Cable Capacity

[Chris Electron]

Hello,

Can anyone tell me please if you can use the figures given in BS7671 cable current carrying capacity when using 12V?

eg- so 1.5 multicore clipped direct can take 19.5A. Is this what the cable can carry for both 12V and 230V- or any other voltage?


Any help greatly appreciated!

Thanks,

Chris.

 

[Silly Sausage]

Yes. The voltage is irrelevant.

 

[moose]

Weard, but i have forgot the maths to work out Amps im using a 50v Lighting system and cable is 2.5 not the usual 1.5 as amps are more in this system...

Hope to see some great reminder Calcs here plz not done them for years...

 

[Chris Electron]

Thank you for this. I find it hard thinking that amps at both 12v and 230v are the same thing. If you grabbed hold of a cable with 12v and say 10A running through it you would live to tell the tale. But not 230V.

 

[AP Electrical]

Thank you for this. I find it hard thinking that amps at both 12v and 230v are the same thing. If you grabbed hold of a cable with 12v and say 10A running through it you would live to tell the tale. But not 230V.

so in saying that when a police man tazers you you'd drop dead......

 

[AP Electrical]

This is why we go to college to learn the theory. And to understand what it is we work with.

 

[Robert Turnbull]

How do you calculate current when you have the voltage and power??

Think about your answer

 

[jeremy]

My palms have enough resistance to withhold a ELV pd, but 230 will just jump on in there! 0nce it gets through the difficult bit ( the old calloused skin), the wet fleshy bit isn't so resistive and just lets it flow (man!) Current will kill you, but only if it can get in there. Enough rubber, or any other insulator, will inhibit the path, a Faraday cage gives it an alternative path

 

[FAA Greenie]

Thank you for this. I find it hard thinking that amps at both 12v and 230v are the same thing. If you grabbed hold of a cable with 12v and say 10A running through it you would live to tell the tale. But not 230V.

It's irrelevent what current the cable is carrying. Your body has a certain resistance, 12v at your finger tip will produce 20x less current flow through your bits and pieces than that of a 240v pd (and we all know that it's the current that will fry you). Simple ohms law.

 

[Robospark]

Chris - If you where hit by a lorry, the amount of damage caused (power transfered) would be a result of both the size and speed of of the vehicle. Think size is the current in amps and speed the voltage. The product of these would be the energy/power transfered measured in watts. Hence

Power(watts) = Current (amps) x Voltage (volts).

A basic calculation to derive the amount of current a load draws would see this formula swapped round to

Current drawn = Power/Voltage

 

[Robert Turnbull]

Put some
Figures in that equation and see the difference

100 watts/12 volts = 8.33 amps

100 watts/230 volts = 0.43 amps

100 watts/130 000 volts = 0.0008 amps

 

[Robospark]

As RT illustrates above, this is why power line transmission voltages are high.

The power lost across the length of a power line is equal to the square of the current carried multiplied by the cable resistance or

P=I2R

As the cable resistance is fixed higher voltages are used to transmit the same power at a lower current to limit the power lost across the line.

 

[Robert Turnbull]

Imagine the cable size also it they transmitted at 230v

 

[Engineer54]

Put some
Figures in that equation and see the difference

100 watts/12 volts = 8.33 amps

100 watts/230 volts = 0.43 amps

100 watts/130 000 volts = 0.0008 amps

Haha!! reminds me of the numerous times i've seen drawings for swimming pool lighting with wiring sizes totally inadequate for the load and distance. 2 X 300W/12V lamps these pools usually take, so drawing around 25A a piece. ...lol!! Often seen cable sized at 1.5 / 2.5mm, obviously a junior at work, forgetting they are 12V when doing his calculations...lol!!

 

[johnboy6083]

Don't forget to calculate the Zs for MCB's ect using the new voltage, not 230.

As E54 and others have mentioned, the current draw of the load should be calculated using the applicable voltage, and this is then compared to the BS7671 tables, once installation derating factors are taken into account.
You should then check volt drop. In ELV installations, volt drop and Zs are the prime constraints on cable size.

 

[Chris Electron]

Thank you for all the messages. Am new doing a 12v installation within a domestic house 230v. Apart from sticking up recessed down lights. Have done all volt drop calcs etc. When Zs has been mentioned above it reminded me that I will need a circuit breaker. So you get 12v fuses? I guess they must be out there.

 

[KAS1]

B&Q for your 12v fuses

Thank you for all the messages. Am new doing a 12v installation within a domestic house 230v. Apart from sticking up recessed down lights. Have done all volt drop calcs etc. When Zs has been mentioned above it reminded me that I will need a circuit breaker. So you get 12v fuses? I guess they must be out there.

 

[Marvo]

So you get 12v fuses? I guess they must be out there.

The primary function of a fuse is to limit the current that can flow in the circuit so you select an appropriate fuse according to its rating in Amps.

As long as the voltage rating on the fuse is higher that your circuit voltage it's fine. You can use a fuse with a 500v rating on a 12volt circuit.

 

[p11jor]

Chris have you managed to work out the Adiabatic for the earthing size running up to the transformer?

 

[Chris Electron]

This 12v install is oddly off a leisure battery! It is all a bit odd. I have backed off from any 'design' now on this 12v circuit as it is all a bit DIY for my liking.
Any info on anything that I might need to add if using a leisure battery in this install?!