Help....gone blank!!!

[sparky b]

Hi,

I went to look at a swimming pool today, the chap wants the heater hooked up to a cu in his shed..

The heater needs a 63a D type mcb and 30ma rcd protection at shed cu. the cable feeding the shed cu is 10mm swa running 20 meters to the house into a 4 way cu containing a 30ma rcd and mcb.

My long winded question is if I was to take the 30ma rcd from house end of cable and swap it for a 100ma rcd, then putting the 30ma in the shed cu, what type/rating of mcb would i need at the house end bearing in mind the heater would be running on a 63a D type mcb?

I know this is basic stuff, but i really am having a 'blonde moment' :Blushing

many thanks

b

 

[septiclecky]

Don't forget the volt drop as well with a 63A load, so the existing 10mm might not be big enough.

 

[sparky b]

Doh! I really am having one of those days....can't see wood for trees.....

bad day explode

 

[green-hornet]

Volt drop would look ok on 10mm given 63amp as maximum required.

Looks like a drop of 5.64v for 20 meters.

Supply voltage = 230 Volts

Ib - design current = 60 Amps

Protective device type = MCB typeD

In - protective device rating = 63 Amps

Length of run of cable = 20 meters

Power factor = 1

Cable Type : Thermosetting 90

 

[m4tty]

Nice one gh. Can you explain the last paragraph for me please as I'd like to understand why that is if you don't mind mate. Is it because zs? ThAnks

 

[green-hornet]

Yes a type d mcb has a maximum Zs, the calculation I did was for 0.4 sec disconnection which is only applicable to circuits up to 32amp, but I prefer to edge on the side of caution.

A RCD or RCBO reacts to a fault differently to a MCB and can disconnect faster when a typical installtion has a problem with high Zs readings, which is why RCD's are required for installations that are TT.

 

[sparky b]

Thanks Mr Hornet,

If i install rcbo at shed cu, what would i install at the house end of the 10mm swa to protect that?

 

[Blue Duck]

Volt drop would look ok on 10mm given 63amp as maximum required.Looks like a drop of 5.64v for 20 meters.

Supply voltage = 230 Volts

Ib - design current = 60 Amps

Protective device type = MCB typeD

In - protective device rating = 63 Amps

Length of run of cable = 20 meters

Power factor = 1

Cable Type : Thermosetting 90

 

[septiclecky]

Volt drop would look ok on 10mm given 63amp as maximum required.Looks like a drop of 5.64v for 20 meters.

Supply voltage = 230 Volts

Ib - design current = 60 Amps

Protective device type = MCB typeD

In - protective device rating = 63 Amps

Length of run of cable = 20 meters

Power factor = 1

Cable Type : Thermosetting 90

 

[bluetobits]

Why type D for a heater?

 

[sparky b]

Cause it's a heat pump, basically an air conditioning unit running in reverse. The unit itself only needs 6mm conductors but a large type d mcb because of the start up surge (according to manafacturers instructions....) ray