Ohms Law & Cable length

[keith13672]

Hi guys,,

Brand new here so easy going ok!

Something is baffling me and i cannot for the light of me figure it out even after study of GN3.


I'm 41 and been around a while so not wet behind the ear.

If 55m of 2.5mm cable has a known resistance of 7.41m ohms per metre this equates to 0.41ohms.
Well using ohms law V over IR,,, 230 / 0.41 i get 560amps. I'm having a blonde moment surely as 560 amps don't ring correct.

My point is i suppose is how much current will flow with a given cable resistance.

The mind boggles - Help.

 

[telectrix]

simple. using ohms law. a dead short across the L and N ( bear in mind your value of R is doubled) would give rise to a fault current of 230/0.82. = 280.5A.

if just using that single 55m length shorted across a 230V supply, then you would get 560A and a rather warm piece of 2.5mm.

 

[keith13672]

Doh!!

simple. using ohms law. a dead short across the L and N ( bear in mind your value of R is doubled) would give rise to a fault current of 230/0.82. = 280.5A.

if just using that single 55m length shorted across a 230V supply, then you would get 560A and a rather warm piece of 2.5mm.

Cheers.I was thinking of the cable as a resistor yet it is only if with a fault of negligable impedance is applied.

 

[i=p/u]

In fault conditions . V =IR on a 10A breaker max volt drop of 4.1...

Im just typing

 

[telectrix]

what's volt drop got to do with melting the cable with a fault current of 560A? just for the record,VD would be 230V.

 

[i=p/u]

I x R =V

- - - Updated - - -

You sco use yank Merry christmas .

 

[telectrix]

I x R =V

- - - Updated - - -

You sco use yank Merry christmas .

ok. I =560A. R = 0.41 ohms so V = 560 x 0.41 = lo and behold. 229.6V.

 

[R1CH-G]

The current will flow up to the limit of the fuse, and then the fuse will disconnect.

Current handling capacities of cables can be found in the big green book of electrical wisdom.

Ib<=In<=Iz<=It

Sorry - am I missing the point here?

 

[i=p/u]

Fault current has nothing todo with volt drop, I was only adding another scenario as he's wet behind ears and wants shock in enamel.

 

[telectrix]

was no mention of fuses in OP. he was confused then, now it's getting better. next he'll be installing 240mm cable on a 500A breaker for the bedroom sockets.

 

[i=p/u]

He won't get the job as could wire house for the price of few sockets .

 

[telectrix]

@ OP. just to put you out of your misery, the current is limited by the load resistance, which will be much higher than that of the cable. e.g. an immersion heater is about 20 ohms, so current will be 230/20 = 11.5 A.

 

[ackbarthestar]

Quite right Tel, the OP forgot to mention the load.
Once that has been established, the volt drop due to the conductor resistance can be considered......

 

[telectrix]

Quite right Tel, the OP forgot to mention the load.
Once that has been established, the volt drop due to the conductor resistance can be considered......

and if a Electrical Trainee, ignored.

 

[ackbarthestar]

yea right....lol...

'So madam I have calulcated the size of cable required. It will be 95mm2 ... Would you mind if it was done in singles because the chap down at the trade counter could not find a listing for T&E.......'

Now what sort of light fitting would you like ?..................................................