Forums » Electrical Engineering

Sub panel

    • 37 posts
    February 10, 2019 12:54 PM PST
    Carl Coulter said:

    Rajish Rakish said:

     

    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

    Rajish -

    I need some clarification

    Is the sub-panel 400V, 3 ph?

    The 160A breaker @ the main panel - is that the CB feeding the sub-panel?

    The 160A breaker @ the sub panel - is that the incoming CB at the sub panel?

    I will need to understand this to help at all

     

    Yes, Sub panel 400V, 3ph

    Correct, 160A @ the main panel feeding the sub panel

    Correct, its location inside the sub panel 160A breaker

  • February 10, 2019 1:05 PM PST

    Rajish Rakish said:

    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

     

    Rajish -

    I am not clear on the loads or the CB ratings:

    Is the panel 400V, 3 phase?  If not, what?

    "Breaker @ main panel 160A"   Is this the load on the CB feeding the sub panel?  Is this the rating of the CB feeding the sub panel?

    "Breaker @ sub panel 160A"   Is this the load on the sub panel incomer CB?  Is this the rating of the sub panel incomer CB?

     "Cable Size = 50 mm2"   I am not familiar with mm2 cable ampacities.  It is about 1/0 in size and that would be 150A ampacity .  What is the rating for 50mm2 cable?

     

    carl

    • 37 posts
    February 10, 2019 1:13 PM PST

    max current flowing from main to sub 123A

    Breaker rating at main panel 160A

    Breaker rating at sub panel 160A

    50 mm2 Cable current capacity 130A as below photo

     


    This post was edited by Rajish Rakish at February 10, 2019 1:14 PM PST
  • February 10, 2019 1:43 PM PST

    So:

    There is no 80A CB feeder CB.  Main Panel feeder CB is 160A.

    The sub panel has a 160A main CB, not 125A?

    Load calc is 68kw at .8pf, 400V, 3 phase  =  123A

    Feeder Cable current carrying capacity is 130A.  The conductors could be a bit small.  I don't know what your local regulations are for continuous load and using next size up for the conductor overcurrent protection.  I don't see an issue for this instance.  Even if the conductors were a touch undersize, I don't see that contributing to causing the CBs to trip.

     

    The system looks fine.  Having the two CB is series is not a good design, but not bad or unsafe.  If one or both trip it doesn't matter.  It doesn't take out any additional equipment.  It is just minor inconvenience to have to reset two CBs.  And that should not even matter.  The CBs should not be tripping.  If one (or both) are tripping - even occasionally, is not good.  Something is broken.  The tripping CB is a good candidate.  Or you mis-measured the load - as in there is a sneak load jumping in when you are not looking.

    Tell me again:  Which CB is tripping?

     

    carl

    • 37 posts
    February 10, 2019 10:35 PM PST
    The CB which was tripping, was old design 80A. This above information is a new proposal for updating the existing system. In new design I have changed cable size to 70mm2. Now looks completely fine.
  • February 11, 2019 8:00 AM PST

    Sounds perfect.  Good luck with getting it implemented

    • 19 posts
    February 16, 2019 6:28 AM PST

    I went through this discussion. just one tip

    no need a circuit breaker 125A in Subpanel. Just replace it with a disconnector. It safes cost.