3G2.5 · 3G6 · 4G4… This is the question we are asked most before anyone buys an appliance: "Is my existing cable enough?" On this page we explain step by step what cable cross-section is, how to identify the cable you already have, and which Veito model needs which cable. You do not have to be an electrician.
Cable cross-section is the thickness, in square millimetres (mm²), of each copper conductor inside the cable. The thicker the conductor, the more current it carries and the less it heats up. Instant water heaters are high-power appliances, so a lighting or socket cable is often not enough.
Official figures: Blue S / Blue E → 4G4 NYM (3×32A fuse) · Flow E9 → 3G6 NYM (40A) · Flow E7 · Glow · V1200 → 3G2.5 NYM (32A). These are the minimum requirements; if the run is long your electrician may recommend the next size up.
You do not have to read everything below. Send a clear photo of the cable end on WhatsApp and we will look at it and tell you — free of charge.
If you are going to touch the cable, first switch off the main breaker at the consumer unit. If you are only going to look, touch nothing — a photo from a distance is enough.
On the grey/white outer sheath of the cable there is a marking such as "NYM-J 3G2.5" . Take the photo close enough to read it. If the marking has worn away, photograph the cut end of the cable so the number of cores is visible.
Add which appliance you have in mind . Is the cable enough, is the fuse right, is a new line needed — we will tell you honestly.
This is the most reliable method. There is a marking repeated along the outer sheath of the cable. It looks like this:
| Part | What it means | Example |
|---|---|---|
| NYM | Cable type — a fixed indoor installation cable with a rigid PVC sheath. It is the standard cable used on instant water heater circuits. | NYM, NYA, NHXMH |
| -J | There is a green-yellow earth core. -O means there is no earth — not to be used in a bathroom. | NYM-J |
| 3 · 4 · 5 | Number of cores (conductors). How many separately insulated conductors come out of it. | 3G… · 4G… |
| G | means the earth is included in the core count. X means there is no earth core. | 3G2.5 · 3X2.5 |
| 2,5 · 4 · 6 | Cross-section. The copper thickness of each core, in mm². This is the number you are actually looking for. | …2,5 · …4 · …6 |
So 3G2,5 = 3 cores (one of them earth), 2.5 mm² each. 4G4 = 4 cores (3 phases + earth), 4 mm² each. On some cables you will also see 3×2,5 written; it means the same thing — in our correspondence we use the 3G2,5 format.
This is what you see when you cut the cable. The orange part in the middle is copper and around it is insulation. The colour arrangement can differ from building to building — what you should really rely on is the marking on the sheath.
On older cables the marking on the sheath has often worn away. In that case look at the cut end of the cable: how many separate coloured conductors come out of it?
| What you see | What it means | Which appliance it could be |
|---|---|---|
| 3 cores | Brown + blue + green-yellow → single-phase (220V) line. | Flow E7 · Glow · V1200 · (Flow E9 if the cross-section is 6 mm²) |
| 4 cores | Brown + black + grey + green-yellow → three-phase (380V) line, 3 phases + earth. | Blue S · Blue E |
| 5 cores | 3 phases + neutral + earth. A three-phase line that also has a neutral. | Blue S · Blue E (the neutral core is left unused) |
| 2 cores | No earth. For an instant water heater it cannot be used. | None — a new line is essential |
⚠ Do not trust the colours blindly. Before 2000 the colour standard was different (red/yellow/blue for phases, black for neutral) and in some installations the colours were used incorrectly. Colour is a clue; the reliable information is the marking on the sheath and the electrician's measurement.
If there is no marking and the colours are unclear, what is left is measuring the thickness of the copper conductor. On single-core (rigid, solid copper) cables this is easy: measure the diameter of the copper conductor and find the cross-section in the table below.
The drawing is to scale (the proportions between them are correct), but it grows and shrinks with the screen size — do not hold a ruler against the screen, measure the cable.
| Cross-section | Copper conductor diameter | Where it is usually found | At Veito |
|---|---|---|---|
| 1.5 mm² | ≈ 1.4 mm | Lighting circuit, ceiling-light drop | Not enough |
| 2.5 mm² | ≈ 1.8 mm | Socket circuit, washing machine / dishwasher | Flow E7 · Glow · V1200 |
| 4 mm² | ≈ 2.3 mm | Three-phase appliance circuit, electric oven | Blue S · Blue E (4G4) |
| 6 mm² | ≈ 2.8 mm | High-power single-phase appliance circuit | Flow E9 (3G6) |
| 10 mm² | ≈ 3.6 mm | Flat supply / main riser | More does no harm |
How to measure: Switch off the main breaker. Hold a vernier caliper against the stripped copper conductor at the end of the cable. If you have no caliper, press the copper conductor onto a sheet of paper, mark the impression and measure it with a ruler — since even a 0.2 mm error can change the cross-section, the result is
approximate, so have your electrician confirm it.
⚠ On multi-strand (twisted, flexible) cables this method does not work: there the cross-section is the sum of dozens of fine strands. That type of cable must be measured by an electrician.
The figures below are each model's minimum requirement. They are the official values printed on the appliance rating plate and in the user manual.
| Model | Output | Supply | Cable | Breaker | Note |
|---|---|---|---|---|---|
| Blue S | 21 kW | 380V three-phase | 4G4 NYM | 3×32 A | A new line from the consumer unit is essential |
| Blue S 24 kW | 24 kW | 380V three-phase | 4G4 NYM | 3×32 A | Bespoke project work |
| Blue E | 21 kW | 380V three-phase | 4G4 NYM | 3×32 A | Same line as the Blue S |
| Flow E9 | 9 kW | 220V single-phase | 3G6 NYM | 40 A | Will not run from a socket |
| Flow E7 | 7 kW | 220V single-phase | 3G2.5 NYM | 32 A | Can be spurred off an existing socket circuit |
| Glow | 8 kW | 220V single-phase | 3G2.5 NYM | 32 A | Shower unit |
| V1200 | 8 kW | 220V single-phase | 3G2.5 NYM | 32 A | Shower type, shower set included |
4G4 = 4 cores × 4 mm² (3 phases + earth) · 3G6 = 3 cores × 6 mm² · 3G2,5 = 3 cores × 2.5 mm². If the run is long (a long distance between the consumer unit and the appliance) your electrician may recommend the next size up — that is not a shortcoming but a voltage-drop calculation.
The same mistakes have come our way for twenty years. Check these before you buy an appliance — you will save both money and headaches.
The Flow E9 and the Blue series will not run from a socket. A socket circuit is usually 2.5 mm² and carries other loads as well. For these appliances a separate, new line from the consumer unit to the appliance is run. The Flow E7 · Glow · V1200 can be fed from a suitable socket circuit.
This is the most dangerous mistake. The fuse protects the cable, not the appliance. If you fit a large fuse to a thin cable, the cable heats up and it can end in a fire without the fuse ever tripping. The right way round is: first upgrade the cable, then choose the fuse to match.
A two-core line (brown + blue) has no earth. An appliance without an earth must not be operated in a wet room. There must also be a residual current device (life protection) in the consumer unit. If there is none, the electrician must add one before the appliance is installed.
Lines joined inside a junction box to extend the cable heat up at the joint under high load. Wherever possible, an instant water heater circuit should be a single unbroken run from the consumer unit to the appliance.
At the same cross-section aluminium carries less current than copper and works loose in terminal connections over time. On an instant water heater circuit we use copper .
A boiler's electrical supply is usually thin(a boiler draws little electricity, it heats with gas). If an instant water heater is going in its place, that cable is usually not enough. The plumbing can be reused as it is, but the electrical line is renewed.
If you are undecided, do not struggle with it. Send the three photos below on WhatsApp; we will tell you honestly which appliance can be fitted, whether a new line is needed and roughly what it will cost. This check is free of charge.
