new supply

[G_FUNK]

A few quick questions;

incoming 3 phase supply into a 3 phase cut out each phase having a 100 amp bs88 type 3 fuse. Iam i right in saying that i would need a 200 amp main 4 pole incomer?

Each phase to the board having a 25mm csa and less than 3 mtrs in length, neutral and earth having a csa of 35mm? my mind has gone black one this one

installations max demand is 200 amps split evenly across the 3 phase supply.

chrees

martin

 

[jaydee81]

only have a 100 amp supply so 100 amp 4 pole incomer is fine as load is evenly split across phases 25mm neutral 16mm earth is the majority of the load single or 3 phase

 

[G_FUNK]

Yes i see where your coming from there 70 amps a phase 25mm lives earth been half the size of the phase conductor. i was thinking in terms of 3 phase fault current with the increased earth and neutral conductor and also the current the neutral conductor will carry if the installation was at or near to its max demand over the 3 phases pulling 200 amps around 67 amps per phase.

 

[jaydee81]

is most of the load 3 phase or single phase circuits?

 

[G_FUNK]

They are all single phase circuits. no 3 phase loads at all. largest loads are heating and water heating loads.

 

[jaydee81]

not an expert but im sure if all 3 phases equally loaded then neutral current would be 0

 

[keniff]

You will never balance 100%

 

[snowhead]

Yes i see where your coming from there 70 amps a phase 25mm lives earth been half the size of the phase conductor. i was thinking in terms of 3 phase fault current with the increased earth and neutral conductor and also the current the neutral conductor will carry if the installation was at or near to its max demand over the 3 phases pulling 200 amps around 67 amps per phase.

The current in the Neutral of 3ph will never exceed the maximum current in any 1 of those phases regardless of whether the loads are 3 phase or not.
So in your case the max in the neutral will be 67amps.

If it was ever possible for it to be higher then the neutral cable would always have to be 3 times the capacity of a phase cable

 

[SirKit Breaker]

You will normally find that 3 phase isolator switches are 125A. Some are 4 pole and some are 3 pole with a connector for the Neutral. Which one you need depends on the type of supply you have.

Cheers...........Howard

 

[Knobhead]

Neutral current

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[G_FUNK]

Neutral current

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

what does Ic stand for? And could you tell me where this calc is in the regs so i could have a read over it

cheers

 

[snowhead]

what does Ic stand for? And could you tell me where this calc is in the regs so i could have a read over it

cheers

I doubt it's in the regs, it's basic 3ph theory.

 

[Knobhead]

Ia Ib Ic the current in 3 phase.

Chuck the reg's away, it doesn't have any rules on basic physics.

Correction, it does, but most of them are wrong.