Breaker up or down

[resistance]

I seen an HVAC unit (Combination type). The unit requires an MCA of 44.9 amps [fuse or breaker @ 45 amps). No MOCD is provided (Meaning the name plate has it listed, but left it blank). Are you allowed to down size the OCP to 40A. Or can it be upsized to 50. Note: They sell a 45 amp breaker, but it seems the original installer installed a 40A breaker instead. Obviously (to me) the 45 is hard to come-by. Your input?

 

[normbac]

i always use the mca to size wire The cb is always larger Check their website specs so you dont void warranty

 

[ActionDave]

440.4 says MCA and MOCP are supposed to be marked. I think contacting the manufacturer is the next step.

 

[resistance]

the unit is 12 years old, so it’s already out of warranty.
@Action,
Exactly, I’m also aware of that section.

I guess my question is, can the breaker be downsized? I would think that calculations have already been done—seeing that this is a combination unit (Has a heater, compressor, fan). So, at least we know that the minimum size has to be 44.9 amps.

 

[resistance]

I believe there is a way to calculate the MOCPD

 

[ActionDave]

Aha, so now we get the full story.

Sure, you can always go down in you breaker size.
When it comes to calculating MOCP I also believe there is a method to do it the hard way. I am sure I have not tried it since I studied for my license.

 

[kwired]

Aha, so now we get the full story.

Sure, you can always go down in you breaker size.
When it comes to calculating MOCP I also believe there is a method to do it the hard way. I am sure I have not tried it since I studied for my license.

Not that hard. We do pretty much the same thing with art 430 motor loads. Difference is with hermatic compressors there normally is no rated HP. You have to use the rated load in place of full load amps.

With air conditioning equipment those calculations are usually done for you and the results is what is marked as MCA and MOCP on the name plate. If you go through and do the calculations yourself you will come up with same results as marked.

If for some reason you do not have these values you need to take a look at 440.22 for short circuit and ground fault protection and 440.32, 440.33, 440.34 or 440.35 for conductor sizing. It is very similar to same methods used for calculating same thing for motor circuits.

I will add that if the MCA is 44 amps and the majority of load is a compressor the overcurrent device typically is almost double the MCA minimum to allow trip free starting of the compressor. If you have resistive heating load on same circuit it possibly is the majority of load and will change the resulting calculations.

 

[Strife]

I seen an HVAC unit (Combination type). The unit requires an MCA of 44.9 amps [fuse or breaker @ 45 amps). No MOCD is provided (Meaning the name plate has it listed, but left it blank). Are you allowed to down size the OCP to 40A. Or can it be upsized to 50. Note: They sell a 45 amp breaker, but it seems the original installer installed a 40A breaker instead. Obviously (to me) the 45 is hard to come-by. Your input?

MCA is for wire size.
MOCP is for breaker/fuse size.
If MCA is 44.9, I'm pretty sure the MOCP is AT LEAST 50A. Most likely 60A
You can downsize the breaker to 40A, but with an MCA of 45 I think you'll have problems with breaker tripping.

 

[stickboy1375]

I seen an HVAC unit (Combination type). The unit requires an MCA of 44.9 amps [fuse or breaker @ 45 amps). No MOCD is provided (Meaning the name plate has it listed, but left it blank). Are you allowed to down size the OCP to 40A. Or can it be upsized to 50. Note: They sell a 45 amp breaker, but it seems the original installer installed a 40A breaker instead. Obviously (to me) the 45 is hard to come-by. Your input?

You size down, if the unit name has a MCA of 44.9 amps, you can just multiply that number by 1.75 to give you the MAX. breaker size, 70 amps... if that does not hold when the unit starts, you can multiply by 2.25, which would be a 100 amp breaker...

 

[kwired]

You size down, if the unit name has a MCA of 44.9 amps, you can just multiply that number by 1.75 to give you the MAX. breaker size, 70 amps... if that does not hold when the unit starts, you can multiply by 2.25, which would be a 100 amp breaker...

You go to 440.22 and follow the appropriate subsection. In most cases the compressor is the largest load on the circuit and you follow subsection (B)(1). That section says to multiply compressor rated current by 175% and add remaining rated currents of other loads to come up with OCP value. MCA is for determining conductor size not overcurrent devices.

 

[stickboy1375]

You go to 440.22 and follow the appropriate subsection. In most cases the compressor is the largest load on the circuit and you follow subsection (B)(1). That section says to multiply compressor rated current by 175% and add remaining rated currents of other loads to come up with OCP value. MCA is for determining conductor size not overcurrent devices.

I can use the motor-compressor rated load current OR branch circuit selection current to add my 175% for short circuit and ground fault protection, I bet you I am right there with my numbers.

 

[kwired]

I can use the motor-compressor rated load current OR branch circuit selection current to add my 175% for short circuit and ground fault protection, I bet you I am right there with my numbers.

For typical A/C condensor unit with single compressor and single fan you may be close, as the fan is probably only about 1 amp max. The value of MCA already contains 125% of compressor rating so you will be high no matter what.

 

[david luchini]

I can use the motor-compressor rated load current OR branch circuit selection current to add my 175% for short circuit and ground fault protection, I bet you I am right there with my numbers.

But in this case, we are given neither the rated load current NOR the branch circuit selection current. It would be impossible to determine the MOCP without the rated load current or branch circuit selection current of the compressor along the the full load current of the other motors in the unit.

 

[stickboy1375]

But in this case, we are given neither the rated load current NOR the branch circuit selection current. It would be impossible to determine the MOCP without the rated load current or branch circuit selection current of the compressor along the the full load current of the other motors in the unit.

Look, we have the MCA of 44.9 amps, that number isn't magical and just appear, its 125% of the rated load current.... and lets be honest, I'm willing to bet the farm the sub cooling motor is under 2 amps..

 

[david luchini]

Look, we have the MCA of 44.9 amps, that number isn't magical and just appear, its 125% of the rated load current.... and lets be honest, I'm willing to bet the farm the sub cooling motor is under 2 amps..

The MCA is 125% of the rated load current PLUS the full load currents of the other loads in the unit. The other loads may be less than 2 amps, but there is no way to tell from the information given.

Could be a 20A compressor and 19.9A of other load to give an MCA of 44.9 and an MOCP of 50.
Or it could be a 35A compressor and 1.15A of other load to give and MCA of 44.9 and an MOCP of 60.

There's no way to tell with the information given.

 

[stickboy1375]

The MCA is 125% of the rated load current PLUS the full load currents of the other loads in the unit. The other loads may be less than 2 amps, but there is no way to tell from the information given.

Could be a 20A compressor and 19.9A of other load to give an MCA of 44.9 and an MOCP of 50.
Or it could be a 35A compressor and 1.15A of other load to give and MCA of 44.9 and an MOCP of 60.

There's no way to tell with the information given.

I know, but.... I'm still betting I'm right in my assumption.

 

[suemarkp]

Resistance - you should post a link to the unit nameplate values so they can see how the manufacturer provided values for this one and other units. I see almost no increase for MOCP -vs- MCA other than rounding up to the next standard breaker size.

This is a combination compressor, heater, fan unit with the following values:
Compressor RLA 12.8
Outside Fan 0.9A
Inside Fan 2.0A
5KW Heat 20.8 FLA

Summation MCA is 44.9

 

[stickboy1375]

Resistance - you should post a link to the unit nameplate values so they can see how the manufacturer provided values for this one and other units. I see almost no increase for MOCP -vs- MCA other than rounding up to the next standard breaker size.

This is a combination compressor, heater, fan unit with the following values:
Compressor RLA 12.8
Outside Fan 0.9A
Inside Fan 2.0A
5KW Heat 20.8 FLA

Summation MCA is 44.9

UGH.... i read right over the "combination" part of the original post... :slaphead:

 

[resistance]

Note: this is a combination unit, so all electrical equipment will be sized per nameplate. I will be going to see the unit personally. I will report back on Weds..
Note:I had a guy out looking at it (This is why I said I seen), so I’m reporting what he told me. He took pictures, but some information is missing. I also had a spec sheet emailed to me, but I will go take a look at the nameplate myself.

 

[resistance]

Resistance - you should post a link to the unit nameplate values so they can see how the manufacturer provided values for this one and other units. I see almost no increase for MOCP -vs- MCA other than rounding up to the next standard breaker size.

This is a combination compressor, heater, fan unit with the following values:
Compressor RLA 12.8
Outside Fan 0.9A
Inside Fan 2.0A
5KW Heat 20.8 FLA

Summation MCA is 44.9

The question becomes, are you allowed to round up from 45 amps, if a 45A breaker is available per NEC 240.6? Note: Keeping mind this is a combination unit