Forums » Electrical Engineering

Sub panel

    • 37 posts
    February 3, 2019 9:39 AM PST
    125 A sub panel was fed from 80A breaker main panel. Does any standard allow this?
    This post was edited by Rajish Rakish at February 3, 2019 9:40 AM PST
  • February 3, 2019 11:11 AM PST

    Good morning.  New poster.

    Assumptions: A sub-panel with a 125A main CB is fed from an 80A CB. NEC applies

    The NEC would not have a problem.  The conductors from the main panel 80A CB must have sufficient ampacity to be protected by the 80A CB.  The load on the 125A sub-panel must be within the ampacity of the feeder conductors.

    The 125A sub-panel main CB is not part of the overcurrent protection.  It merely functions as a local disconnect - although, for high SCC, it might trip concurrent with the 80A feeder CB.

    Note:  "Sub-panel" is not defined in the NEC.  Using a colloquial definition.

    Interesting site.  I'm appreciative.

    carl

     

    • 143 posts
    February 3, 2019 11:28 AM PST

    I agree with Carl.  I couldn't have said it better myself.  The 80 amp breaker is going to be the protective device.  You could have a 200 amp panel and it wouldn't matter unless the lugs on the panel would not accept a small conductor

    • 19 posts
    February 3, 2019 11:32 AM PST

    Any size sub panel you can run as long as the feeder is 100A or less so the main breaker should not trip

    • 37 posts
    February 3, 2019 12:29 PM PST
    If the sum of load in sub panel reaches to 90 Ampere, will not the main breaker of main panel trip due to over load ??
  • February 3, 2019 12:33 PM PST
    Rajish Rakish said:
    If the sum of load in sub panel reaches to 90 Ampere, will not the main breaker of main panel trip due to over load ??

    Yes, however,

    "The conductors from the main panel 80A CB must have sufficient ampacity to be protected by the 80A CB.  The load on the 125A sub-panel must be within the ampacity of the feeder conductors."

     

    Edit:  Whoops, I didn't say that right:

    The load on the 125A panel has to be with in the 80A CB.  As in, non-continuous + 1.25 x continuous less than or equal to 80A.

     

    carl


    This post was edited by Deleted Member at February 3, 2019 12:38 PM PST
    • 64 posts
    February 3, 2019 1:49 PM PST
    Rajish Rakish said:
    125 A sub panel was fed from 80A breaker main panel. Does any standard allow this?

    Technical I am not agree on this. There will not be a selectivity or coordination between 80A and 125A. will the  device trip the 80A breaker at the service equipment before it will trip the 125A breaker ?. I am not sure on this, unless I have full diagram picture. 

  • February 3, 2019 2:25 PM PST
    Reynaldo Danilo said:
    Rajish Rakish said:
    125 A sub panel was fed from 80A breaker main panel. Does any standard allow this?

    Technical I am not agree on this. There will not be a selectivity or coordination between 80A and 125A. will the  device trip the 80A breaker at the service equipment before it will trip the 125A breaker ?. I am not sure on this, unless I have full diagram picture. 

    Absolutely correct.  The 80A CB is the panel overcurrent protection.  The 125A Main CB is a no-extra-cost-unhandy option.  Yes, the two trip curves are going to overlap, especially in the instantaneous range.  However, that is a design issue, not a code issue.

    One could fix this by getting a Main Lug Only panel, or a disconnect switch in place of the 125A Main CB.  Generally what I have seen is either of these two are not readily available, and cost twice as much.

    My work is all industrial, so likely what I would see is as you said.  They would buy an MLO panel - because that is their design.

     

    carl

    • 37 posts
    February 3, 2019 8:46 PM PST
    Code does not recommend a better design ?
  • February 3, 2019 9:09 PM PST
    The NEC really is not a design guide - except where the code panels get stupid and sneak into the design side. It is a set of minimum specs the should not catch fire and is “pracatibaly” safe. The install doesn’t necessarily even have to work good. This is a paraphrase of their words. Article 90.3 (or so - I don’t have a code book with me)

    In this case, it really isn’t a bad design - just not great. If there is a fault, there are two CB’s to reset instead of just one. Consider, that if there is a fault, having series CBs that both tripped is not high on the list of concerns.

    As for designing it like that, I would suspect that the actual install was done because that was the material that was on the shelf and did not have to be ordered.
    This post was edited by Deleted Member at February 3, 2019 9:10 PM PST
    • 37 posts
    February 3, 2019 9:51 PM PST
    Thank you
    • 143 posts
    February 4, 2019 3:10 AM PST

    I don't understand it.  What is the likely hood a 125 amp breaker would trip before an 80 amp breaker and if it di so what.  I assume the load would not be greater than 80 amps anyway. 

  • February 4, 2019 8:11 AM PST

    Dennis -

    As you can see, I hand sketched an 80A Time current Curve overlay on a 125A QOM main.  It is not real accurate - no light table.  I 'm not sure I've seen a light table in the last 40 years.  And I don't have any TCC software on this computer.  Hopefully it is close enough to illustrate.

     

    Coordination is defined as "daylight between the curves".

    If there is a fault down stream of the 125A panel, and the short circuit current is above 800A (or so) the two trip curves overlap.  So in that range, (>800A)  both may trip. 

    However, it is also possible only the 80A may trip.

     

    carl

  • February 4, 2019 8:19 AM PST
    Dennis Alwon said:

    I don't understand it.  What is the likely hood a 125 amp breaker would trip before an 80 amp breaker and if it di so what.  I assume the load would not be greater than 80 amps anyway. 

    1.  Yes. I agree.  It is a "So what?"

    2.  And yes, I agree.  The circuit breaker does not limit the load on the panel, one limits the load on a panel by design.  So, if the load is greater than 80A then either there is a design bust, or a fault.

    For the former, I find beating the designer with a nerf bat helps my attitude (doesn't help theirs much).  For the latter, fix the fault, reset both CBs, carry on - all worked as per design.

     

    carl

    • 10 posts
    February 4, 2019 9:01 AM PST

    According to my point of view the breaker can handle and will not trip on the continuous rating of the device. It will trip at typically 135% rating. Sometimes at 300% can a breaker trip typically in the range of 30 to 90 seconds, even in some points 8 times rating. Most of the breakers have adjustable trip characteristics.

    • 200 posts
    February 4, 2019 12:56 PM PST

    Thanks for the suggestion for this topic.

    Since no more details were given, it is assumed that voltage is less than 2000V, 125A is too low for a "more industrial" design so residential (dwelling) figures and minimum temperature of 60oC/140oF can be used, less expensive conductors are also installed.

    Also it was considered a (Sub-)Panel as a common Panel installed in a branch feed by a (Main) Panel. 

    The code used in this post is NEC/NFPA 2017, since no location was provided. And NEC says about such installation:

    240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).

    (B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

    (1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
    (2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
    (3) The next higher standard rating selected does not exceed 800 amperes.

    310.15 Ampacities for Conductors Rated 0–2000 Volts.

    (A) General. - (1) Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables as provided in 310.15(B) or under engineering supervision, as provided in 310.15(C).

    (B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B) (16) through Table 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by 310.15(B)(1) through (B)(7).

    (7) [chg17: Single]-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4).{NAT...}

    (1) For a service rated 100 through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.

    Let's do some calc, first the minimum Conductor Ampacity (min AMPcond):

    AMPsrvc x 83% = AMPcond  -  125A x 83% = 104A = min AMPcond  -  in Table 310.15(B)(16) we need to select:

    {insert Image: Table 310.15(B)(16) Allowable Ampacities of Insulated Conductors Rated Up to 2000 Volts=imgsnap098.png}

     

    1 AWG Cooper (60oC/140oF), 110A = min AMPcond or

    2/0 AWG Aluminum (60oC/140oF), 115A = min AMPcond;

    Now the Overcurrent Device (OCD), as 110A < 115A < 800A - as the next higher standard OCD rating above the ampacity of the conductors being protected, it can be found in Table Table 240.6(A):

    {Table 240.6(A) Standard Ampere Ratings for Fuses=imgsnap088.png}

    OCDrtng = 125A;

    That is just an opinion: if the Conductors comply with AMPcond >= 110A, then a OCDrtng = 80A is OVER-protecting the 125A (Sub-)Panel, wasting valuable resources on such Panel (45A or more than 30% underused), and depending of proposed usage the 80A Breaker can (and maybe will) trip easily even under normal conditions.

    Regards


    This post was edited by Alex de Moura at February 4, 2019 1:28 PM PST
  • February 4, 2019 2:40 PM PST
    Alex de Moura said:
    ...  OCDrtng = 125A;

    That is just an opinion: if the Conductors comply with AMPcond >= 110A, then a OCDrtng = 80A is OVER-protecting the 125A (Sub-)Panel, wasting valuable resources on such Panel (45A or more than 30% underused), and depending of proposed usage the 80A Breaker can (and maybe will) trip easily even under normal conditions. ...

    Alex -

    I'm going to disagree.  Overall, your presentation was excellent.  The are a few places that don't match, but they are inconsequential, nit-pick stuff.  The main issue is direction of the calculation.  The place to start is not the 125A sub-panel main CB.

    Rather, start with the load calc.  What are the design loads?  Non-Continuous load + 1.25 X Continuous load.  That is the minimum rating of the sub-panel.  Larger rating is fine and often they are.  Note at this stage we don't care if the panel is MLO or MCB.  Next we size the feeder conductors for the ampacity to meet or exceed the calculated load.  Last we size the Feeder CB to protect the conductors.

    Now, the panel has to have a disconnect.  Since the feed is from another panel, this disconnect is the feeder CB.  We still don't care if the panel is MLO or MCB.

    From the OP we can only assume the 80A feeder was selected because that met the design load.  If not, apply:

    Coulter's Law:  "If the system is poorly designed, poorly installed, poorly maintained, expect the system to perform as planned."

    So, anyone coming in after the fact and saying, "Oh look there are 16 empty spaces in the 125A sub-panel.  Lets add another 8 each 16A continuous loads on 2-pole, 20A CBs.  Refer to Coulter's Law.

    Now, if one wanted to say that the 125A sub-panel main is mis-leading and someone might check the existing load, not bother check the feeder, and figure there is 45A of headroom, well, refer to ....

    The excess ampacity is not particularly valuable.  Really, it is not.  Small panels are pretty cheap.  More often than not, the number of spaces is the limiting factor - lots of circuits, a few amps apiece.  Generally, if an MCB was installed, that is because that was the cheapest and least lead time for the number of spaces needed.

    I don't do house wiring, (well except mine, my brother's, my buddy down the street).  And as mentioned, if this was an industrial installation, say attached office lighting loads, my inclination would be to install an MLO with sufficient spaces + 10% (or more), and if transformer \ feeder panel headroom allowed, oversize the feeder (including feeder CB) a bit.

    The OPs question was, Is this a code/regulatory issue?  No it isn't.

    Will it trip under normal operation?  Not if they bothered to design, build, maintain properly.  I still contend, the Feeder CB does not limit the load - the design does (except for one case).

     

    carl

    (edit for minor punctuation)


    This post was edited by Deleted Member at February 4, 2019 3:10 PM PST
    • 200 posts
    February 4, 2019 3:33 PM PST

    Carl Coulter said:

    Assumptions: A sub-panel with a 125A main CB is fed from an 80A CB. NEC applies

    The NEC would not have a problem.  The conductors from the main panel 80A CB must have sufficient ampacity to be protected by the 80A CB.  The load on the 125A sub-panel must be within the ampacity of the feeder conductors.

    Carl Coulter said:

    I'm going to disagree...  The are a few places that don't match, but they are inconsequential, nit-pick stuff.  The main issue is direction of the calculation.  The place to start is not the 125A sub-panel main CB.

    Rather, start with the load calc.  What are the design loads?  Non-Continuous load + 1.25 X Continuous load.  That is the minimum rating of the sub-panel.   Next we size the feeder conductors for the ampacity to meet or exceed the calculated load.  Last we size the Feeder CB to protect the conductors.

    From the OP we can only assume the 80A feeder was selected because that met the design load.  If not, apply:

    Coulter's Law:  "If the system is poorly designed, poorly installed, poorly maintained, expect the system to perform as planned."

    So, anyone coming in after the fact and saying, "Oh look there are 16 empty spaces in the 125A sub-panel  Lets add another 8 each 16A continuous loads on 2-pole, 20A CBs...

    And the first question was:

    Rajish Rakish said:
    125 A sub panel was fed from 80A breaker main panel. Does any standard allow this?

    And the reply starts:

    Alex de Moura said:

    Since no more details were given, it is assumed that... 

    I saw no reason, as you did on your first reply, to consider Rajish's design as inconsequential, nit-pick stuff or a poorly designed system. If they already select a 125A (Sub-)Panel it is because the rating is matching the load connected to this (Sub-)Panel - with no prejudice at all.

    Alex de Moura said:

    Also it was considered a (Sub-)Panel as a common Panel installed in a branch feed by a (Main) Panel.

    And I assumed the 80A Breaker at (Main) Panel, in question, is connected to this 125A (Sub-)Panel only. In other words, I assumed that the (Main) Panel has more than one branch - in a tree design (very common) - also already calculated for all loads distributed in (Sub-)Panels - again with no prejudice.

    Consequently, it is assumed each branch is connected to its specific Breaker, one of those has an 80A Breaker to a 125A (Sub-)Panel - and Rajish wants to know if that is possible by ANY code. It is possible, but why over-protect a 125A designed branch with a 80A OCD??? All loads were poorly summed???

    I think that is too much for a Forum: a design from the scratch, and sometimes many of questions posted here are about things that already installed and after years of changes, correct and incorrect maintenance, etc. It gives to anyone the right for assumptions the same way as in previous reply.

    About the saying, "Oh look there are 16 empty spaces in the 125A sub-panel  Lets add another 8 each 16A" - it is also possible, but it was NOT one of my assumptions and it does not match with the Member's level of this site, as far as I know, it would be prejudice to Engineers and Technicians readers of this Forum. 

     

    • 143 posts
    February 4, 2019 4:08 PM PST

    Do we even know if the sub panel has a 125 amp breaker?  It was not mentioned.  We often feed 200 amp ML sub panels with 100 amp feeder partailly because they are available and we need the spaces.

    • 200 posts
    February 4, 2019 4:33 PM PST

    Dennis, I understand your point, but for sure you know that in this Forum there are (almost) no details in most of questions - no, it was not mentioned anything except "can I use a 80A Breaker for a 125A Panel?"... just like many questions here...

    If we have the assumption that everything is wrong from the scratch in everybody's question, so we can also assume that we are living in an apocalyptic world deep into darkness - no power at all - so better close Z4E, pray and prepare ourselves to live back into caves.

    All the Engineers and Technicians must assume the responsibility for careful reading, search for more knowledge sources, and do a correct interpretation, in mind always the motto: "SAFETY FIRST". 

    By the way it is a very time-consuming job, and demands a lot of research & reading to reply questions with no details at all - and that was the main reason why I "left" the old Z4I - not any quarrel. I promised to help Hameed as I can, so I am trying hard. But after February I suppose I will "leave" Z4E - so just as Guns n Roses said: "... just a little patience", everyone.

    • 143 posts
    February 4, 2019 5:34 PM PST

    Alex you put a lot of work into your replies.  I don't have the time for that much response and I am half retired.  LOL

    • 7 posts
    February 4, 2019 10:02 PM PST
    Alex de Moura, personally I do not know you but I always read your replies. I appreciate your efforts and I am happy to be a part this community. I hope more guys like you, Carl and Dennis to join.
    • 7 posts
    February 4, 2019 10:44 PM PST
    Alex, I am not agree with your statement. You can not operate a breaker as a over loaded breaker. Sub panel is usually designed including demand factor for 125A, and operating continuously. This causes to trip 80A breaker before burns main feeder. I will not allow such a system for design. I agree with Carl for his coordination curve post.
    This post was edited by Rechart Grand at February 4, 2019 10:48 PM PST
    • 37 posts
    February 5, 2019 9:01 AM PST
    Hi all, Thank you for comment. I was upgrading a sub distribution panel and found such a existing design . I shared with you, if it is allowed by standards.
    • 200 posts
    February 5, 2019 11:50 AM PST

    Rajish, finally, I'm glad to see you back. That is what you FOUND (ALREADY INSTALLED) isn't it?