# Calculating Max Demand in a 3phase Domestic Installation



## ozzbash (Nov 5, 2012)

Hey guys I am a 4th year electrical apprentice doing my final exams and I have a question to as you professional sparkies out there before I make a fool myself infront of the whole class.

We are doing Maximum demand calculations on a 3 phase domestic house situation and the dilemma is that the teacher is dividing the lights calculations equally across all 3 phases and balancing it out. My question is why balance the lights in all 3 phases when you can just put all the lights on 1 single phase. We use 240/415 volts in Australia. 

When you balance the lights on all three phases, there can be a dangerous situation arising at a light switch where say its a 3 gang light switch, and its fed by 2 phases. Say two switches on the 3 gang switch are in 1 phase and the remaining switch is on another phase. There could potentially be 415V across the two phases of the common of the light switch. 

So my solution is just two have all the light circuit in the one phase rather then splitting them up and balancing the lights on all three phases. I dont know if I put this question up right or if you guys even understand what I am saying but if any contractor out there has had this problem pliz advise back 


cheers from aus


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## Aussielec (Apr 1, 2012)

ozzbash said:


> We are doing Maximum demand calculations on a 3 phase domestic house situation and the dilemma is that the teacher is dividing the lights calculations equally across all 3 phases and balancing it out. My question is why balance the lights in all 3 phases when you can just put all the lights on 1 single phase. We use 240/415 volts in Australia.


The idea behind it is that if you loose a phase every single light doesn't go out. Same deal with power circuit's if one of the service fuses blow or the SA drop's a phase upstream you don't loose everything. 



ozzbash said:


> When you balance the lights on all three phases, there can be a dangerous situation arising at a light switch where say its a 3 gang light switch, and its fed by 2 phases. Say two switches on the 3 gang switch are in 1 phase and the remaining switch is on another phase. There could potentially be 415V across the two phases of the common of the light switch.


That's fine aslong as the switch mech's both have shrouds over them.

With that said you already got a 240 volt potential at the light switch anyway which is just as lethal, so in that respect I don't think it really matters.

Btw welcome to the forum:thumbup:


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