RTU Overcurrent protection

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Wiscoz

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Electrician
I'm working a job that has 4 rooftop units. The one line calls for a 225 amp breaker for each. With the conductors sized for 150 amps (1/0 conductors). The units have a maximum overcurret protection device listed at 125 amps. They than call for a fusible disconnect with 125amp fuses. My understanding is that the conductors between the breaker and the disconnect would have to be rated for the full 225 amps. The engineer is stating that they want this installation In case in the future larger units are installed. Is there anything that would permit the conductors between the 2 overcurrent devices to be sized like this?
 
Welcome to the forum.

Read NEC Art. 240.4(G) and its reference to Art. 440.

Added: I believe you're questioning whether the conductors between the breakers and the disconnects are feeders or branch circuit conductors. It has been thoroughly debated here, and I'm not sure we ever came to a consensus.

As I said, welcome to the forum. :sneaky:
 
What is the conductor length between the 225 amp OCPD and the fused disconnect? Could a tap rule apply?
 
It is over 150' i do not see the tap rule Applying. The fused disconnects are on the roof with the units.
 
That's what I can't stop thinking. The minimum circuit current of the units are 100 amps, They are multiple compressor units. Listed with 125 MOCPD. I just don't see how we could even apply any previsions to increase the breaker size to begin with. Let alone have it oversized for future with undersized conductors.
 
If the MaxOCPD were 225 amps then you would be fine but since it's 125 amps I don't see how the conductors between the 225 and the 125 amp OCPD can be only #1/0. If the conductors were solely on the outside of the building then the 150' would not be an issue with the tap rules.
 
Yeah I'm just trying to think of something that would allow this but I can't. The conductors are ran through the building and Penetrate through the curb of the RTU'S.
 
The conductors between the breaker and disconnect meet the definition of a feeder and must follow 215.3.
  1. Change the 225A breaker to 150A and you have met article 215 requirements for the feeder.
  2. Change the 150A rated wire to 225A rated wire and you have met article 215 requirements for the feeder.
  3. OR you can change the disco to a non fused and install a 125A rated breaker because article 215 goes away.
  4. OR install a 125A rated breaker and leave the fused disco.
I see no other code compliant options.
 
LarryFine said:
Welcome to the forum.

Read NEC Art. 240.4(G) and its reference to Art. 440.

Added: I believe you're questioning whether the conductors between the breakers and the disconnects are feeders or branch circuit conductors. It has been thoroughly debated here, and I'm not sure we ever came to a consensus.

As I said, welcome to the forum. :sneaky:
Click to expand...
The code is pretty clear about what constitutes a feeder circuit and what constitutes a branch circuit. It is pretty unequivocal that The circuit going to the fuse disconnect is a feeder circuit. We don't know what's after the fused disconnect so it could be a feeder circuit downstream of the fuse disconnect too.

I don't see any way to make this compliant using 1/0 conductors between the circuit breaker and the fused disconnect unless you used two of them per phase.
 
Thinking some more, you could put the fusible disco "tap-rules" close to the 225amp breaker and an unfused disco at the RTU for maintenance; that would allow the smaller wire for the long run. It also seems like a daft idea.
 
The RTU'S have built in disconnects, the easiest and best way in my opinion would be to change the breakers. I am just getting conflicting information on how this current setup would be code acceptable.
 
Wiscoz said:
The RTU'S have built in disconnects, the easiest and best way in my opinion would be to change the breakers. I am just getting conflicting information on how this current setup would be code acceptable.
Click to expand...
I think that if someone reads this thread they will see that everyone agrees, the current setup of 225 amp OCPD and #1/0 conductors is not code complaint. Your suggestion of changing the 225 to a 150 will make it code complaint.
 
Nuber said:
The conductors between the breaker and disconnect meet the definition of a feeder and must follow 215.3.
  1. Change the 225A breaker to 150A and you have met article 215 requirements for the feeder.
  2. Change the 150A rated wire to 225A rated wire and you have met article 215 requirements for the feeder.
  3. OR you can change the disco to a non fused and install a 125A rated breaker because article 215 goes away.
  4. OR install a 125A rated breaker and leave the fused disco.
I see no other code compliant options.
Click to expand...
Nice
 
zbang said:
Cannot do that if the RTU spec's fusible, and there lies the problem.
Click to expand...

Agreed - although in my experience most modern HVAC units are not limited to fuses only by their labeling. A good thing to check.
 
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