DIY Doctor UK DIY Forums

greetings

there is a 16 amp mcb fuse in the main cu that used to power up a water tank.the water tank is no longer in use because we now get hot water from central heating boiler.we have 16 amps that is not being used and no other items depend on the 16 amps. the water tank had its own 16 amp.the switch for the water tank is over 9 mtrs away from the boiler cause the switch is in the hall way upstairs.is it alright to extend the wires in the switch by 2 mtrs and connect to a secondary cu? the secondary unit has max 40 amps, 2 fuses, 1 16 amps and the other 6 amps.i just need 6 amps for a light outside in the garden.

can you please tell me the formula that is required to find out how much amps flow through electrical wires, if for example they are 2 3 4 ect mtrs long?

To use the redundant supply to power other items is OK. But the way you describe it is not possible to work out if within regulations as to extent one must follow a set of rules as to direction and protection required and cables must run in Zones and connections either visible junction box or with set permitted methods if hidden.
All new installation would need RCD protection and with a garden then registering with the council under Part P there is a link in projects section.
As to amps this is controlled by what you are powering and the protection device and although there is a cable length consideration the length is more to do with volt drop and earth loop impedance. If this is what you are trying to work out then 2.5mm cable will drop 18 mV/A/m so 2.5mm cable limited to 6 amp would have a maximum of 64 meters for lighting (3% volt drop) or at 16 amp 24 meters. Assuming the ELI is within limits which with a RCD it would likely be OK.
It seems you are well out of your depth and I would suggest it may be better to get a registered electrician!
If you do want to DIY first step is down load Part P and see what you are letting yourself in to. Once you have done this then ask some more questions.
Eric

greetings Eric

i will take your advice and consult an electrictian.at the moment i am not messing around with the electrics, just learning the basics.please can you help with with the following (this is not for real, only an example),

you have mains box in the front room
there is a 6 amp mcb fuse
15 mtrs of wire (6 amp 1.5 mm) is connected to the mcb fuse
the wire is powering up 1 light in the garden.
please explain how to work out how much current is passing through the wire.
is it something like resistance * length * amps?

If for example you have a 50 watt lamp and the nominal voltage is 230 volts assuming a power factor correction of unity then 50/230 = 0.217391 amp. Also using ohms law 230/0.217391 = 1.058KΩ if we take the resistance of 50 meters of 1.5mm cable at 0.029 ohms per meter then adding that to the resistance then we find the bulb is only drawing 49.93157 watt instead of 50 as you can see a little pointless!
Eric