Three phase distribution

13 Mar 2010
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United Kingdom
Distribution of circuits in commercial settings

I have little experience of three phase supplies and how it is distributed, and how it is balanced.

How are single phase boards usually fed from a three phase supply, in (light) commercial settings

I understand they are different ways of how it is done, but, what is the norm...

Would the three phases come into a head, then onto a three phase isolator, then a ryefield distribution board or similar, with a fuse for each phase.
Would the three supplies then go into isolators, then on to the individual single phase boards.

I presume with this method, balancing is not a concern, you just have to be aware of the maximum demand, and the supply fuse. Like a domestic situation

The reason I ask

Im doing some work in a light commercial building
I was just swapping out the old halogen security lights for LED

The Board supplying power to my lights is fed off one phase, from a distribution board somewhere in the building (The board is labeled Red phase!)

There is a firm also working in the building, installing a washing machine, dryer , and a 14.2 kw oven.
So quite an additional load on this phase.

Now if this phase is straight off the supply , as it were, balancing would not be something we would be concerned about, or could do anything about.

But, what if this phase is off a three phase distribution board, i.e the phase is off the red phase of a distribution board.
Then balancing would come into play, as a larger load is now on the main boards red phase.

So the main board would need balancing, i.e moving loads around to compensate for the increased load on the red phase.?

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For the most basic arrangement - 3 phase incomer, metering, 3 phase DB.
Both 3 phase and single phase loads can be connected to a 3 phase board.
If power is also required at some distant location, it's generally advisable to take the 3 phase there and fit another DB.

The presence of single phase CUs, particularly plastic ones, indicates that DIY bodgers and those who only know domestic work have been in.

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