:

Option A — Dynamic Load Management (DLM). A CT clamp on the incoming supply monitors real-time household demand and the charger's control electronics reduce or pause output to keep total demand within the supply limit. This doesn't change the BS 7671 diversity calculation on paper, but it is the recognised engineering solution that makes an install viable in practice on a constrained supply. Document the DLM device's make, model and settings in the installation certificate, since this is the safety justification for proceeding on a supply that the raw ADMD figure says is inadequate.

Option B — Reduce the charger to 16A (3.6kW). Halves the charger's contribution to ADMD (16A instead of 32A) at the cost of roughly doubling charge time. Continuing the example: 104.2A + 16A = 120.2A — still tight against a 100A supply, illustrating that a lower-rated charger alone doesn't always solve a heavily loaded property.

Option C — DNO supply upgrade. Request an upgrade from the DNO (commonly 80A to 100A, sometimes single-phase to three-phase for heavily loaded all-electric properties). Standard single-phase upgrades are often free or low-cost from the DNO; three-phase upgrades are a significant cost (£2,000–£8,000) and a multi-week to multi-month lead time — see ev charger pricing guide for full cost detail.

Step 4 — Multiple charge points on a shared supply

Where more than one charge point shares a single incoming supply — a house with two EVs on separate chargers, an HMO with a shared driveway, a small block of flats, or a workplace car park — you cannot simply multiply the single-charger 100% figure by the number of chargers and expect the DNO connection to support it. This is where genuine diversity across multiple charge points comes in, but it is not a fixed BS 7671 table percentage — it is assessed through:

• The DNO connection agreement, which specifies the maximum simultaneous import the site is permitted, agreed under Engineering Recommendation G100 for demand-side connections
• A site-wide load management system that allocates available capacity dynamically across all connected charge points — commonly by equal share, by priority queue, or by a configurable cap per point
• Manufacturer-specific multi-charger control systems (increasingly common on workplace and fleet installations) that manage this allocation automatically

Worked example — small workplace car park, 6 charge points, 100A three-phase supply available for EV charging (after other site loads accounted for):

• Full theoretical demand if all 6 chargers ran at 32A simultaneously: 6 × 32A = 192A — far beyond the 100A available
• A load management system capping total EV demand at 90A (leaving headroom) and dividing dynamically across active vehicles: each of 6 cars charging simultaneously would receive roughly 15A each (a reduced but functional rate), while fewer simultaneous vehicles would each receive a higher share up to the individual charger's 32A maximum
• This is a load management design decision, not a diversity lookup — the actual allocation strategy (equal share vs. priority vs. first-come) is configured in the load management system, agreed with the DNO connection capacity, and documented at commissioning

Never present a multi-charger diversity assumption to a customer or building control as if it were a fixed BS 7671 percentage — it is a system design outcome that must be engineered and documented for the specific site, not read off a table.

Frequently Asked Questions

Why doesn't BS 7671 give a diversity discount for a single EV charger like it does for a shower?

Because the risk profile is different. A shower runs for a few minutes; even if several household members shower in sequence, the overlap is brief, so a diversity allowance reflecting "not everyone showers at once" is statistically reasonable. An EV charger left plugged in can run continuously for 6–10 hours overnight, coinciding with any other overnight load (immersion heater, storage heaters) for the full duration — there's no statistical basis for assuming it won't be at full demand for the period it's connected.

If I fit a DLM-capable charger, can I write a reduced ADMD figure on the certificate?

You should document what the DLM device actually does and the maximum current it will permit under worst-case conditions, rather than writing an arbitrary reduced figure. The safety case for a DLM installation on a constrained supply rests on the device's real-time management functioning correctly, not on a paper diversity calculation — make sure the certification reflects the device's specified operating limits, not an assumed "typical" reduced demand.

Does the property's existing supply always need upgrading if ADMD plus the charger exceeds it?

Not always — a DLM-managed charger or a reduced-rate 16A charger can both make an install viable without a DNO supply upgrade, and are frequently cheaper and faster than waiting for a three-phase upgrade. Which option is right depends on the customer's actual charging needs (how much daily range they need replenished overnight) versus the cost and lead time of a supply upgrade — this is a conversation to have before committing to a specification, not an automatic escalation to "you need a bigger supply."

Do heat pumps and EV chargers compound the same way?

Yes — heat pumps are also assessed at 100% of rated input when running, with no diversity discount, for the same reason as EV chargers: sustained, potentially overnight or all-day operation. A property adding both an ASHP and an EV charger should have both added at their full rated current to the existing ADMD, and the combined figure checked against supply capacity — this combination is one of the most common triggers for a genuine need for load management or a three-phase supply in all-electric retrofits.

Regulations & Standards

• BS 7671:2018+A4:2026 — IET Wiring Regulations; Appendix 1 contains domestic diversity factors, Section 722 covers electric vehicle charging installations specifically

• IET Code of Practice for Electric Vehicle Charging Equipment Installation — detailed installation and diversity guidance; edition current at time of writing — **** with the IET before citing a specific clause

• Engineering Recommendation G100 — governs DNO connection and demand assessment for EV chargers and similar high-demand loads, including multi-charge-point sites

• Building Regulations Part S — EV chargepoint infrastructure requirements for new buildings and major renovations

• Part P of the Building Regulations — new EV charger circuits are notifiable work

• IET — BS 7671 Wiring Regulations — Appendix 1 diversity factors and Section 722

• IET — Code of Practice for Electric Vehicle Charging Equipment Installation — installation and diversity guidance (check current edition)

• Energy Networks Association — G100 — DNO demand-side connection assessment

• Gov.uk — EV Chargepoint Grant guidance — eligibility and technical requirements

• electrical load — full domestic ADMD calculation across all load types, worked examples

• ev charger — EV charger circuit design, DNO notification and PME earthing requirements

• part s ev charging — Part S requirements for new-build EV chargepoint infrastructure

• ev charger faults — diagnosing EV charger faults including PEN and RCD issues

• ev charger pricing guide — pricing EV charger installation including supply upgrade costs

• cable sizing — cable sizing for EV charger circuits and meter tails