NEUTRAL TAIL SIZE ON A 3 Ph SUPPLY etc?

Piratepete · Jan 23, 2011

The supply at a farm is 3 phase PME, currently with 2x60Amp and 1x100Amp cutouts. The 3 phase supply feeds via a 40Amp switchfuse to a board in farm buildings - the load seems to consist mainly of lighting distributed across the 3 phases. As well as supplying the outbuildings the 100 Amp leg is also connected to a 40 Amp spn workshop supply and the house consumer unit.

I have to get an 80 Amp spn supply to a remote bungalow (60 metres away).

I'm proposing an upgrade to the cutouts all to 100 Amp (DNO says ok), splitting the house, workshop and bungalow across the 3 phases to try to balance the load.

I have to install new tails - I can see that the 3 lives will be 25sqmm, but what should the neutral be? Answers to this and constructive comments to my masterplan welcome!

Deleted member 26818 · Jan 23, 2011

If your line conductors are to be 25mm² then the neutral should also be 25mm².

ezzzekiel · Jan 23, 2011

agrees with above

Piratepete · Jan 23, 2011

If the majority of the load is single phase, won't the return current be far greater than the 114 Amps that 25 sqmm is rated for? Ah! Just put brain into gear - it's 3 phase DOH!

johnboy6083 · Jan 23, 2011

even if the majority of the load is 1ph, that will be balanced across the phases, and therefore cancel eachother out. if all ophases are exactly balanced, then the resulting neutral current will be 0A. in the worst case scenario, if all loads are on one phase, then the neutral current will equal the phase current. This is then no different to a single phase supply

Dean182 · Jan 23, 2011

By balancing the loads across three phases you reduce the net neutral current, providing they are evenly balanced.

Piratepete · Jan 23, 2011

johnboy6083 said:
if all ophases are exactly balanced, then the resulting neutral current will be 0A. in the worst case scenario, if all loads are on one phase, then the neutral current will equal the phase current.

Surely the neutral current can only be zero if you're not using the neutral (all 380 volt kit connected line to line)? If everything is 230 volt using the neutral, the current will be max 100Amp because the return current is 120 deg out of phase on each line.

Dean182 · Jan 23, 2011

johnboy6083 said:
if all loads are on one phase, then the neutral current will equal the phase current. This is then no different to a single phase supply

Not sure if thats quite true. If your overloading one phase of a three phase supply, can't the neutral current be 1.73 times that of the phase. Which is why it's important to keep them as balanced as possible. On single phase supplies the phase current will be equal to the neutral because upon distribution its balanced. So if there 9 houses, 3 are fed from L1, 3 from L2, and 3 from L3, all sharing a common neutral back to the transformer, so in theory balances itself out.