Currently working on a HVAC control panel that has a “standard” 3phase and neutral 400V 100A supply.
The panel supplies multiple heaters and fans in a warehouse, all the heaters and fans are single phase with individual loads of between 2 & 6A. The load is fairly well distributed between the phases as follows-
L1=92AFLC
L2=78AFLC
L3=78AFLC
My question is does the neutral cable to the panel need to oversized to cope with the total load across the 3 phases?
Or am I being a dingbat!

TT and TN-S schemes

Single-phase circuits or those of c.s.a. ? 16 mm2 (copper) 25 mm2 (aluminium): the c.s.a. of the neutral conductor must be equal to that of the phases
Three-phase circuits of c.s.a. > 16 mm2 copper or 25 mm2 aluminium: the c.s.a. of the neutral may be chosen to be:

– Equal to that of the phase conductors, or
– Smaller, on condition that:
– The current likely to flow through the neutral in normal conditions is less than the permitted value Iz. The influence of triplen(1)
harmonics must be given particular consideration or
– The neutral conductor is protected against short-circuit, in accordance with the following Sub-clause G-7.2
– The size of the neutral conductor is at least equal to 16 mm2 in copper or 25 mm2 in aluminium

http://www.electrical-installation.org/enwiki/Sizing_the_neutral_conductor

If the three phases remain perfectly balanced then you don’t need a neutral at all, have a look at the latest regs for actual sizing in your circumstances

Footflaps has it.

You’ve essentially 3, single phases. Size conductors for the maximum load/voltage drop on the highest loaded phase as per normal. Same size for neutral. If concerned, oversize all incoming conductors to give spare capacity. ie 25mm armour ref. method C gives 102 A capacity. 35mm gives 125A. This would account for the extra size needed for harmonics (plus I like to overengineer!).

It’s covered in Section 524 of BS7671 says (paraphrased) ….

a) neutral not less than that of the line conductor where the line conductors are <= 16mm (25mm for aluminium).

b) >16mm – may have a reduced neutral in certain circumstances.

c) neutral may need to be bigger if there are significant harmonic currents.

Note: I’m fundamentally a domestic electrician. This is my interpretation of the regs. I would suggest that you check this before relying on my knowledge and if it all goes seriously bent remember it was footflaps who suggested it first. 😆

Rich.

Further to my last post, which was perhaps a bit unclear given the time of the morning and the rush I was in to get my son to college…..

From the OPs original post I’m assuming you’re thinking that the neutral might neeed to be sized for the sum of the three loads – 92+78+78=248 amps.
The answer to that is no. Because it’s three phase, the currents are going to be 120 degrees out of phase with each other. So, if L1 is at it’s peak, L2 and L3 will be 120 degrees before and 120 degrees after L1 in their cycle so will in fact be reducing the overall current in the neutral by a proportion of each of their maximum currents. For arguements sake, lets say that is 39 amps each (( 78*Cos(60) )then your actual neutral load is 92-78 = 14 amps.

A fully balanced 3 phase load will have no neutral current. If L1, L2 and L3 are all 78 amps then the neutral current is (78 – 2*(78Cos60)) which is 78-(2*39) which is zero.

So, sizing of the conductors is based on the highest phase conductor; in this case 92 amps. Neutral is sized by the regs to be the same size as the line conductor. Where harmonics are present there may be a factor in sizing to be applied to derate the cable (up to 0.86 of the cable rating I believe from a very quick look at the regs). So by increasing the cable size, the current rating goes up to cover the extra harmonic induced currents in the neutral.

Hopefully this makes sense and I’m not talking rubbish (it wouldn’t be the first time).

Rich.