MCA vs Feeder ampacity

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Andrzej

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Location
Glendale, CA
We are in the process of installing new refrigeration system.
per name plate data FLA=590A and MCA=600A.
I have spare 600A circuit breaker in Main Switchboard.
This circuit breaker has continious rating of 80%.
Is the circuit breaker sufficient to supply load requiring minimum circuit ampacity of 600A?
If I was to install 800A 80% rated breaker what would be required minimum feeder ampacity?
640A?
 

jumper

Senior Member
MCA is the minimum ampacity of the conductors feeding the unit. It should have the continuous loads already sized at 125%.

In this case maybe 2 sets of 350 CU.

The OCPD is usually given in the specs also, I imagine a 600 amp breaker may not hold on start up.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
What jumper said + look closely at the nameplate/data sheet as some refrigeration units call for fuse protection only. You should see a notation for max OCP (and sometimes minimum)
 

Andrzej

Member
Location
Glendale, CA
What jumper said + look closely at the nameplate/data sheet as some refrigeration units call for fuse protection only. You should see a notation for max OCP (and sometimes minimum)

How about second part of my question?
If I use 800A CB rated 80% what size conductors should be used?
 

roger

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Fl
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Why, MCA is 600A, 800*.8=640, this means CB continious rating is greater than MCA.
However if I use 800A CB rated 80% what ampacity conductors I should use 800A or 800*.8=640A?
Have you read through article 440?

Roger
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
As Roger notes, Art 440 should provide the answers you seek. In 440.4(B) you will find that the manufacturer should have provided a nameplate showing MCA and MOCP (size and type)
440.35 will tell you the conductor must not be less than the MCA.
Part III of Art 440 will guide you in selecting the BS-SC device.

It's normally a case of following the manufacturer's nameplate info.
 

Andrzej

Member
Location
Glendale, CA
Part III of Art 440 will guide you in selecting the BS-SC device.

?????
What does BS stand for?
You mean Branch Circuit?
440 part III is a good start, however I am puzzled by possible conflict with requirements of article 240.4 regarging conductors OC protection.
In my case MCA=600A, RLA=595A,
if I use 600A fusible disconnect with 600A TD fuses I can install (2) sets of 350kCM and meet requirements of articles 240.4 and 440 part III
however if I want to use 600A Molded case CB, I may have a problem on a startup.
let's say I need to use 800 A CB with LT trip set @ 750A, now my (2) sets of 350kCM are not protected according to article 240.4 (2*310=620 and next standard CB rating is 700)
does it mean I have to increase conductor size to 500kCM?
with todays CU prices it is very expensive.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Part III of Art 440 will guide you in selecting the BS-SC device.

?????
What does BS stand for?
You mean Branch Circuit?
440 part III is a good start, however I am puzzled by possible conflict with requirements of article 240.4 regarging conductors OC protection.
In my case MCA=600A, RLA=595A,
if I use 600A fusible disconnect with 600A TD fuses I can install (2) sets of 350kCM and meet requirements of articles 240.4 and 440 part III
however if I want to use 600A Molded case CB, I may have a problem on a startup.
let's say I need to use 800 A CB with LT trip set @ 750A, now my (2) sets of 350kCM are not protected according to article 240.4 (2*310=620 and next standard CB rating is 700)
does it mean I have to increase conductor size to 500kCM?
with todays CU prices it is very expensive.

I think Augie and Roger are trying to get you to realize something here. Hint: the wire does not need to match the breaker.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Part III of Art 440 will guide you in selecting the BS-SC device.

?????
What does BS stand for?
You mean Branch Circuit?
440 part III is a good start, however I am puzzled by possible conflict with requirements of article 240.4 regarging conductors OC protection.
In my case MCA=600A, RLA=595A,
if I use 600A fusible disconnect with 600A TD fuses I can install (2) sets of 350kCM and meet requirements of articles 240.4 and 440 part III
however if I want to use 600A Molded case CB, I may have a problem on a startup.
let's say I need to use 800 A CB with LT trip set @ 750A, now my (2) sets of 350kCM are not protected according to article 240.4 (2*310=620 and next standard CB rating is 700)
does it mean I have to increase conductor size to 500kCM?
with todays CU prices it is very expensive.

You size the conductors to the MCA. The OCPD can be larger than the ampacity of the conductors to allow for starting current.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Part III of Art 440 will guide you in selecting the BS-SC device.

?????
What does BS stand for?
You mean Branch Circuit?
440 part III is a good start, however I am puzzled by possible conflict with requirements of article 240.4 regarging conductors OC protection.
In my case MCA=600A, RLA=595A,
if I use 600A fusible disconnect with 600A TD fuses I can install (2) sets of 350kCM and meet requirements of articles 240.4 and 440 part III
however if I want to use 600A Molded case CB, I may have a problem on a startup.
let's say I need to use 800 A CB with LT trip set @ 750A, now my (2) sets of 350kCM are not protected according to article 240.4 (2*310=620 and next standard CB rating is 700)
does it mean I have to increase conductor size to 500kCM?
with todays CU prices it is very expensive.


My error: Art 440 Part III Branch-Circuit Short Circuit and Ground Fault Protection SC-GF
That, incidentally, is the key... SC-GF protection is provided by the breaker. OL protection by the units overload devices. This allows you to use a protective device greater than the conductor ampacity.
You need to look a little deeper at Art 240.4 ..note in the 1st sentence "unless otherwise permitted" and 240.4(G) includes Art 440 as one of the "otherwise" situations.
In summary..what Infinity is telling you.
 

Andrzej

Member
Location
Glendale, CA
Thank you all for valuable input.
I red articles 240 and 440, I am starting to get an idea how code appritches this issue.
Now it all makes sense and seams to be logical.
I would like to elaborate on one more aspect of branch circuit design for HACV equipment.
Lets say two units each with MCA=400A and max OCPD = 500A are to be fed from the same panelboard. How MCA and max OCPD shoud be calculated for combined load? Will article 440 still apply? Additionally would I use manufacturer data ie MCA and max OCPD for each unit or I need data for each component within each unit?
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Adding up the MCA's will give you a safe number for the feeder (e.g. 800A with 800A overcurrent protection). Then two 500A breakers using 400A ampacity wire to each unit. There is no allowance in the feeder calculation to oversize the breaker -- that only applies to the branch circuit. To make the absolute smallest feeder, you could go through all the parts of the HVAC unit and then add 25% to the one largest motor or compressor. Seems like a lot of work, and you may not have all the data available to you, especially when there are electronic controls buried inside.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Adding up the MCA's will give you a safe number for the feeder (e.g. 800A with 800A overcurrent protection). Then two 500A breakers using 400A ampacity wire to each unit. There is no allowance in the feeder calculation to oversize the breaker -- that only applies to the branch circuit. To make the absolute smallest feeder, you could go through all the parts of the HVAC unit and then add 25% to the one largest motor or compressor. Seems like a lot of work, and you may not have all the data available to you, especially when there are electronic controls buried inside.

I think this is open to discussion. As I see it Art 440 does not address "feeders", however, 440.3(A) does reference Art 430 by stating that Art 440 amends Art 430. From that, my thinking is that you can go back to 430 Part II and Part V. In fact in 430.62 for sizing feeder SC/GF proetction, 440.22(A) is referenced.

{I'm still "learning" that section :) but if I understand it, from the OP's post 13, you could take the MOCP device for the 1st unit (500 amp) and add the MCA for #2 (400) and use a 1000 amp OCP device (next standard size) with 800 amp conductors}
 
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jumper

Senior Member
I think this is open to discussion. As I see it Art 440 does not address "feeders", however, 440.3(A) does reference Art 430 by stating that Art 440 amends Art 430. From that, my thinking is that you can go back to 430 Part II and Part V. In fact in 430.62 for sizing feeder SC/GF proetction, 440.22(A) is referenced.

{I'm still "learning" that section :) but if I understand it, from the OP's post 13, you could take the MOCP device for the 1st unit (500 amp) and add the MCA for #2 (400) and use a 1000 amp OCP device (next standard size) with 800 amp conductors}

What about these? I am not seeing the place where I get to upsize the feeder OCPD in this install.

430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.



240.4(C) Overcurrent Devices Rated over 800 Amperes. Where
the overcurrent device is rated over 800 amperes, the ampacity
of the conductors it protects shall be equal to or
greater than the rating of the overcurrent device defined in
240.6.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
jumper, I'm sorry I didn't see your point at first.... but now you have me thinking.
I agree there is no provision to "upsize" the OCP device on the feeder as in "next higher rating".
I do think the provisions of 430 will allow the OCP of the feeder to exceed the ampacity of the conductors.
In his last scenario he had (2) units with a MCA of 400 and a MOCP of 500. Art 430 would allow you to select the feeder OCP based on the 500 OCP unit + the sum of the other MCAs. The conductor ampacity would be based on the MCAs.

240.4(C) would be "trumped" by 240.4(G) which allows OCP ampacity to exceed conductor ampacity in Art 430 and 440 situations, IMHO, however in this case the feeder OCP would calculate to 900 amps and I was incorrect (ouch) in thinking he could use a 1000 amp device. He could, I believe, use a 900 if he cared to pursue that path.

thanks
 

jumper

Senior Member
jumper, I'm sorry I didn't see your point at first.... but now you have me thinking.
I agree there is no provision to "upsize" the OCP device on the feeder as in "next higher rating".
I do think the provisions of 430 will allow the OCP of the feeder to exceed the ampacity of the conductors.
In his last scenario he had (2) units with a MCA of 400 and a MOCP of 500. Art 430 would allow you to select the feeder OCP based on the 500 OCP unit + the sum of the other MCAs. The conductor ampacity would be based on the MCAs.

240.4(C) would be "trumped" by 240.4(G) which allows OCP ampacity to exceed conductor ampacity in Art 430 and 440 situations, IMHO, however in this case the feeder OCP would calculate to 900 amps and I was incorrect (ouch) in thinking he could use a 1000 amp device. He could, I believe, use a 900 if he cared to pursue that path.

thanks

My bad, I should have been more clear, I was referring to up sizing the breaker to the "next higher rating". Someday I may learn how to write clearly.

I agree that a 900 amp feeder breaker, or really a 1000 dialed down, is compliant.
 
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