Forums » Electrical Engineering

calculating wire and breaker size

    • 2 posts
    January 12, 2020 11:45 AM PST

    we are upgrading the facility and looking forward to calculate the wire and breaker size for 3 phase 300 kVA. The voltage level 120/208Y step up transformer.
    Thank you.

    • 24 posts
    January 12, 2020 11:49 AM PST

    Just quick rough:

    300 kVA / 1.732 * 208V = 832A

    Breaker = 1000A

    cable size = 2(4x150mm2)

    • 149 posts
    January 12, 2020 12:20 PM PST
    David Welson said:

    Just quick rough:

    300 kVA / 1.732 * 208V = 832A

    Breaker = 1000A

    cable size = 2(4x150mm2)

    for cable, I would select and go with 3(4x120mm2). lets see for others.


    This post was edited by Hameedullah Ekhlas at January 12, 2020 12:21 PM PST
    • 70 posts
    January 12, 2020 3:10 PM PST

    my calculation

    • 67 posts
    January 13, 2020 10:59 AM PST

    Willy -

    What code are you working to?  208/120, 3 phase sounds sort of North American.

    And I'm not clear on what you mean by a "step up transformer".

    Under the NEC:

    As noted by others, for 300KVA, 208/120 secondary current is 833A.

    For Primary 1000V or less (example:  480V primary/208V secondary)

    Depending on the primary protection, transformer secondary OCP may not be required.  For transformer primary OCP 125% FLA (next size up is okay), the secondary OCP is not required.  However, the secondary conductors and panel will still require protection. 

    If secondary protection is required, (Primary OCP up to 250% FLA), Secondary OCP shall be no more than 125% (next size up is okay).  Secondary OCP  = 833 x 1.25 = 1041.  One could go as high as 1200A.  One usually sets the secondary conductor and panel OCP to cover the transformer secondary OCP.

    Secondary Conductors:

    Normal design is to set the secondary conductor ampacity to 125% X Secondary FLA = 1041.  However, there is no next size up for the CB and the panel must also be protected at its rating.  There are some length limitations as to how far the secondary CB can be from the transformer.

    So, for a 1000A panel, with a 1000A main, conductors are:

    4 - 250kcmil (yes, per phase), 75C terminations = 1020A

    Note:  250kcmil is 127 mm2

    Reference 2017 NEC

    450.3

    240.21

    And, if you are not under the NEC, none of this counts


    This post was edited by Carl Coulter at January 13, 2020 11:03 AM PST
    • 32 posts
    January 14, 2020 9:21 AM PST

     Carl: what is 4-250kcmil? 250kcmil = 120mm2. Did you mean 4x(4x120mm2) ?

    • 67 posts
    January 14, 2020 2:38 PM PST

     Carl: what is 4-250kcmil? 250kcmil = 120mm2. Did you mean 4x(4x120mm2) ?

     

    4 - 250kcmil (yes, per phase), 75C terminations = 1020A

    Note:  250kcmil is 127 mm2

    Not specified, but implied:

    4 - 250 kcmil means 4 paralleled per phase.  The neutral is unspecified, could be somewhat reduced, depends on the loading, Equipment Bonding Conductor unspecified.

    The area of 250 Kcmil is 127 mm2

    Raceways could be:

    4 each conduit  -  each with 3 - 250 kcmil individual phase conductors, a neutral (possible reduced) and maybe an Equipment bonding conductor

    Or perhaps:

    cable tray with 12 - 250kcmil individual phase conductors, 3 each unspecified neutrals, no EBC,  Cables grouped up (tied) in three phase conductors and 1 neutral.

    Or

    4 each 4Conductor - 250 kcmil, MC-HL w/g (Cable contains a full size neutral and usually segmented interstitial grounding (bonding) conductors.  

    Could be in cable tray, PVC conduit, or strapped to the wall

    CEC TEK cable is real similar to MC-HL

    This is likely what you call "4x(4x120mm2)"

     

     

    • 149 posts
    January 16, 2020 4:38 AM PST

    Hi Carl,

    What is the Ampacity capacity for 250kcmil cable?

    • 67 posts
    January 16, 2020 9:08 AM PST

    NEC T310.15.B.16

    75C terminations

    No more than 3 current carrying conductors in the raceway, cable, direct burial

    250 kcmil = 255A

    edit to add

    That would be for copper conductors


    This post was edited by Carl Coulter at January 16, 2020 9:10 AM PST
    • 149 posts
    January 18, 2020 5:14 AM PST
    Carl Coulter said:

    NEC T310.15.B.16

    75C terminations

    No more than 3 current carrying conductors in the raceway, cable, direct burial

    250 kcmil = 255A

    edit to add

    That would be for copper conductors

    Carl, but why not using manufacturer catalog for ampacity and then applying derating factor for parallel in a cable tray. I am saying this because manufacturer catalog has higher ampacity.