Re: neutral conductor sizing for 3 phase panelboard
Bob, it is really a design issue and it starts at the entrance. From the MDP I divide the loads up in to either non-sensitive, sensitive, critical, lighting, or environmental loads, and then separate them via feeders to the respective sub-panels. Most everything I deal with is 480/277, but the same principle can be applied to 208/120.
Environmental loads are HVAC, UPS, and rectifiers. I use straight forward 480 3-phase distribution.
Lighting panels get a full size neutral on feeders, and individual neutrals on branch circuits.
Sensitive loads that are not on UPS I take to a K13 transformer and use 150% over-sized neutral on feeders, and use separate neutrals for all branch circuits.
Non sensitive loads which are GP outlets, office PC?s, frigs, microwave, etc I use a K4 transformer and full size neutral on feeders and multi-wire branch circuits. If I know there are going to be a lot of PC?s, printers, etc. I will use an individual neutral on branch circuits or a super-neutral multi-circuit cable. Really depends on budget.
Critical loads are either fed by UPS or DC battery plant. The UPS systems uses a custom PDU?s located close to where power is utilized with separate neutral an all branch circuits. Battery plant distribution is top secret.
I have taken a lot of readings over the years in data centers. I do find a few installations where the neutral current exceeds the phase conductor current. So I would sum it up this way. Always consider single-phase loads to be non-linear and at least use a full size neutral on feeders, at least a K 4 rated transformer to trap harmonics, and at least a full size neutral on multi-wire branch circuits. If you know it is going to be a heavy or all non-linear loads use K 13 transformers, 150% over-sized neutral on feeders, and separate neutrals on branch circuits. Last, get the grounding right or you will shoot yourself in the foot.
