What size OCP SCP and conductors to feed fused disconnect

[milwaukeesteve]

I have a question in relation to DOUBLE FUSING but a different question and different topic, so I started a new thread.

If I have a HACR load, with a fused disconnect, what size OCPD and Short Circuit Protection do I install for the circuit feeder conductors that feed the fused disconnect.

Example:
I have a A/C unit outside
max fuse size 40AMPs
Min circuit Ampacity 19Amps
Fused disconnect with 40AMP Time Delay Fuses
Conductors from disconnect to A/C unit are #12s

The above is legal as is from disconnect to unit.

Now my question is the 'feeders' that feed the disconnect. IF they are from a source that cannot have HACR rating (ie. Zinsco, FPE, Pushmatic), what size breaker do you need, what size can you install, what size conductors minimum?

If the Minimum circuit ampacity is 19 Amps, the conductors should never see a running load of more than 19Amps (here we are not worried about in rush current at startup). We should be able to keep #12s as our conductors.

What size breaker (Non inverse time breaker) is needed to allow for inrush current at startup? If I focus on the conductors and only install 20 Amp breakers, the breaker will trip. Am I allowed to install a 30, 40 or even a 100 Amp breaker supplying the 'feeder' conductors, since the Fuses protect the conductors on Overcurrent?

Is it fine to install a 30, 40 or 100Amp Breaker? Now what size conductors? Again I say only #12s.

Any thoughts?


Editted with additional thought: I don't know if any of those breaker manufactures listed above do make HACR rated breakers. I have never seen one. If so, please let me know, too. There may be more also, those were off the top of my head.

 

[hillbilly]

The post by starbright (within the last few days) is very helpful.
steve

 

[infinity]

The unit is marked so that you can use fuses or an HACR breaker. Since you have fuses the HACR is not required. The CB can be any size between the minimum and maximum listed on the unit. I would use a 40 amp.

 

[LarryFine]

The unit is marked so that you can use fuses or an HACR breaker.

I disagree:

max fuse size 40AMPs

That means fuse only.

The "feeder" is not a feeder, it's a branch circuit with supplemental OCP. The #12 on a 40a breaker is correct.



Usual disclaimer: Of course, if someone proves me wrong, then never mind.

 

[George Stolz]

Usual disclaimer: Of course, if someone proves me wrong, then never mind.

LMAO

Good one, Larry.

 

[infinity]

I disagree:

That means fuse only.

The "feeder" is not a feeder, it's a branch circuit with supplemental OCP. The #12 on a 40a breaker is correct.



Usual disclaimer: Of course, if someone proves me wrong, then never mind.

So how do I install fuses in my 100 amp Sq D panel?

 

[LarryFine]

So how do I install fuses in my 100 amp Sq D panel?

You can't, obviously. You can use a fusible disconnect. If you really wanted to, you could mount a fused switch next to the breaker panel, but since a disco is required at the unit, why not place the fuses there?