Transformer wiring

[MAXIWANG]

I was sizing the wires for transformer by using the data sheet. Then I saw the things like this:

Three Phase 600-120/208v
30Kva, Primary 3#8-16mmC, Secondary 4#2+GND-35mmC.
Single Phase 600-120/240v
25Kva, Primary 2#6-21mmC, Secondary 3#1-41mmC.

My questions are
1. why we need grounding for the secondary of three phase transformer but not for primary?
2. why we do not need any grounding for single phase 600-120/240v?
thank you in advance.

 

[waross]

Three phase;
Three phase wires plus neutral. The neutral should be grounded. If the load is all three phase then the neutral conductor is not needed as a neutral but is still required for grounding.
Single phase. Two line wires plus neutral. The neutral should be grounded. If the load is all 240 V then the neutral conductor is not needed as a neutral but is still required for grounding.
Whoever listed the information was inconsistent in his use of the word "Ground".
yours

 

[dwftx34]

Waross this high lights a problem that the NEC is trying to correct. The confussing use of ground,grounded and neutral the 3-phase system really by definition does not have a neutral only the single phase 240/120 has a neutral conductor. all that said waross is once again correct in that the secondary side of your transformers needs to have a grounded/neutral conductor. This grounded conductor serves two proposes 1. to provide a ground fault path back to the source to insure overcurrent/ fault current devices operate. 2.voltage balancing of the secondary if imbalance loads are used (120v to ground). The primary being delta does not have a neutral or grounded conductor but should have a grounding conductor to a common equipment grounding point.