Forums » Electrical Engineering

Sub panel

    • 200 posts
    February 5, 2019 12:19 PM PST

    And in this configuration the GLOBAL protection is tripping BEFORE the LOCAL protection, isn't it?

    It is the same to say:

    - A 10-family building into the darkness just because a girl decided to use her new hairdryer after shower...

    - A 15-store Business center shuts everything down just because one guy decided to turn an air conditioning on in a hot day;

    No, it is NOT ALLOWED, no matter how safe the system is now with current configuration, no matter how strong any code can be!!!

    It was a first sight: no PDMS/AutoCAD Eng./Tech. guy would never ask such a question. You are NOT in a "Design lysergic trip", you are in a "Maintenance real-world journey" as I did many times during 15-year maintenance experience.

    You cannot change or demolish an "installation already done", with few strokes in a calculator and 5 clicks in a mouse. You must work with what you already have.

    You need a guidance in this journey, and who will be your guide? Not me, or Dennis, or Carl, or Rechart...

    Your guide will be the CONDUCTORS ALREADY INSTALLED

    The primary purpose of a Circuit Breaker is to protect the CONDUCTOR. Protection to the final equipment is a secondary purpose.

    Also, a Circuit Breaker shall protect the CONDUCTOR AFTER its position (usually, although not always).

    Please read carefully the calculations I posted and my "opinion" (not statement), you need to check the CONDUCTORS, if they match to something similar to the diagram maybe you will need to replace the 80A CB.

    Regards


    This post was edited by Alex de Moura at February 5, 2019 12:29 PM PST
    • 132 posts
    February 5, 2019 12:29 PM PST

    Dear all, I do not want to interrupt you. Some information may be useful from my side. Please let me know, if you have any comment

    Based on Rajish given information. This design is not applicable.  The Simulation with errors are as follows. 

     

    Single Line Diagram:

     

    After Simulation Errors

     

    63 A is not Selective with 80A Breaker

     

    50A break is not selective with 80A breaker

     

    Result: it is not possible to have such a system

  • February 5, 2019 3:43 PM PST
    Rajish Rakish said:
    125 A sub panel was fed from 80A breaker main panel. Does any standard allow this?
    Dennis Alwon said:

    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.

     

    Rajish -

    Dennis has an excellent point.  Does the 125A panel have a main CB, or is it a 125A Main Lug Only.

    It doesn't change my answer any, but it would make more sense as to why the system was installed that way.

    • 37 posts
    February 5, 2019 9:32 PM PST
    Carl , it is same as first picture. The distance between main and sub panel is 50m. As far I know, it was not running with full current in the past. Now they have problem, and it is asked to upgrade this system.
    • 132 posts
    February 5, 2019 11:22 PM PST

    Carl, It does not matter the first picture or the second

    The question is the main breaker (80A) is not sized properly. It needs to be changed to at least 125A

    Here is screen shots

    Wrong Designed

    Simulation Errors

     

    Correction to upgrade 80A breaker to 125A. Still there will be a question, is the 125A downstream breaker would be selective to 125A main breaker? If not you need to go one step farther to have 160A or 200A. Or you need to have a time delay setting for main 125A breaker.


    This post was edited by Hameedullah Ekhlas at February 5, 2019 11:43 PM PST
    • 17 posts
    February 6, 2019 3:14 AM PST

    It happens to me several times and it is OK.

    It is not the best solution but it works and is safe if the breaking capacity ,of both Circuit Breakers (CB), is the same (For 80 and
    125A CB). The installation is covered by the code however if the CB 80 is going off due to overload then it is necessary to upgrade the
    installation. Some times, there are several parties that are working together and these issues happens.

    • 17 posts
    February 6, 2019 3:45 AM PST

    Another key insight, the Circuit breakers are protecting the cable and the downstream circuits and there are not designed to protect the load or minimise the interruption operation.

    Some circuit breakers have special relays addressed to them to give special functions. I guess that this is not the case.

    Coordination is for maximum protection with minimum interruption, it is an economical and/or environmental upgrade, not a code statement.

  • February 6, 2019 8:31 AM PST

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

    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.

    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.

    As I said in my first answer, as long as the load is within the ampacity of the feeder (80A CB and feeder conductors) the design/install is fine - not great, but it meets code (NEC), is safe, and will work.

    Rajish Rakish said:
    Carl , it is same as first picture. The distance between main and sub panel is 50m. As far I know, it was not running with full current in the past. Now they have problem, and it is asked to upgrade this system.

    I am translating this as the 80A CB is tripping occasionally.  If not, disregard the rest.

    This tells me that someone went in and added load to the 125A sub-panel, and didn't bother to do a load calc and make sure the load was within the feeder CB/conductor ampacity.  This is the dreaded, "Oh there are empty spaces in the panel, we can add more load."  (Rakish - This last sentence is humor.  I know it wasn't you that added the load with out checking)

    Others have said, you need to upgrade the feeder to match the 125A sub-panel rating.  I don't think that is the answer, because I don't know what the load is.

    My recommendation:

    First: Do a load calc for the 125A sub-panel.  Maybe the 125A panel is too low of a rating.  Maybe they need a 150A - we don't know.

    Second:  Size the feeder conductors to meet the calculated load.

    Third:  Select the Feeder CB.

    For numbers 2 an 3, you may choose to oversize the conductors (and CB to match).  Most of my clients want excess capacity.  Yes, that adds to the job cost, however, the clients think it is cheaper in the long run to have the ability to add small loads.  It could easily be that upgrading the Feeder CB/conductors to meet the 125A subpanel CB is an excellent answer.  Then again, maybe it needs a larger sub-panel.  We don't know the load.

    Fourth:  Consider checking the Main Panel load.  Someone has added load with out performing a load calc.  If the system was marginal to start with - it is worse now.  This is judgment call - we can't see your system.

    Hopefully I helped and did not add confusion.

     

    carl

    • 8 posts
    February 6, 2019 9:01 AM PST

    Agree with Carl to do load calculation.

  • February 6, 2019 9:51 AM PST

    Hameedullah Ekhlas said:

    Carl, It does not matter the first picture or the second

    The question is the main breaker (80A) is not sized properly. It needs to be changed to at least 125A

    Yes. of course the feeder CB (and feeder conductors) need to be changed.

    Because:

    Rajish Rakish said:
    Carl , it is same as first picture. The distance between main and sub panel is 50m. As far I know, it was not running with full current in the past. Now they have problem, and it is asked to upgrade this system.

    The load is now tripping the Feeder CB (my assumption from Rakish's post)  As Rakish said, that wasn't a problem in the past. 

    So, someone added load and didn't do a load calc.  We don't know why the original installation was an 80A feeder.   125A feeder could easily be the answer.  But Rakish won't know until -.................................... (drum roll) until a load calc is done.

     

    Hummmm ... another thought - Just thinking:  Maybe no one added load.  Maybe it is the same as it was.  Maybe the 80A CB  is malfunctioning.  We won't know until the load calc is done.

    I am getting repetitive - getting time for me to quit on this one. 

     

     minor format edit for clarity - no change in content


    This post was edited by Deleted Member at February 6, 2019 10:59 AM PST
    • 7 posts
    February 6, 2019 10:18 AM PST
    Alex de Moura said:

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


    In your diagram I see more connected load in Main panel, while the main breaker has already overloaded. Can you explain farther?
  • February 6, 2019 10:45 AM PST

    A few notes on sizing a feeder to a sub-panel with a 125A main CB. 

    Based on 2017 NEC - other country codes will most certainly vary

    Alex - Your presentation on selecting conductors was excellent.  However, this is a "feeder", not a "service.  The reduction in ampacity allowed for services by 310.15.B.7.1 does not apply.

    The 125A sub-panel main CB is rated for 125A non-continuous.  We don't have to go any farther - pick a conductor ampacity of 125A or more.

    Conductor temperature rating will be 75C and CB termination will be 75C.  For conductors above #10, that has been the standard for the last 30 years (maybe even 40 years.)  Yes, I am aware of NEC 110.14.C.1.   

    As shown in the Tables Alex posted,

    #1CU is good for 130A

    And could be protected by a 150A CB in the Main panel.

    Caution:  Depending on Main panel type/model, there may not be 150A CB available.  There may not be a 125A CB available.  If not, I don't have any decent suggestions.

     

    Thanks for the site.  I'm enjoying it.

    carl

    edit to add:

    The trip curve for a 150A CB may still overlap with the 125A sub-panel CB trip curve - especially in the higher current ranges, and both CB could trip together.  Hard to get away from that with series CBs.


    This post was edited by Deleted Member at February 6, 2019 10:54 AM PST
    • 37 posts
    February 6, 2019 10:03 PM PST
    Yes Carl, that’s what I did. Load calculation.
    • 7 posts
    February 7, 2019 4:10 AM PST
    Careful with sub panels. I don’t know how or where it is to be used but NEC Is not in favor of subpanels because it is easy to abuse the use of such. It is called “Power Housing” and typically it is NOT allowed.
    • 200 posts
    February 7, 2019 4:20 AM PST
    Rajish,
    The load calculation is always good to make to monitor the entire system, but in your case, the Load values are good ONĹY if you are replacing the CONDUCTORS (Cables) and all Panels, if your Customer requested to do because he wants to add more load, and that is VERY expensive it is an entire system upgrade.

    Please do NOT use Load values to select a new Breaker if the rest of the system remains with SAME CONDUCTORS (Cables), and Panels. This is a dangerous mistake!

    The Engineering Practice rules are simple:
    1. Load defines CONDUCTORS (Cables);
    2. CONDUCTORS (Cables) define their respective Circuit Breakers;

    If you are NOT replacing CONDUCTORS (Cables) then Load calculation is an interesting information ONLY, then you need to check the AWG of CONDUCTORS (Cables) from 80A CB to 125A Sub Panel.

    Depending on this CONDUCTOR (Cable) ONLY you can select the new CB. I think it will be more than 80A and less than 125A (MAXIMUM to protect the 125A Sub Panel, secondary purpose of a CB).

    I had improved the diagram a bit I will send in private message. Good luck and regards
  • February 7, 2019 8:00 AM PST
    Paul Travers said:
    Careful with sub panels. I don’t know how or where it is to be used but NEC Is not in favor of subpanels because it is easy to abuse the use of such. It is called “Power Housing” and typically it is NOT allowed.

    Paul -

    Tell me about this.  I've never heard of the term "power housing".  It does not show up in the NEC (2017 edition), nor IEEE 100.

    "Subpanel", (or "sub-panel", or "sub panel") in the context of an NEC 408,part III, panelboard, does not show up in either IEEE 100 or the NEC.

    I am using a colloquial definition, A panelboard fed from an overcurrent protective device in another panelboard or switchboard.

    My area is heavy industrial, not commercial or high-rise.  Subpanels are really common, 480V as well as 208/120V. 

    Educate me a bit.  I understand you said, "I don’t know how or where it is to be used ...  So, tell us what you remember about what exactly is "Power Housing", what is the "abuse"

     

    carl

     

     

    • 132 posts
    February 8, 2019 9:03 AM PST
    Paul Travers said:
    Careful with sub panels. I don’t know how or where it is to be used but NEC Is not in favor of subpanels because it is easy to abuse the use of such. It is called “Power Housing” and typically it is NOT allowed.

    Dear Paul,

    could you please give some farther explanation on Power Housing? 

  • February 8, 2019 12:47 PM PST
    Rajish Rakish said:
    Yes Carl, that’s what I did. Load calculation.

      Don't leave us hanging.  What did you get?

    • 143 posts
    February 8, 2019 1:17 PM PST

    Carl, I get the feeling that some of the others got the numbers just by looking at their diagrams but I may be wrong.

  • February 8, 2019 2:23 PM PST

    Hummmmm --

    You're telling me I'm lost again.  Sigh, okaycry

    • 7 posts
    February 8, 2019 5:44 PM PST
    Sub panels provide a function. Distribution of the main power to another side of a building for convenience . It takes said portion of power off the main panel and that wiring is sized appropriately for the sub breaker and sub panel & thats it!!! Many engineers become separated from the main panel and assume a sub panel is the main panel and “Sub” off again into another location. Meanwhile the main panel suffers an overload with its own wiring because it supports remote panels. You need to down size your breaker by 50% off a main panel & wire up 25% in gauge for future errors .
    Sub panels, unless electrically limited, someone will sub off it to support another appliance. When it comes to factories and cities it is extremely important to layout schematics with future addons to support. Sub panels arbitrarily thrown up are a hazard by themselves!
    • 132 posts
    February 9, 2019 3:19 AM PST
    Paul Travers said:
    Sub panels provide a function. Distribution of the main power to another side of a building for convenience . It takes said portion of power off the main panel and that wiring is sized appropriately for the sub breaker and sub panel & thats it!!! Many engineers become separated from the main panel and assume a sub panel is the main panel and “Sub” off again into another location. Meanwhile the main panel suffers an overload with its own wiring because it supports remote panels. You need to down size your breaker by 50% off a main panel & wire up 25% in gauge for future errors . Sub panels, unless electrically limited, someone will sub off it to support another appliance. When it comes to factories and cities it is extremely important to layout schematics with future addons to support. Sub panels arbitrarily thrown up are a hazard by themselves!

    Thank you Paul.

    • 37 posts
    February 10, 2019 11:14 AM PST
    Carl Coulter said:
    Rajish Rakish said:
    Yes Carl, that’s what I did. Load calculation.

      Don't leave us hanging.  What did you get?

     

    Sorry for late reply. I came out with the following results

    Final load including demand factors @ Sub panel = 68kW @ 0.8 p.f

    Distance from Main panel to Sub panel = 50m

    Cable Size = 50 mm2

    Breaker @ main panel 160A

    Breaker @ sub panel 160A

     

    I would be thankful for any suggestion.

     

    regards

    Rajish

     

     

     

     

     

    • 64 posts
    February 10, 2019 11:51 AM PST

    One point, increase main panel breaker to 200A.

  • February 10, 2019 12:49 PM PST

    rewriting - I was not clear


    This post was edited by Deleted Member at February 10, 2019 12:53 PM PST