Question 1

What cable size do I need for a 10.5kW shower in an insulated loft?

Accepted Answer

A 10.5kW shower draws 45.7A (10500 / 230). Under Method 101 (enclosed in thermal insulation for over 0.5m), a 10mm² cable is derated to roughly 32A — insufficient. You need 16mm² T&E, which retains approximately 43A capacity in insulation. Always verify voltage drop on runs over 15m.

Question 2

Can I upgrade from an 8.5kW to a 10.5kW shower on existing 6mm² cable?

Accepted Answer

No. An 8.5kW shower draws 37A and is marginal on 6mm² even under Method C (47A rating). A 10.5kW unit draws 45.7A, exceeding the 6mm² rating once any correction factor is applied. You must rewire with 10mm² minimum, and check the MCB is uprated to 45A or 50A.

Question 3

Why does my shower feel weak — could it be a voltage drop issue?

Accepted Answer

Likely yes. Long cable runs on undersized conductors cause excessive voltage drop. BS 7671 allows a maximum 5% (11.5V at 230V), but even within this limit the shower may underperform. At 45A, a 20m run of 6mm² drops about 6.6V — within limits but noticeable. Shorter runs or thicker cable improve performance.

Question 4

Do I need RCD protection for a shower circuit?

Accepted Answer

Yes. All bathroom circuits require 30mA RCD protection under Regulation 701.411.3.3. Amendment 3 (2022) further requires a Type A (or Type F) RCD where electronic controls are used, which covers most modern electric showers with digital temperature regulation.

Question 5

Is supplementary bonding still required in bathrooms?

Accepted Answer

Not always. Regulation 701.415.2 permits the omission of supplementary bonding provided all circuits in the room have RCD protection, and the main protective bonding and automatic disconnection times are confirmed satisfactory by testing. In practice, many electricians retain it for belt-and-braces protection.

Question 6

What size cable do I need for a 9.5kW electric shower?

Accepted Answer

A 9.5kW shower draws 41.3A at 230V. A 10mm² T&E cable (rated 64A, Method C) on a 45A MCB is standard. A 6mm² cable at 47A is technically adequate but leaves almost no margin after correction factors. Industry best practice is 10mm² for showers above 9kW.

Question 7

What MCB size do I need for an electric shower — 40A or 45A?

Accepted Answer

For showers up to 9kW (39.1A), a 40A MCB is adequate. For 9.5kW and above (41.3A+), a 45A MCB is required. The MCB rating must always be at least equal to the design current (Ib ≤ In) per Regulation 433.1.

Question 8

Can I use 6mm cable for a 10kW shower?

Accepted Answer

No. A 10kW shower draws 43.5A. While 6mm² is rated at 47A (Method C), this leaves only 3.5A headroom — any correction factor reduces capacity below the design current. 10mm² cable is the minimum safe choice for 10kW showers per Table 4D5.

Question 9

Does an electric shower need its own circuit?

Accepted Answer

Yes. Every electric shower must have a dedicated radial circuit from the consumer unit. The circuit must be protected by an appropriately rated MCB and a 30mA RCD per Regulation 701.411.3.3.

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