# Harmonic Distortion limite and Neutral cable sizing

Please I would like to know what percentage of cross section area should my neutral cable be with respect to the live conductor for a total harmonic distortion limit of less then 5% at nominal frequency of 50Hz

Parents
• I did wonder why this has not been moved to 'Wiring and the Regulations' as it's BS 7671-related ?

• Interestingly, the design software I use automatically provides a Total Harmonic Distortion percentage based on the type of load, which can be quite helpful.

• is your load 3-phase 3-wire, like most motor drives, or a collection of single phase connections? if the disturbing load isn't connected to the neutral, then it can't draw any current through the neutral

• if the three phase 3-wire load has one of it`s lines broken, I think it would create an unbalanced load in the neutral even though ithe neutral isn`t connected to it

• if the three phase 3-wire load has one of it`s lines broken, I think it would create an unbalanced load in the neutral even though ithe neutral isn`t connected to it

Think again - there can't be current in the N if there's no N conductor. Loads can either be connected directly between lines, in which case there's just current flowing in two lines if one is broken - potentially with two of the loads connected in series between the two remaining lines. If loads are connected in star without a N connection, then the voltage on the star point will move away from 0V until things equalize (similarly some loads connected in series across lines).

- Andy.

• if the three phase 3-wire load has one of it`s lines broken, I think it would create an unbalanced load in the neutral even though ithe neutral isn`t connected to it

Think again - there can't be current in the N if there's no N conductor. Loads can either be connected directly between lines, in which case there's just current flowing in two lines if one is broken - potentially with two of the loads connected in series between the two remaining lines. If loads are connected in star without a N connection, then the voltage on the star point will move away from 0V until things equalize (similarly some loads connected in series across lines).

- Andy.

Children
• Think again - there can't be current in the N if there's no N conductor.

That's an interesting perspective, but I'm not sure that's always the case either .... if there are single-phase 400 V loads (or broken three-phase 400 V delta loads) connected alongside fully balanced single-phase 230 V loads, and 400 V TP&N loads, in a three-phase 4-wire system, harmonics can still travel in the 4-wire neutral upstream of a 3-wire circuit, and unbalance even in only delta-connected or line-line connected loads could cause neutral current based on volt-drop causing unbalance in the star-connected and single-phase loads.

• Sorry I was trying to say A 3 - phase 4-wire system

• is there such a thing as a 400V single phase load?

• is there such a thing as a 400V single phase load?

I understand some welders are (connect between two lines of a 3-phase plug, as they require more power than 230V single phase can comfortably deliver, but can't easily make use of 3-phase) also some stadium or sports field lighting (where the large distances make 230V a problem with voltage drop).

- Andy.

•    - Andy.

• is there such a thing as a 400V single phase load?

You can't rule it out ... they used to be more common than they are now, and mainly industrial or heating/lighting loads in commercial applications.

•    - Andy.

That would, of course, need a suitable ballast ... but agreed