protective circuit-breaker

[RR449A]

Dear members, I have a question for which I would like to Know your opinion.

I would like to mention that I am not an electrician by trade.

I am wondering when we are setting the protective circuit-breaker for a compressor, which current should we take into account? Is it MCC, RLA, or the maximum operating amperage which corresponds with envelope?

[pecmsg]

MCC

[RR449A]

I've read this at Copelan's website:

RLA is used by an electrical systems designer to size compressor contactors, breakers and wire size. Increasing the factor F will decrease the RLA value for a given MCC. It should be noted that since the MCC is refrigerant and application specific, the RLA and the selections of components based on RLA are also refrigerant and application specific. For purposes of calculating RLA to size breakers and wire sizes, it is acceptable to use the UL recommended value of 1.56, taking into account appropriate factors of safety over the component rating. RLA = MCC/F

What does this mean? Do they say that we should size (set it up) by RLA?

[crazzycajun]

If I’m reading this correctly they are saying that mcc is derived from rla x 1.56. Which is the formula at the end of the paragraph in reverse. Like I said that’s how I read it. Time builder or some one else will be along and confirm or deny my interpretation. I know your new to this site, but if Tb chimes in I would treat his knowledge as the gospel when it comes to electric.

[icemeister]

Copeland's Electrical Handbook has a section defining fuse sizing:

Copeland Fuse Sizing.pdf

[timebuilder]

In order to answer I need to know if you are designing a system, or installing a system that has been manufactured by others. The Copland information is for designers.

If you are an installer, NONE of that is relevant to you.

If you are installing equipment, there are only two things you need to know, from the equipment label:

1) The Minimum Circuit Ampacity. This is the current value you take to the appropriate NEC table to determine conductor size. Since this is an open forum, that's as far as I am going.

and

2) The Maximum Over Current Protection Device value. That value tells you the maximum fuse or breaker size to be used with the aforementioned conductors.

In this area of equipment, the breaker DOES NOT protect the conductors. That protection is provided by the internal overloads of the motors.

The breaker instead protects against ground faults and short circuit currents.

This is from NEC Article 440.

NOTE:

This IS NOT THE SAME as choosing conductors and breakers for "lighting and general use." Article 440 supersedes all of what non-electricians think they know about the electrical trade.

So...are you designing a system, or are you installing a system?

[RR449A]

I m not installer, I don't install Products myself, others do it for me.
I decide which condensing units + evaporators are going to be installed.
I also decide which electrical panels which contain circuit breakers and contactors are going to be used.

Since I am the one who could have problems if the compressor burns out, I would like to Know why are some people telling me that the circuit breaker should be set by MCC, when this current is nowhere to be found in the operating envelope.

Bitzer says:

The thermal protection system will not allow the compressor to run higher than the amperage value that is referred to as Maximum Continuous Current (MCC)

MOA should be taken in to consideration when sizing electrical components including inverters.

That makes sense to me since MOA is what we find in the operating envelope.

But, then again, Bitzer also says this:

Nuisance tripping of the circuit breaker can occur if sizing strictly on RLA. For this reason, the MOA should be considered when sizing the circuit breakers. For example, the selection should be at least 110% of MOA or even up to 125% of MOA (when possible based on applicable standards or codes).

I am totaly confused.

P.S. I am not allowed to post links, therefore, I cannot put a link to a quoted document.

[icemeister]

...I decide which condensing units + evaporators are going to be installed...

The system components you select should have the information you're looking for listed on their data plate. For example, Copeland's condensing units usually include "Max Fuse" as the Maxiumum Overcurrent Protection Device which timebuilder referred to above.

[pecmsg]

Emerson’s site / OPI (apps) has all the #’s you need

[RR449A]

Thanks. But, still, I would like to Know why is the breaker sized by MCC when we are pushing the compressor out of envelope limits.

[pecmsg]

How are you pushing out if it’s limits?

[RR449A]

If MCC is the current under which the thermal Protection activates, what use do we have from breaker?

Aren't the limits defined by envelope?

[pecmsg]

The two are not the same.

[pecmsg]

MCC insures enough current is available to start without neusence trips.

[RR449A]

What do you mean?

[pecmsg]

The unit has a MCC size. Your not allowed to exceed that.
The thermal overload is to protect the compressor from excessive heat, not Amperage. You can install a smaller breaker and chance Nuisance trips but WHY?

[timebuilder]

I m not installer, I don't install Products myself, others do it for me.
I decide which condensing units + evaporators are going to be installed.
I also decide which electrical panels which contain circuit breakers and contactors are going to be used.

Since I am the one who could have problems if the compressor burns out, I would like to Know why are some people telling me that the circuit breaker should be set by MCC, when this current is nowhere to be found in the operating envelope.

Bitzer says:





That makes sense to me since MOA is what we find in the operating envelope.

But, then again, Bitzer also says this:



I am totaly confused.

P.S. I am not allowed to post links, therefore, I cannot put a link to a quoted document.

From what I am able to surmise, you are directing the installation of manufactured equipment.

The information I talked about is on the label.

That is the ONLY information you can use and be in code compliance.

Have your men size the co0nductors and the protection according to what I explained. Any good electrician will know what to do.

If you do not know which table to use in the NEC, you must consult with someone who does, and not post questions on line. Clearly, you are out of your area of experience, so you must access that experience locally. We cannot provide step by step help to people in the open forums, because they are indexed by search engines.

Article 440 is well explained in a youtube video by Mike Holt. He will tell you the same things that I have.

[icemeister]

If MCC is the current under which the thermal Protection activates, what use do we have from breaker?

Aren't the limits defined by envelope?

The operating envelope defines the limits of normal operation as determined by the compressor manufacturer.

The compressor's thermal/overload protection is designed to protect the motor from over-current and high internal temperatures.

The circuit breaker or fuse is for over-current and short circuit protection.

[timebuilder]

I just caught that you are installing Bitzer equipment.

They have all of the answers you need.

[idontgetit]

There is a difference in "overload protection" and "overcurrent protection" despite it being somewhat common for folks to use them interchangeably.

There are a lot of things needed to be fully understood before trying to design electrical and mechanical systems to work together.

One example, if your name plate says Max Fuse Size, you are required to use fuse protection not breakers it's not just an amp rating, it is indeed specific to using fuses only.