overcurrent protection question

[mblaquière]

Hi all, I'm fairly new to all this so bear with me if it's a ridiculous question...

In my text book that I'm studying for my course (c&g 2365) it has symbols that you use to define whether the protective device will work for a given circuit (I think? please correct me if I'm wrong).

The symbols are
Iz (current carrying capacity of conductor)
It (value of current stated in regs for type of cable and install in amb temp of 30c)
Ib (current intended to be carried by circuit)
I2 (operating current of protective device)

Then it gives me a formula of conditions that must be satisfied for circuit

Ib<In<Iz and I2<1.45 x Iz

Where does the In come from? Am I being stupid?
And also how would you work out much current appliances and cookers will use on a circuit?

Thanks

 

[Brightspark2]

Rating of protective device...

And you get the information from the appliance or cooker itself. They differ you see...

 

[telectrix]

In is the rating of the fuse/MCB. so that Ib<In (so the ocpd won't trip zt normal load). and In< Iz ( so that the ocpd will trip before the cable is overloaded). I2 is the current which will cause the ocpd to trip ( as ocpd's will stand overload. I2 is generally 1.45 x In).

 

[shagbite]

Hi all, I'm fairly new to all this so bear with me if it's a ridiculous question...

In my text book that I'm studying for my course (c&g 2365) it has symbols that you use to define whether the protective device will work for a given circuit (I think? please correct me if I'm wrong).

The symbols are
Iz (current carrying capacity of conductor)
It (value of current stated in regs for type of cable and install in amb temp of 30c)
Ib (current intended to be carried by circuit)
I2 (operating current of protective device)

Then it gives me a formula of conditions that must be satisfied for circuit

Ib<In<Iz and I2<1.45 x Iz

Where does the In come from? Am I being stupid?
And also how would you work out much current appliances and cookers will use on a circuit?

Thanks

In = nominal rating of overcurrent protective device.

appliances usually come with the Kw rating attached. use the formula I = P/V to work out the current demand. remember to apply diversity where allowed.

 

[mblaquière]

Thank you very much! That makes much better sense. Became pretty obvious once you said it! Haha
So if you have numerous appliances on a circuit is the total current all of them added together?
Thanks again!

 

[Andy78]

Thank you very much! That makes much better sense. Became pretty obvious once you said it! Haha
So if you have numerous appliances on a circuit is the total current all of them added together?
Thanks again!

Yes, but note Shagbite's reminder to to apply diversity where allowable.

 

[GLENNSPARK]

Yes, but note Shagbite's reminder to to apply diversity where allowable.

this is it...3 KW cookers being a prime example...
you can usually knock a KW off....

 

[GLENNSPARK]

but remember..no diversity allowable for instantanious water heaters....