# Adding A Submain



## chrisb271 (Jul 6, 2007)

Hi Gav

Don't put your 6mm cable on the 63A mcb,the mcb is a little too big,you want either a 32A or max 40A ( depending of course on the distance of your run,if the sub is only short then you could get away just with a 50A.

In my opinion i would go to a 16mm and leave the 63 Amp mcb in,you can guarantee they will need something extra on this board at a later date.

Regards 

Chris


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## Gavin Adams (Aug 22, 2008)

Hi chris
thanks for coming back to me. i had a look in the regulations before coming on her and i new that a 6mm would not carry 63 amps. and i understand how cable calculations work also. that is why i thought i would come and ask . the baurd is only a two way and has one way left on it. . just as a rule of thumb. does the breaker at the baurd suplying the sub need to be the same or lettle biger than the largest breaker in the sub. how does one work it out...
thanks bud


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## chrisb271 (Jul 6, 2007)

Gav,its all about something they call discrimination ( not the name calling of other races either :laughing: )

Basically ( BUT THERE ARE OTHER THINGS TO CONSIDER TOO)

Say you start off at the mains intake with a 100 amp fuse your first submain can be protected by an 80 amp fuse then your next sub main from that same board could be on a 63 amp fuse ( or mccb ) then from that board you could use a 50 amp to protect that submain from there.

Now some of the other things mentioned above are Earth loop impedances ( circuit resistances ) distances,installation routes,amount of cables bunched with,temperatures........

Its not as clear cut as just wangin in a circuit and blobbin a fuse on the end,it does need some thought.

Chris


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## Gavin Adams (Aug 22, 2008)

hello again chris
nice to here from you again. i do understand the thought prossess of electrical design as i do have the 2400 quoly. i understand discrimination too. however. as lets say, a sub main has two circuits. those circuts are already protectin by the fuses in that sub main. i understand that you work out ib then In then It and then volt drop and then iz then pfc. but how do you do all that with a sub main. how does the cable calculation work with you want to add a sub main. its late. sory for asking so many questions....hope to here from you soon


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