Sub panel to load breaker and feeder protection

[Rides Engineer]

I have a load (machine) with a 800A frame breaker set to 650A. The facility has a 1200A sub panel to feed the load, we want to set a 700A trip CB and feed with 700A wire. But the inspector is requiring an 800A breaker and 800 amp wire because the load has a 800A breaker capability.

Is there any code that supports the inspectors requirement.

The issue is the existing conduit will not handle the wire for 800A but can for 700A

 

[charlie b]

The inspector is wrong. You are talking about a feeder, not a branch circuit. Article 215.2(A)(1) says that a feeder shall have an ampacity not less than required to supply the load as calculated . . . . However you calculate the load, you are not gointg to come with an answer of 800 amps. You have a load that is below 650 amps.

Welcome to the forum.

 

[Rides Engineer]

Thanks Charlie

Thanks Charlie

Thank you, I thought I was missing something.

The initial design had undefined loads, so we assumed worst case and built the cabinet with a 800A adjustable trip CB. The calculated FLA is 525A, so we set the trip at 650A. The frame would still be 800A, but we could have used a 700A trip unit instead of 800A.

The hang up is the feed to the cabinet is from an existing service and the facility does not want to have to install larger conduit if it can be avoided with a 700A CB on that end protecting the feeders.