AC condenser Name Plate Rating Calculations

[Pierre C Belarge]

This does not make sense to me.

For whatever reason I use a fused disconnect (fused meaning fuse or circuit breaker), it is the last overcurrent device between the conductor and the utilization equipment. As per the definition (which doesn't say where the OCPD has to be located - in panel, outside, inside, etc...) the conductors between the panel to the fused disconnect are feeders, and the conductors between the fused disconnect and the A/C unit are branch circuits. I believe this would have an effect on the sizing of the conductors in both locations.


"Branch circuit - the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)."

In the example we are discussing, the circuit actually starts at the fused disconnect, and the conductors supplying the fused disconnect are feeders.


This is similar to the discussion of tap conductors and feeders involving transformers. I do not agree with the CMP's decision on the proposal by Mr Dollard in regards to this, but the difference is there is more distinction here, the two definitions do not overlap as in the tap and feeder definitions.

 

[peter d]

Pierre,

Don has convinced me also. The fusible disconnect is simply supplementary overcurrent protection. Nothing more, nothing less.

 

[don_resqcapt19]

Pierre,
The fuses at the unit do not protect the circuit as they are not required by a code rule to protect the circuit between the branch circuit OCPD and the fuses. They are only there to provide a specific protection for the equipment. If the equipment did not say fuse only, the disconnect would be a non fusible one. The requirements of the equipment for fuse protection of the equipment does not change the circuit from a branch circuit to a feeder.
Don

 

[iwire]

For whatever reason I use a fused disconnect (fused meaning fuse or circuit breaker), it is the last overcurrent device between the conductor and the utilization equipment.

I agree it is the last over current device between the conductor and the equipment.

As per the definition (which doesn't say where the OCPD has to be located - in panel, outside, inside, etc...) the conductors between the panel to the fused disconnect are feeders, and the conductors between the fused disconnect and the A/C unit are branch circuits.

That is not the entire definition you are (like I was) focusing on ' the final overcurrent device'.

You are not including the last part of the definition.

'protecting the circuit and the outlet(s).'

This fused disconect is not protecting the circuit or the outlet. It is protecting the equipment per UL listing and labeling.

Now lets think about what makes sense.

We have an HVAC unit that has an MCA of 30 and a fuse size of 60.

If we feed this from a fused switchgear we can run 10 AWG for the whole circuit using 60 amp fuses at the switchgear and have a non fused disconnecting means at the unit.

Now you say if I happen to have installed a fused disconect I would have to run 6 AWG from switchgear to disconect than I can run 10 AWG from there to the unit.

That does not make any sense to me.

The main point in my mind is that disconect at the unit is not ever required by the NEC to 'protect the circuit and the outlet(s).

That job is completed at the panel.

In the example we are discussing, the circuit actually starts at the fused disconnect, and the conductors supplying the fused disconnect are feeders.

I can not agree with that above for the reasons I gave above.

I do not see the connection to the transformer conductors.

Bob

 

[Pierre C Belarge]

Just for clarification: the fused disconnect I am talking about is not part of the A/C unit, it is the disconnect typically mounted at the building, with conductors installed from it to the A/C unit.

Lets say the A/C unit does not require a fused disconnect.

I install the 6 AWG to the disconnect, of which I install a circuit breaker type, not nonfused.
From this circuit breaker type disconnect, I can install the 10 AWG conductors as per the example given in this thread. What is wrong with this?

 

[don_resqcapt19]

Pierre,

I install the 6 AWG to the disconnect, of which I install a circuit breaker type, not nonfused.
From this circuit breaker type disconnect, I can install the 10 AWG conductors as per the example given in this thread. What is wrong with this?[/quote\
In the original post the minimum circuit ampacity is 25 amps. This would permit the use of #12 wire from the branch circuit OCPD to the unit. There is no reason to install wire larger than that between the panel OCPD and the unit disconnect, even where the unit disconnect is a fuse. The fuse is not the branch circuit OCPD, it is only there to protect the equipment.
Why would you want to install a circuit breaker at the unit? What purpose would it serve?
Don

 

[Pierre C Belarge]

Don
I would not install the breaker type disconnect, I am trying to understand the difference of why this would not be a feeder/branch circuit type of installation based on the definitions if the disconnect had some type of OCPD.

 

[benaround]

Pierre,

Compare it to the branch circuit to a parking lot pole light, and at the

pole inline fuses are installed. It's still a branch circuit feeding the poles.

Think of it this way, the motor has overloads that limit the current on

the circuit, so the branch breaker is really for short circuit or fault current.

The starting current of the motor requires the larger breaker at the panel

and that is only for a few seconds,if by chance it takes longer the motor

overloads or the fuses in the disconnect will open.

As said before this is Code and Code is minimum.

 

[iwire]

I am trying to understand the difference of why this would not be a feeder/branch circuit type of installation based on the definitions if the disconnect had some type of OCPD.

Per the definitions you are referring to the OCPD at the unit is not protecting the conductors of the circuit.

I know you have read this but I don't think you have been reading it.

Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

An OCPD installed at the unit is not protecting the circuit, it is if anything protecting the equipment.

The circuit is protected from short circuits and ground faults by the OCPD at the panel, the circuit is protected from overload by the OLs in the unit.

Bob

 

[George Stolz]

One other section that supports this reasoning is 240.10. The fuses in the disconnect would be protecting the appliance as supplementary overcurrent protection, as described in that section.

 

[briselec]

An OCPD installed at the unit is not protecting the circuit, it is if anything protecting the equipment.

As a point of interest, here in Australia an overload in a start/stop station installed next to a motor is regarded as the OCPD for the circuit with the breaker in the panel doing the job of short circuit protection only. That is the code compliant way you are meant to do it here.

 

[benaround]

briselec,

I recently worked with an Austrialian electrician an he told me that the

inspection process has been eliminated, and is now a 'self inspection'

type of deal, Is there truth in this??

 

[briselec]

briselec,
I recently worked with an Austrialian electrician an he told me that the
inspection process has been eliminated, and is now a 'self inspection'
type of deal, Is there truth in this??

It differs from state to state but here in queensland they supposedly inspect a randomly chosen 10% of new installations.
I haven't had a job inspected in years.

 

[LarryFine]

I'm movin' down unda!

Who's with me?

 

[tincan44]

It differs from state to state but here in queensland they supposedly inspect a randomly chosen 10% of new installations.
I haven't had a job inspected in years.

HOLY COW :shock:
I wonder what the statistics are there for sloppy dangerous work. Around here it seems like 1 'electrician' in 10 can't read, 8 in 10 don't own a code book, some spend their days waiting for "beer 30" and pay-day-Friday. But EVERY permitted job gets inspected for minimum code compliance.

 

[briselec]

I wonder what the statistics are there for sloppy dangerous work.

Who knows! It's near impossible to tell. All I know is that I haven't struck any so far that was done by an electrician. There is a lot of onus on the contractor to make sure jobs are done to the rules and they will jump on you hard if they find otherwise. The last boss I worked for had his contractors license cancelled and would find it difficult to get one again.