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Thread: CB Sizing and safety factor

  1. #1
    Join Date
    Jan 2018
    Location
    LA, Ca
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    1

    CB Sizing and safety factor

    I've got an Inverter 330kW with 3 output legs which I'm using 175A fuses for each leg. Can I use 2/0 in place of 3/0?
    For 175A fuse, 2/0 (195A) provides 11.4% safety factor. Using the ambient temp correction factor of 0.96 (NEC 310.6), I get 187.2A on a 2/0 which gives me 10.69% safety factor.
    Please let me know your thoughts would greatly appreciate also please reference any NEC/UL clause.
    Thanks

  2. #2
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,725
    Quote Originally Posted by qwertyRP View Post
    I've got an Inverter 330kW with 3 output legs which I'm using 175A fuses for each leg. Can I use 2/0 in place of 3/0?
    For 175A fuse, 2/0 (195A) provides 11.4% safety factor. Using the ambient temp correction factor of 0.96 (NEC 310.6), I get 187.2A on a 2/0 which gives me 10.69% safety factor.
    Please let me know your thoughts would greatly appreciate also please reference any NEC/UL clause.
    Thanks
    What kind of inverter is this, for PV? What is the system voltage? I am having trouble figuring out a system voltage that would possibly fit with the given information. No one really adds safety factors above a given OCPD size for conductors. In fact, for 800 amps and less, we often have conductors with LESS ampacity then the rating of the OCPD. Note that continuous loads require a 25% safety factor which applies to conductors and OCPD's. What is the load?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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