- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
We are having a circuit run for a new machine. The machine requires 230V 3pf and we only have 208 3ph. The EC (his crew) is installing a field installed autotransformer, using 2 transformers. They will be fed from a panel right next to them, and then feed straight to the machine. They planned on using an 80A breaker for OCP. This was based on info that was given them before the machine arrived. They had wired another smaller machine we have a few years back and was given the same specs as was used then. I was looking at the wiring diagram and requirements for the 208V-230V configuration. It said to fuse (or breaker) the primary side at 125V. I believe the paper said under this set up it would be 30KVA open Delta. I know from the code in Table 450.3(B) that the primary protection should be 125% of the transformer rated current. I'm not very good with transformers but I know that 125% is more than the 80A they were planning on using for OCP. I kindly pointed out to the lead man the info on the wiring diagram, and if that is correct, then the conductor size would also need to be sized accordingly. I don't think I made him very happy or at least his boss. He called his boss and told him what I showed him and his boss said well the machine only needs 80A protection. That would be true had we not had to use a transformer to get that voltage. I didn't tell him he had to do anything nor did I argue with him. I told him I was simply pointing out what is on the transformer requirements. Does what I pointed out sound correct or am I way off base?