I have an existing 1200A circuit breaker for a chiller. New chiller is 700 MOCP. Can I use the existing 1200A breaker and provide a fused disconnect switch with 700A fuses with 2 sets of 500kcmil? Or change out the breaker? Code reference please.
Chiller MOCP and Conductor sizing
- Thread starter Dwight W
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
How close will the new disconnect be to the existing 1200 breaker? Tap rules will apply if your 2 set of 500's are protected at 1200 amps.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Don't most 1200a breakers have adjustable trips units, for the past 30 years?
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
My thoughts exactly. Even before that, it might have a rating plug that can be changed. A disconnect switch and fuses that size is not going to be insignificant in both cost and physical size.
Last edited by a moderator:
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
Sure. There is no code reference, you have complied with the 700 max OCPD required. Now as infinity said, if you come off the 1200A with less than 1200 of conductor, that is a tap and would need to meet one of the tap rules in 240.21(B)
There are three breakers in question for three chillers. Two, in a main switchboard, we can change the trip unit and one in a QED panelboard is fixed without trips or adjustments. The disconnects will be outside by the unit with underground feeders 35 to 40 feet from the source.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Since your conductors are completely outdoors the unlimited length tap rule would apply. You can come off of the 1200 amp OCPD with two sets of 500's and terminate in a switch with 700 amp fuses.