In design stage of the larger installation, I think, it is quite difficult to decide which (three-phase) distribution circuit has third harmonic current.
But BS7671 523.6.3 says that if the total harmonic distortion due to third harmonic current or multiple of third harmonic is greater than 15% of the fundamental line current, the neutral conductor shall not be smaller than the line conductor.
I imagine that the sub-main to a distribution board which supply many articles of elecronic equipment has the third harmonic current.
But how do you estimate that the circuit has third harmonic being greater than, for example, 15% of the line conductor.
I think there must be an established method or a traditonal practice or a rule of thumb that a few like me don’t know.
I am very grateful if someone tell me what phase is appropriate to design the distribution circuits containing 3rd harmononic current and how to estimate 3rd harmonic content of line current in a design stage, if any.
