# Disconnect sizing for chiller

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#### acolella

##### Member
I am trying to get to the bottom of a discussion about the proper fused disconnect size that is needed for a chiller.

The MCA is 316.3 amps with an MOCP of 400 amps. The unit has 2 compressors with RLA of 119 amps each and 8 condenser fans with RLA of 5.4 amps each.

Total RLA is 281.2 amps. If I understand correctly, minimum disconnect size shall be not less than 115% of RLA which would be 323.38 amps.

I would think that a 400 amp disconnect would suffice but I have heard that a fused disconnect can only be utilized up to 80% of its rating for continuous loads. Is that correct?

Can I use a 400 amp fused disconnect in this application or do I need a 600 amp disconnect?

Also, I have a question about the MCA. Compressor 1 = 119 amps. Compressor 2 = 119 amps. 8 condenser fans = 43.2 amps total. I would think I calculate it as such. Largest load times 1.25 plus all other loads.

119 x 1.25 = 148.75

148.75 + 119 + 43.2 = 310.95

How was the 316.3 amp MCA determined?

Thanks!

#### paulengr

##### Senior Member
The rules for a group motor setup are not the sum of FLC times 125%. Read article 430 more closely.

Also as an assembly you do not look “inside the box” for MCA. The chiller has a design utilization that is less than name plate FLA. Thus it exceeds name plate when loaded vs unloaded. Thus MCA reflects not only this but the mechanical load irrespective of the installed motor: The way it is determined is to build one and measure average current on a test stand. In the same way a motor is on a test stand with a dyno while measuring temperature.

As to disconnects a fused disconnect uses name plate rating. Non-fused you derate. This is to accommodate the 125% factor used for OCPDs.

#### kwired

##### Electron manager
Group motors is one thing, one listed assembly with MCA and MOCP on the nameplate is another. Plus this is not only something with motor loads it is also an Art 440 application.

#### kwired

##### Electron manager
I am trying to get to the bottom of a discussion about the proper fused disconnect size that is needed for a chiller.

The MCA is 316.3 amps with an MOCP of 400 amps. The unit has 2 compressors with RLA of 119 amps each and 8 condenser fans with RLA of 5.4 amps each.

Total RLA is 281.2 amps. If I understand correctly, minimum disconnect size shall be not less than 115% of RLA which would be 323.38 amps.

I would think that a 400 amp disconnect would suffice but I have heard that a fused disconnect can only be utilized up to 80% of its rating for continuous loads. Is that correct?

Can I use a 400 amp fused disconnect in this application or do I need a 600 amp disconnect?

Also, I have a question about the MCA. Compressor 1 = 119 amps. Compressor 2 = 119 amps. 8 condenser fans = 43.2 amps total. I would think I calculate it as such. Largest load times 1.25 plus all other loads.

119 x 1.25 = 148.75

148.75 + 119 + 43.2 = 310.95

How was the 316.3 amp MCA determined?

Thanks!

328.38 amps is not the continuous load it is 115% of the continuous load. From what you mentioned the continuous load is 281.2 which is less than 80% of 400.

#### infinity

##### Moderator
Staff member
Good news, when the unit has a nameplate that states the MCA and the MaxOCPD there isn't much to do as far as calculations. The disconnect when sized to the MCA will already be large enough to cover the 115%. Forget continuous versus non-continuous it's all been done for you. If you're using a fused disconnect and you need fuses sized to the MaxOCPD then you may need a larger disconnect. In your example if the MaxOCPD were 450 amps then you would need a 600 amp disconnect.

#### acolella

##### Member
328.38 amps is not the continuous load it is 115% of the continuous load. From what you mentioned the continuous load is 281.2 which is less than 80% of 400.
Let’s say, for example, that the continuous load was 345 amps. Minimum disconnect size would be 396.75 amps. However, I would need a 600 amp disconnect since I would be above 80% of 400 amps?

#### acolella

##### Member
The rules for a group motor setup are not the sum of FLC times 125%. Read article 430 more closely.

Also as an assembly you do not look “inside the box” for MCA. The chiller has a design utilization that is less than name plate FLA. Thus it exceeds name plate when loaded vs unloaded. Thus MCA reflects not only this but the mechanical load irrespective of the installed motor: The way it is determined is to build one and measure average current on a test stand. In the same way a motor is on a test stand with a dyno while measuring temperature.

As to disconnects a fused disconnect uses name plate rating. Non-fused you derate. This is to accommodate the 125% factor used for OCPDs.
I understand that it is not the sum of FLC times 125%. From what I understand about calculating MCA, at least for refrigeration equipment, is that it is 125% of the largest compressor plus all other loads.

This unit's nameplate is as follows:
1 x compressor circuit A, RLA 119 amps
1 x compressor circuit B, RLA 119 amps
8 x outdoor fans, RLA 5.4 amps

MCA = (119 x 1.25) + 119 + (8 x 5.4) = 310.95 amps

I'm not necessarily looking "inside the box" for MCA. I'm looking at the nameplate of the unit. I'm not looking at the nameplates of the individual components. I'm just wondering why the unit's nameplate MCA is 5.35 amps higher than the calculated MCA.

The EE that I work with reviewed this and agrees that my calculations are correct and is also unsure why the unit's listed MCA is 5.35 amps higher than calculated. He thinks that perhaps the control power is included in the MCA but is not labeled on the nameplate.

#### topgone

##### Senior Member
I understand that it is not the sum of FLC times 125%. From what I understand about calculating MCA, at least for refrigeration equipment, is that it is 125% of the largest compressor plus all other loads.

This unit's nameplate is as follows:
1 x compressor circuit A, RLA 119 amps
1 x compressor circuit B, RLA 119 amps
8 x outdoor fans, RLA 5.4 amps

MCA = (119 x 1.25) + 119 + (8 x 5.4) = 310.95 amps

I'm not necessarily looking "inside the box" for MCA. I'm looking at the nameplate of the unit. I'm not looking at the nameplates of the individual components. I'm just wondering why the unit's nameplate MCA is 5.35 amps higher than the calculated MCA.

The EE that I work with reviewed this and agrees that my calculations are correct and is also unsure why the unit's listed MCA is 5.35 amps higher than calculated. He thinks that perhaps the control power is included in the MCA but is not labeled on the nameplate.
For the nth time, I'd say follow what the manufacturers say it is! It's been done for you and you are not required to "calculate" again. The MCA and the MOP are all you need for a safe install. Keep It Stupidly Simple!

#### infinity

##### Moderator
Staff member
For the nth time, I'd say follow what the manufacturers say it is! It's been done for you and you are not required to "calculate" again. The MCA and the MOP are all you need for a safe install. Keep It Stupidly Simple!
It's really that simple. Follow the nameplate and move on.

#### acolella

##### Member
It's really that simple. Follow the nameplate and move on.
I understand that it is that simple. I work for a manufacturer. We do this all the time. This one just didn't make a whole lot of sense to me. I'm just asking if anybody has any ideas about how that particular MCA was arrived at. This is just to satisfy my curiosity. Nothing more, nothing less.

#### paulengr

##### Senior Member
I understand that it is that simple. I work for a manufacturer. We do this all the time. This one just didn't make a whole lot of sense to me. I'm just asking if anybody has any ideas about how that particular MCA was arrived at. This is just to satisfy my curiosity. Nothing more, nothing less.

I’ll bet they make several chiller models with different requirements and a fairly common control panel. The extra current might be an extra fan if it’s installed in say a food plant engine room where it’s 90 degrees even in winter.

#### StarCat

##### Industrial Engineering Tech
I need a detailed explanation as to why anyone would ever put a " Fused " disconnect ahead of any 3 phase compressor load.
This could lead to a discussion about how much costly destruction this has caused on the historical time line and how systems that were properly set up with non-fused disconnects and breakers can be diagnosed and gotten back on line much quicker in most cases, and with far less wasted time and expense. Inherent 3 phase protection may be much more common these days, but it is still not standard on everything.

#### topgone

##### Senior Member
I need a detailed explanation as to why anyone would ever put a " Fused " disconnect ahead of any 3 phase compressor load.
This could lead to a discussion about how much costly destruction this has caused on the historical time line and how systems that were properly set up with non-fused disconnects and breakers can be diagnosed and gotten back on line much quicker in most cases, and with far less wasted time and expense. Inherent 3 phase protection may be much more common these days, but it is still not standard on everything.
In my experience, fused disconnects are installed on areas where there are very high fault currents available and no circuit breaker are availably capable of breaking those very high fault currents! Each protective device has its own niche, IMO.

#### Wyolectrical

##### New User
I need a detailed explanation as to why anyone would ever put a " Fused " disconnect ahead of any 3 phase compressor load.
Many non-fused disconnects have a short circuit rating (SCR) of 10kA unless series-rated with a specific breaker/fuse combination. If the available fault current (AFC) is greater than 10kA, it is very possible, even likely, the non-fused disconnect is not rated for the AFC.

#### StarCat

##### Industrial Engineering Tech
Thank you for the responses. The scope of my point would refer to commercial equipment from fractional up to say 250 tons of refrigeration.
Fuses tend to complicate matters in these settings, whether on board or out board. There are also all kinds of on board breakers specific to the unit control panel that do a fine job of isolating faults that only take down " part " of the system in many cases. I have seen quipment subsystem fuse blocks retrofitted to equivalent circuit breakers that have solved a world of problems. Especially as used in conjunction with phase monitors and very dirty power situations. Those problems were stopped cold with breakers as opposed to fuses.

#### Jraef

##### Moderator
Staff member
Like it or not, get used to seeing more fuses on anything with power electronics., like VFDs, Soft Starters Inverters and UPS. New listing rules with regard to the harmonization of UL and IEC standards is leading manufacturers to start relying more on newer style "high speed" current limiting fuses, a.k.a. "semiconductor" fuses, in attaining the SCCR values typically required (i.e. 65kA and up). These fuses are VERY expensive so they are typically over sized so as to not nuisance clear, but the disconnect mfrs have not yet caught up to making disconnect switches that hold them yet. Still, most power electronics mfrs are plowing forward on that program anyway, so you will see in the next wave of new versions of things like VFDs that circuit breakers will no longer be acceptable OCPDs (unless there are these fuses in series).

So be aware of this issue, OEMS will likely want to ignore it and make it the installer's problem, one that will be difficult to do in the field. If you get a chance to influence buying decisions, be sure to INSIST on the equipment suppliers providing equipment that is ALREADY listed for the required SCCR, meaning THEY have deal with the issue of fusing requirements in their own assembly (it's easier for them to address it in the design). A lot of schlocky suppliers are pretending they don't know this is an issue. I just ran into that with some 600HP VFDs; one supplier included the fuses, another one with a lower cost "said" they were unnecessary (but didn't put that in writing). So I read their manual, saying they WERE required to get more than a 10kA SCCR! The fuses alone (Bussman 170M) were \$440 each and there is no listed disconnect switch for them, so the installer would have been in a bind, having to find a custom equipment supplier to build and list one for them. VERY expensive.

#### acolella

##### Member
I need a detailed explanation as to why anyone would ever put a " Fused " disconnect ahead of any 3 phase compressor load.
This could lead to a discussion about how much costly destruction this has caused on the historical time line and how systems that were properly set up with non-fused disconnects and breakers can be diagnosed and gotten back on line much quicker in most cases, and with far less wasted time and expense. Inherent 3 phase protection may be much more common these days, but it is still not standard on everything.
The chiller is being temporarily installed on an outdoor test pad where there are already existing 400 amp fused disconnects. That initially prompted my original question. I was unsure how much continuous load a 400 amp fused disconnect was rated to carry. There was a discussion with my team about this. Some thought we could use one of the 400 amp disconnects, others thought that it may need to be 600 amp. As far as I know, they are fused disconnects in this location for at least two reasons. 1. They frequently serve smaller equipment that has MOCP well below 400 amps so need to be fused accordingly. 2. Available fault current is higher than 10kA. 1500 kVA 13800-480/277 unit substation with 5.77% impedance so I think around 31kA.

#### kwired

##### Electron manager
The chiller is being temporarily installed on an outdoor test pad where there are already existing 400 amp fused disconnects. That initially prompted my original question. I was unsure how much continuous load a 400 amp fused disconnect was rated to carry. There was a discussion with my team about this. Some thought we could use one of the 400 amp disconnects, others thought that it may need to be 600 amp. As far as I know, they are fused disconnects in this location for at least two reasons. 1. They frequently serve smaller equipment that has MOCP well below 400 amps so need to be fused accordingly. 2. Available fault current is higher than 10kA. 1500 kVA 13800-480/277 unit substation with 5.77% impedance so I think around 31kA.
How temporary? Will it ever demand full load rating? If temp and won't ever see full rating during intended use can any portion of the load be disabled or adjusted so it can't draw full rating?

Just a few thoughts, and things you may be able to get by with since it is temporary.

#### acolella

##### Member
How temporary? Will it ever demand full load rating? If temp and won't ever see full rating during intended use can any portion of the load be disabled or adjusted so it can't draw full rating?

Just a few thoughts, and things you may be able to get by with since it is temporary.
About 2.5 months. It will be ran at full load the entire time but most likely won’t approach its maximum rated current. These machines usually only get up to the maximum when ran full load at an outdoor ambient of 115F-125F which it certainly won’t see on the test pad. I like to supply these machines with a source that can handle the maximum current even if we don’t expect it to get up that high. Size for worst case. I just think it’s good practice in our situation to size for the max current. In the past we have undersized relative to the nameplate and just demand limited the machine. Either way, 281.2 amps being the max, the 400 amp disconnect should be acceptable.

#### Mgraw

##### Senior Member
I understand that it is that simple. I work for a manufacturer. We do this all the time. This one just didn't make a whole lot of sense to me. I'm just asking if anybody has any ideas about how that particular MCA was arrived at. This is just to satisfy my curiosity. Nothing more, nothing less.
If you work for a manufacturer why don't you ask them? My understanding is they conduct "worst case" test and calculations on the equipment and that determines mca.

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