Fitting a higher kW showera plain-English guide
A bigger shower is not a like-for-like swap. The cable and the breaker decide what you can fit, and that also decides whether the job is notifiable.
Before you start
What this means
This is the question behind an enormous number of electric shower searches and it is usually answered badly, because the honest answer has two halves and most sources give one. The appeal is real: the kW rating of an electric shower sets how much water it can heat to a usable temperature, so a 10.5 kW unit genuinely gives a better shower than an 8.5 kW one, especially in winter when the incoming mains is cold. The constraint is equally real and it is not in the shower. The unit is fed by a dedicated circuit sized for the load it was installed to carry, and moving from 8.5 kW to 10.5 kW takes the current from roughly 35 amps to roughly 44 amps at the 240 V the ratings are quoted at, and Triton’s own table moves the breaker from a 40 A to a 45 A MCB for the same step.
If the cable and the breaker were sized for the smaller unit, they are now undersized for the larger one, and the failure mode is not that it does not work — it is that everything warms up under load. That is the same mechanism as the burning-smell page. So the sequence is fixed: find out what the circuit is before choosing a shower, not after.
The legal position is worth getting right too, because it is commonly overstated. Swapping a shower for another one on the existing circuit is a replacement, and replacements are not notifiable under the Building Regulations — the Welsh Approved Document P says so about this appliance by name. The moment the job needs a new circuit, because the existing cable will not carry the bigger unit, it becomes notifiable work that is normally done by an electrician registered with a competent person scheme. Same bathroom, same afternoon, two different regulatory answers, and which one applies is decided by the cable rather than by the shower.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Existing cable sized for the old shower | The usual blocker. A cable adequate for 8.5 kW is not automatically adequate for 10.5 kW, and the run length and how it is installed both matter. | Common |
| Breaker rating below the new load | A 32 or 40 amp breaker feeding a shower that now wants 46 amps will nuisance-trip at best. Uprating the breaker without uprating the cable is the dangerous shortcut. | Common |
| Isolating switch not rated for the new current | The pull cord is part of the circuit and has a rating. It is the component most often left in place during a shower swap. | Occasional |
| Incoming water pressure will not support the flow | A bigger element does not help if the supply cannot deliver. Manufacturers publish a minimum running pressure and it is the other half of the specification. | Occasional |
What to try yourself
Read the rating plate on the shower you have. The kW figure is printed on it, and it tells you what the circuit was installed to carry.
Look at the consumer unit and note the rating of the shower breaker — 32, 40 or 45 amp. That number, not the shower, is what limits your options.
Get an electrician to confirm the cable size and its condition before you buy anything. The breaker can be right while the cable is not, and only one of those is visible.
Do not fit a larger breaker to stop a new shower tripping. The breaker protects the cable, and uprating it removes that protection rather than solving anything.Check the running water pressure against the new unit’s minimum. Bristan quote 0.7 bar for theirs; a shower starved of water will not deliver its rating whatever the element.
If the circuit will not take the bigger unit, price the new circuit as part of the decision. That is the notifiable half of the job and it is usually the larger cost.
Get a qualified electrician to look at your electric shower
We do not pass enquiries to a repair firm, so there is no form here. When you book a qualified electrician, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£150–£700
Time on site
2 hours
A like-for-like swap on a sound existing circuit is the low end, unit included. A new circuit from the consumer unit to the bathroom, which is what a real uprating usually needs, is the high end and it is notifiable work.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related electric shower fault codes
- Shower keeps tripping the electricsStop — call an engineerA shower that trips the breaker or RCD is reporting a real electrical fault. Do not keep resetting it to get one more shower out of it.Related: electrical-supply
- Shower completely deadGet it checkedNo neon, no water, nothing when you pull the cord. On an electric shower a total dead is almost always the supply rather than the appliance.Related: electrical-supply
- Burning smell from the shower or pull cordStop — call an engineerHot plastic, scorching or discoloured fittings on a shower circuit is an overheating connection. Switch the circuit off at the consumer unit and leave it off.Related: electrical-supply
- Shower runs coldGet it checkedWater flows but never heats. On an electric shower this is nearly always the supply, the flow through it, or an element that has stopped working.Related: same brand
- Thermal cut-out has operatedGet it checkedThe safety device has taken the elements out of circuit. Whether you can reset it yourself depends entirely on the brand, and the answers differ.Related: same brand
- Water suddenly scaldingGet it checkedAn electric shower that runs too hot is usually short of flow rather than making too much heat. The exception is a thermostatic model, which should not do this at all.Related: same brand
Related guides
Common questions
Can I fit an electric shower myself?
Legally, a like-for-like replacement on an existing circuit is a repair and is not notifiable — the Welsh Approved Document P names cookers and electric showers specifically. Whether it is sensible is a different question: it is a 40 amp connection in a wet room.
When does it become notifiable work?
When it needs a new circuit. Approved Document P makes installing a new circuit notifiable, as well as any addition or alteration to existing circuits within the zone around the bath or shower.
Will a 10.5 kW shower fix my weak shower?
Only if the weakness is heat rather than water. More kW means more flow at the same temperature — but if the incoming pressure is the limit, a bigger unit changes nothing and a pumped model or a different system is the real answer.
Is a lower kW shower cheaper to run?
Per minute, yes; per shower, not necessarily. It heats less water per minute, so the same wash takes longer. The saving is smaller than the rating difference suggests.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Trade body guidance
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.