The answer, by charger size
P28 governs voltage fluctuation — how much your equipment makes the mains move when it switches. For charging equipment, Clause 8.6.2 sets the thresholds directly, so for most installations there is nothing to calculate at all.
| Charger | Approx. current | Domestic residence | Anywhere else |
|---|---|---|---|
| 3.6 kW single-phase | 16 A | No DNO reference | No DNO reference |
| 7 kW single-phase the common home charger |
32 A | No DNO reference Clause 8.6.2(b) |
Conditional connection |
| 11 kW three-phase | 16 A/phase | No DNO reference | No DNO reference |
| 22 kW three-phase | 32 A/phase | No DNO reference | Conditional connection |
| 50 kW rapid | ~72 A/phase | Conditional connection at best; usually Stage 2 | |
| 150 kW+ ultra-rapid | > 75 A/phase | Stage 2 assessment, outside Stage 1 entirely | |
Currents are the nominal draw at 230 V single-phase or 400 V three-phase. Every row assumes the equipment conforms to BS EN 61000-3-3 — check the declaration of conformity, because the whole structure rests on it.
Check your specific case
Our free P28 calculator has an EV charger mode. Give it the rating, the supply and whether it is a domestic installation, and it tells you which route applies and cites the clause.
The 32 A domestic allowance
This is the clause worth knowing, because it decides the outcome for almost every home installation and it is routinely missed.
The general Stage 1 threshold in Clause 6.3.2.1 is 16 A per phase. Equipment at or below it, conforming to BS EN 61000-3-3, connects without reference to the network operator and without conditional connection attached.
Clause 8.6.2(b) raises that to 32 A specifically for fixed charging equipment installed at a domestic residence. A 7 kW home charger draws about 32 A, so it sits inside the allowance — which is precisely why the allowance exists.
Two conditions, and both matter
- A domestic residence. Not an office car park, not a fleet depot, not a block's communal parking. The allowance is written for homes.
- Conformance to BS EN 61000-3-3. If the manufacturer declares 61000-3-11 instead, this route does not apply and conditional connection does.
Above 32 A: conditional connection
Once you are past the applicable threshold, the equipment moves to conditional connection under BS EN 61000-3-11 (Clause 6.3.2.2). The word conditional is doing real work: the manufacturer has demonstrated the equipment behaves acceptably provided the supply is strong enough, and demonstrating that is now your job.
It qualifies under Stage 1 if either:
- (a) the supply impedance at the customer's terminals — measured, or from calculated values the network operator supplies — is no greater than the Zmax the manufacturer declares in the instruction manual; or
- (b) the service is confirmed at 100 A per phase or more, the equipment is marked to that effect, and the supply impedance is confirmed by measurement as no greater than:
| Connection | Impedance | Modulus |
|---|---|---|
| Single-phase (phase–neutral loop) | 0.25 + j0.25 Ω | 0.350 Ω |
| Three-phase | 0.15 + j0.15 Ω | 0.212 Ω |
Two mistakes that invalidate the claim
A 100 A fuse carrier is not a 100 A service. The carrier tells you what fits, not what the service can deliver. Capacity has to be confirmed with the network operator.
That single-phase figure is a phase–neutral loop impedance, not an earth-fault loop impedance. They are different measurements and most testers default to the wrong one. Measuring Ze and comparing it against 0.35 Ω is not the test the standard asks for.
For the simpler 16 A route, the assumed maximum supply impedance is 0.40 + j0.25 Ω (|Z| = 0.472 Ω) single-phase and 0.24 + j0.15 Ω (|Z| = 0.283 Ω) three-phase. PME networks typically sit well inside that, so it is rarely the binding constraint — services installed before about 1950 are the ones worth measuring.
Several chargers on one supply
This is where a straightforward job becomes a real assessment, and where the reasoning most often goes wrong.
Clause 8.6.2 directs network operators to give special consideration to multiple charging points at one point of common coupling, including preventing them from switching simultaneously. Six chargers that each individually qualify do not become six individually-qualifying chargers — they become one installation whose combined switching has to be considered.
The intuition to resist is the one that says "each is under the threshold, so we are fine". The threshold was written for a single item of equipment. Whether the combination is acceptable depends on how many can start together, and that is a question about your control system as much as your cable.
The practical route
Load management that staggers or interlocks starts is usually cheaper than reinforcing a supply, and it is exactly what the clause is pointing at. If the chargers physically cannot all switch at once, the case you have to make is far smaller.
Rapid chargers
A 50 kW rapid charger draws roughly 72 A per phase, near the top of the 75 A conditional connection range. Anything larger leaves Stage 1 altogether and needs a Stage 2 assessment — worst-case Pst at the PCC under worst-case supply impedance, against a limit of 0.5.
In practice a rapid charging site is a connection application, not an installation question, and it is worth involving the network operator early rather than presenting them with a finished design. They hold the short-circuit level the whole assessment turns on, and they will have a view about the connection point.
Why there is no percentage to calculate
People expect a P28 answer to be a step voltage percentage checked against 3%. For charging equipment on the Stage 1 routes above, there is nothing to calculate — and that is the design, not an omission.
Where equipment qualifies through conformance to an EMC product standard, that conformance is the demonstration that its voltage fluctuation behaviour is acceptable. The manufacturer has already done the testing, against a reference impedance, and Clause 8.6.2 says such equipment may be connected. Running a separate step voltage calculation and failing the charger on it would contradict the clause that permits it.
So a Stage 1 pass supersedes the step voltage check, the RVC category and the Stage 2 flicker check together. Our calculator shows those figures for information and marks them not required, rather than hiding them or letting them override the verdict.
If it does not qualify
A charger falling outside Stage 1 is not a refusal. It means the documentation route is closed and something has to change — and the cheapest change is rarely the supply. Roughly in order of cost:
- Check the declaration again. The whole structure turns on whether the manufacturer declares BS EN 61000-3-3 or 61000-3-11. Some manufacturers publish both for different models in the same range, and installers frequently work from the range datasheet rather than the model's own declaration.
- Find the manufacturer's Zmax. For conditional connection, route (a) only needs the declared maximum impedance and a measurement to compare it against. That is an afternoon with a tester, not a study.
- Limit the current. Many chargers can be commissioned at a lower rating. A 32 A unit configured to 16 A moves onto the unconditional route entirely — and for a car parked overnight the charging time rarely matters.
- Stagger or interlock, for multiple points. If they physically cannot start together, the case to be made is far smaller.
- Ask about the connection point. Step voltage change is inversely proportional to short-circuit level, so where you connect can matter more than what you connect.
Only after those is a supply upgrade or a full Stage 2 assessment the right answer. The third one in particular is worth raising with the customer before quoting reinforcement: a slower charger they never notice is usually preferable to a bill they certainly will.
What P28 does not cover
P28 is voltage fluctuation only. A charge point installation has other obligations that live elsewhere, and clearing P28 does not clear them:
- ENA notification for EV charge points. A separate process with its own thresholds. P28 saying "no reference required" is not the same as "no notification required".
- Harmonics. Chargers are power electronics and therefore harmonic sources. That is EREC G5, assessed separately.
- BS 7671 — the wiring regulations, including Section 722 for EV installations.
- Load and diversity on the existing service, which is often the real constraint.
The harmonic side
Several chargers on one supply is exactly the case where the harmonic assessment matters as much as the voltage one. Our G5/5 calculator handles that end to end.
Where this guide stops
This is an explanation, not the standard. It paraphrases and cites; it does not reproduce P28's text. For anything you intend to submit, work from your own copy of ENA EREC P28 Issue 2 (2019), available from the Energy Networks Association.