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4 core gets used for transformers that get moved around - 3 phases and the neutral gets sheathed in green/yellow and used as an extra earth because if the tranny gets bashed or the cable pulled and the armouring comes loose from the gland you lose earth, which is no bueno (and the tranny doesn’t need a neutral).
Not sure about not needing a neutral. Balanced loads such as motors don't need neutrals as there is no return current. Site transformers (presumably 110V) aren't going to be balanced across the three phases as each phase has a different load.
It's all about Phasor Diagrams.
Mr_Sworld-
A 3 phase transformer doesn’t have a neutral full stop. And as it’s centre tap, each circuit is a pair of phases (rather than a line and neutral pair), each phase being 63.5V line to earth, which then gives 110v phase-phase. The 8 or so circuits per transformer are split evenly between the phases.
There might be losses, but they’re almost always powered by generators and the loads are relatively small compared to permanent installations. I don’t think it’d be cost effective or realistic to try and do PF correction when the loads are changing so often, and when the installation isn’t even permanent.
But I do love a phasor diagram (not that I’ve ever used one in anger).
Nef
We use 3 core for single phase installations, the cable is glanded and earthed at both ends so it just becomes a parallel path for earth, with your 3 cores as phase, neutral and CPC.
It’s nice to have it in single phase colours because it saves you faffing with tape to sheath them in the correct colours is all.
4 core gets used for transformers that get moved around - 3 phases and the neutral gets sheathed in green/yellow and used as an extra earth because if the tranny gets bashed or the cable pulled and the armouring comes loose from the gland you lose earth, which is no bueno (and the tranny doesn’t need a neutral).
But we regularly use the armour as the sole earthing. Chucked in 90m of 70mm2 x 4 core for a crane supply on Friday 3P+N, . Definitely makes you yearn for twin and earth when you’re wrestling that through a series of bog filled ducts, lol.