my brother's shower

CharlesM · Posted: Thu Mar 15, 2007 8:03 pm Post Subject: my brother's shower

My brother has 10.5kW shower and it keeps on tripping the fuse. It has a 30A fuse and the 6mm cable is about 20m from the consumer unit. My Dad thinks that putting in a 45A fuse may be dangerous as he doesn't think the 6mm cable would be man enough to take such a current. Do you think that he will have to upgrade the cable to a 10mm one if he puts in a 45A fuse?

ricicle · Posted: Thu Mar 15, 2007 8:26 pm Post Subject:

6mm² is rated at 46A so as long as the cable is not installed in thermal insulation anywhere, is not likely to be in ambient temps of >30c then fitting a 45A cartridge fuse or MCB will be ok.(I take it the shower is already protected by a 30mA RCD)

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davy_owen_88 · Posted: Thu Mar 15, 2007 8:27 pm Post Subject:

Under certain circumstances 6mm² is ok for 45A. The best solution though is to replace the cable with 10mm².

If you have a 30A fuse you most probably don't have RCD protection and an old consumer unit. Could you post some pictures?

CharlesM · Posted: Thu Mar 15, 2007 8:38 pm Post Subject:

Thanks, ricicle and Davy. I'll see if I can get a picture - of the consumer unit I assume?

davy_owen_88 · Posted: Thu Mar 15, 2007 8:40 pm Post Subject:

CharlesM wrote:

I'll see if I can get a picture - of the consumer unit I assume?

Yup. Should give us an idea of whether it is suitable for such a load.

You've also got over 6V of volt drop (if my memory serves me well), depending on the voltage drop in the rest of the installation you are pretty close to the limit.

Softus · Posted: Thu Mar 15, 2007 8:59 pm Post Subject:

davy_owen_88 wrote:

You've also got over 6V of volt drop (if my memory serves me well), depending on the voltage drop in the rest of the installation you are pretty close to the limit.

Just curious davy_owen_88 - what's the effect of exceeding that limit?

davy_owen_88 · Posted: Thu Mar 15, 2007 9:00 pm Post Subject:

It' isn't a problem for showers except they will warm the water a tiny bit less. But you will be exceeding the limit set by BS7671 so run for the hills

Softus · Posted: Thu Mar 15, 2007 9:03 pm Post Subject:

davy_owen_88 wrote:

...run for the hills

Um, excuse me being so naive, but is this because it would contravene a standard or because it would be dangerous (or both)?

davy_owen_88 · Posted: Thu Mar 15, 2007 9:06 pm Post Subject:

It wouldn't be dangerous in the case of a shower but if the VD exceeds the 4% limit set out in BS7671 then you have failed to meet one of its 'rules' Motors are more susceptible to VD and can fail to start because of their high in-rush currents.

Softus · Posted: Thu Mar 15, 2007 9:07 pm Post Subject:

Sorry to keep going on about this, but what is the purpose of that rule?

davy_owen_88 · Posted: Thu Mar 15, 2007 9:09 pm Post Subject:

Motors are more susceptible to VD and can fail to start due of their high in-rush currents.

securespark · Posted: Thu Mar 15, 2007 9:10 pm Post Subject:

ricicle wrote:

6mm² is rated at 46A so as long as the cable is not installed in thermal insulation anywhere, is not likely to be in ambient temps of >30c then fitting a 45A cartridge fuse or MCB will be ok.(I take it the shower is already protected by a 30mA RCD)

Or grouped or run in MT...

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Softus · Posted: Thu Mar 15, 2007 9:13 pm Post Subject:

OK. Thanks.

RF Lighting · Posted: Thu Mar 15, 2007 9:15 pm Post Subject:

The problem is that for example if the shower heater element (or whatever the load is) is 10kW then at 240V it will use 41.7A, but if the cable run is too long the and the volts drop to say 220V then the current rises to 45.5A.

It only needs a VD of 8% to drop to 220V.

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ricicle · Posted: Thu Mar 15, 2007 9:17 pm Post Subject:

Rotor overheating can also occur in induction motors due to undervoltage.
I can't quite remember the exact explanation but I know the rotor currents have to increase to offset the fact that the flux is lower from the stator due to reduced voltage. (I can dig out the exact explanation if anyones sad enough to be interested)

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