Mike Dolt Enterprises
Mike Dolt Enterprises
User: Little Bill, Location: Tennessee NEC:2017
Asked on: Thursday, April 21, 2011 at 5:08 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:29 am EDT
Question: 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?
Your observations are generally correct
1. Primary Protection
Transformers are typically rated for a specific voltage and current. The primary side of an autotransformer should be protected at 125% of the transformer's rated current. This ensures that the transformer is adequately protected from overcurrent conditions.
2. Transformer Rating
A 30kVA open delta transformer would typically have a rated current of 144.3 A (30kVA / 208V / √3). 125% of this current is 180.4 A.
3. Conductor Size
The conductor size for the primary side of the transformer should be sized for the protection device rating (180.4 A).
4. Secondary Protection
While the machine may only require 80A protection, the primary side protection must be based on the transformer's rated current. This is to ensure the transformer is protected even if the machine draws less than 80A.
5. Electrical Code Compliance
Table 450.3(B) requires primary protection at 125% of the transformer current.* Using an 80A breaker for the primary side would violate this code requirement.
Conclusion:
Your concerns regarding the primary protection and conductor sizing are valid. The transformer requirements should be followed to ensure the safe and proper operation of the system. The EC should revise the circuit design to comply with the transformer requirements and the applicable electrical code. Flag for review