Summary

Electric showers draw more current than almost any other single circuit in a domestic installation — a 10.8kW unit pulls close to 47A continuously while running, right at the edge of what 10mm² cable can carry. Getting the cable size wrong isn't a nuisance-trip problem like an undersized lighting circuit; it's cable overheating inside a wall cavity, which is a genuine fire risk in a location nobody inspects again once the plaster goes back on.

The number that gets quoted casually on site — "8.5kW is 6mm², 10.8kW is 10mm²" — is a reasonable rule of thumb for a short, unobstructed cable run clipped direct to a joist or wall. It stops being reliable the moment the cable passes through loft insulation, is bundled with other cables, runs a long distance, or the property has an unusually warm roof void. This calculator sets out the full method — design current, current-carrying capacity with correction factors applied, and voltage drop — so the cable size is verified for the actual installation, not just looked up against a rough table. For the full installation picture beyond cable sizing — RCD protection, bathroom zones, Part P notification — see electric shower installation.

This matters for anyone specifying or first-fixing a shower circuit: electricians doing the calculation properly rather than by habit, and builders/kitchen-and-bathroom fitters who need to brief an electrician accurately or sanity-check a quote. Undersizing shows up as nuisance tripping or overheating; oversizing wastes money on cable and a bigger MCB than needed, and can itself cause problems if the protective device no longer matches the accessory's rating.

Key Facts

Quick Reference Table

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Shower current draw and starting-point cable size (short run, clipped direct, no insulation, no grouping — verify against the full method below for the actual installation):

Shower rating Design current (Ib) Cable (clipped direct) Cable (any run through insulation) Min MCB/RCBO
7.5kW 32.6A 6mm² 6mm² (check length) 32A
8.5kW 37.0A 6mm² 10mm² recommended 40A
9.5kW 41.3A 10mm² 10mm² 45A
10.5kW 45.7A 10mm² 10mm² 45A
10.8kW 47.0A 10mm² 10mm² 45–50A
12.5kW (commercial) 54.3A 10mm² 16mm² recommended 63A

Detailed Guidance

Step 1: Calculate the Design Current

Ib = P (watts) / V (230V)

This is the actual current the circuit will carry — showers run near their full rated load whenever in use, so no diversity is applied (unlike a cooker circuit, where diversity reduces the design figure). For a 9.5kW shower: Ib = 9,500 / 230 = 41.3A.

Step 2: Establish the Installation Method Along the Whole Route

The tabulated current ratings in BS 7671 (e.g. 47A for 6mm² clipped direct) apply to a specific reference installation method — typically Method C, clipped direct to a surface with free air around it. The moment any part of the cable's route departs from that — buried in a wall, run through a stud cavity packed with insulation, or bundled through a loft hatch alongside other cables — the rating drops.

Walk the actual route from consumer unit to shower unit and note every section where the installation method changes. It's the worst single section along the whole run — not the average — that determines the cable's effective rating, because that's where the cable will run hottest.

Step 3: Apply Correction Factors

The cable's actual current-carrying capacity (Iz) is the tabulated rating multiplied by the relevant correction factors:

Iz = It × Ca × Cg × Ci

Worked example: 6mm² T&E, tabulated rating 47A, run passes through 400mm of loft insulation for a 1.2m section of its route (fully surrounded).

Iz = 47 × 0.5 (fully surrounded by insulation, run >0.5m)
Iz ≈ 23.5A

An 8.5kW shower needs 37A. At 23.5A this cable fails completely — it must be upsized, most straightforwardly to 10mm², or the route changed so the cable isn't buried in insulation (running it in a batten/conduit above the insulation line, for example, avoids the derating rather than requiring a bigger cable).

Step 4: Check Voltage Drop

Even where current-carrying capacity passes, the cable must also satisfy the voltage drop limit — a separate check using a different table.

Vd = (mV/A/m × Ib × L) / 1000

Where L is the actual route length in metres (not the straight-line distance).

Worked example: 10.8kW shower, Ib = 47A, 18m cable route, 10mm² T&E (mV/A/m = 4.4).

Vd = (4.4 × 47 × 18) / 1000 = 3.7V

The limit for power circuits is 11.5V (5% of 230V) — 3.7V passes comfortably. Repeating the same calculation for 6mm² (mV/A/m = 7.3) gives Vd = 6.2V, which also passes on voltage drop alone — this is why voltage drop can pass on a cable that current-carrying capacity has already ruled out. Always check both; passing one does not mean the cable is adequate.

Step 5: Confirm the Protective Device

Once the cable size is confirmed, the MCB or RCBO rating must satisfy:

Ib ≤ In ≤ Iz

For the 9.5kW shower example above (Ib = 41.3A, 10mm² cable clipped direct, Iz = 65A before any derating), a 45A device satisfies both sides of this rule comfortably. Where derating has reduced Iz significantly — as in the insulation example above — check the device rating still sits at or below the derated Iz, not the clipped-direct figure.

Worked Example: Full Route, Loft Conversion En-Suite

Scenario: 10.5kW shower, consumer unit in the garage, 22m cable route: 8m clipped direct along the garage/hallway ceiling void (no insulation), then 6m through the loft space where it's laid on top of 300mm mineral wool insulation (touching on one side only, not fully buried), then 8m clipped direct down a stud wall to the shower unit.

Step 1 — Design current: Ib = 10,500 / 230 = 45.7A

Step 2 — Trial cable: 6mm² T&E. Tabulated clipped-direct rating = 47A.

Step 3 — Correction factor for the loft section. Touching insulation on one side (not fully surrounded): approximate factor 0.75.

Iz = 47 × 0.75 = 35.25A

35.25A is below the 45.7A design current — 6mm² fails.

Step 4 — Trial cable: 10mm² T&E. Tabulated clipped-direct rating = 65A.

Iz = 65 × 0.75 = 48.75A

48.75A exceeds 45.7A — 10mm² passes current-carrying capacity.

Step 5 — Voltage drop check for 10mm² over 22m:

Vd = (4.4 × 45.7 × 22) / 1000 = 4.4V

4.4V is well within the 11.5V limit — passes.

Result: 10mm² T&E on a 45A MCB/RCBO, 30mA RCD protection. The 6mm² option that a quick rule-of-thumb table would suggest for a shower in this wattage bracket fails once the loft insulation section is properly accounted for.

Frequently Asked Questions

Do I really need to check voltage drop separately, or does passing the current rating cover it?

They're independent checks and both must pass — a cable can have plenty of current-carrying capacity but still exceed the voltage drop limit on a long run, and (less commonly, but it happens) a cable can be within the voltage drop limit while its current-carrying capacity has already been derated below what's needed. Always run both calculations; see voltage drop for the full voltage drop method and additional worked examples across other circuit types.

My cable only touches insulation for a short section — does the full derating still apply?

The reduction depends on how much of the run is affected, not just whether any insulation contact exists. A cable touching insulation on one side for a short section typically takes a smaller reduction than one fully surrounded for a long run. Always check the current BS 7671 tables for the specific length and insulation configuration rather than assuming a flat factor — this is one of the more commonly misapplied corrections on site, usually in the direction of not applying it at all.

Can I avoid the insulation derating altogether?

Yes — the practical fix is often routing the cable above or outside the insulation layer (in a conduit or batten space, for example) rather than burying it. Where the route genuinely can't avoid running through insulation, upsizing the cable is the reliable fix, and the material cost difference between 6mm² and 10mm² is small next to the cost of the shower unit and installation labour.

Is the answer different for a shower on a long extension lead from a garage consumer unit versus a normal domestic circuit?

The method is identical — design current, correction factors along the full route, voltage drop — but long runs from a detached consumer unit (garage, outbuilding) are exactly where voltage drop is most likely to become the limiting factor rather than current capacity, because route length has a direct linear effect on the Vd calculation. Always calculate voltage drop explicitly on any run over roughly 15–20m rather than relying on a short-run rule of thumb.

Regulations & Standards