Mike Dolt Enterprises
Mike Dolt Enterprises
User: buzzr, Location: Tennessee
Asked on: Saturday, April 2, 2011 at 1:54 am EDT
Answered on: Tuesday, May 14, 2024 at 3:39 am EDT
Question: 480vac1ph primary w/150amp CB. 120/208 vac secondary feeding 10 ea 20amp GFCI receptacles & 4ea 2/50amp power receptacles.I know NEC 240-4 tells me I need to have the Secondary OCP (which the Manufacturer neglected to install) So can I install a fuse block in the secondary between the Transformer and the panel lugs. or do I have to install a fusable disconnect switch.If I decide to install a circuit breaker and back feed the panel can I use a 150A breaker.thanks
Secondary Overcurrent Protection (OCP)
According to NEC 240.4, the secondary of a transformer must have OCP.
Options for Secondary OCP Fuse block
You can install a fuse block in the secondary to provide OCP.
Fusable disconnect switch
You can also install a fusable disconnect switch.
Breaker Backfeeding
You cannot backfeed a panel with a breaker. A breaker is designed to interrupt current flow in only one direction. To backfeed the panel, you must use a double-throw breaker or a transfer switch.
Breaker Size
If you decide to install a breaker, you must use a breaker with an ampacity equal to or greater than the secondary OCP. In this case, you would need to install a 150A breaker if you are using a 150A fuse block or fusable disconnect switch.
Recommendation
The best practice is to install a fusable disconnect switch in the secondary. This will provide both OCP and a means of isolating the transformer for maintenance.
Note: It is important to verify the manufacturer's specifications for the transformer to determine the maximum overcurrent protection that is allowed. Flag for review