I am currently upgrading my electric shower. At present, I have 6mm cable running the 8 kW shower with no problems. From the diydoctor guide, BS7671 states that I need 10mm cable. We normally only run the shower on setting 5 or 6 on the thermostat so therefore am I right in saying that we only draw about 18 to 20 Amps ? If this is the case then is this the reason why I we havent had any tripping of the MCB ?
[quote="terminator72"]I am currently upgrading my electric shower. At present, I have 6mm cable running the 8 kW shower with no problems. From the diydoctor guide, BS7671 states that I need 10mm cable. We normally only run the shower on setting 5 or 6 on the thermostat so therefore am I right in saying that we only draw about 18 to 20 Amps ? If this is the case then is this the reason why I we havent had any tripping of the MCB ?[/quote]
GET your part p electrcian 2 install a 10 mm2
cable for u.
The trip is a two in one device.
The magnet part will trip when the power is 3 to 20 times what is written on the MCB and is categorised by a letter a B means 3 to 5 times. This part of the trip is designed for short circuit not overload.
The thermal part is designed for overload and we are given a graph for B type Fig 3.4 page 249 in 17th Edition. From this graph a 32 amp MCB will run at 40 amp for around 1000 seconds and 70 amp for 100 seconds.
Most showers are also two stage where a 8kW shower has two elements of 4kW so can run half or full power but in the main the thermostat controls the water rather than power so the low the temperature the more water flows and all settings use same amount of power. The smaller units may not have duel power setting.
Now most people don’t spend too long in a shower so 1000 seconds = 16 minutes so a 32 amp MCB will run a 40 amp shower for that time without tripping. But each time it does trip it tends to get weaker so after many trips it my trip after only 10 minutes.
The table converts Kw to Amps and as you can see even a 9.5Kw on a 32 amp MCB will likely run for over 10 minutes to start with.
7 = 30.43478
7.5 = 32.6087
8 = 34.78261
8.5 = 36.95652
9 = 39.13043
9.5 = 41.30435
10 = 43.47826
Cable except for flex which should always be exposed to free air does not have a single rating i.e. you can’t buy 30 amp cable. There are tables to work out with the cooling available what it’s maximum current will be and also there are different types of cable that will run at different temperatures so a 6mm flat grey cable (PVC) is normally considered as 32 amp but it could be down to 23 amp again we have Table 4D5 to help us. Using one of the new cables designed for partition walls you could be drawing 45 amp on a 6mm cable.
Because there are so many variables it is very hard to tell anyone what cable they should use and what size MCB but one would hope the original electrician would have matched the MCB to the cable so as long as you don’t up grade the MCB without up grading cable all should be safe but using a larger shower would mean you have to take shorter showers.
Does that explain it all?
Often it is two different people setting out wiring and selecting what showers are fitted likely some buyer has found he can get job lot of 8Kw cheaper than original 7Kw and by time electrician found out first fix already completed so too late to change cables. On some large estates there can be months between putting cables in and finishing off. And you can imagine builders reaction if spark tried to replace cable after plastering. Especially if the builder had informed electrical firm of up grade but they had been slow of the mark telling the sparks on the job.
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