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Thread: Wire sizing for 100% Rated Breakers

  1. #1
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    Wire sizing for 100% Rated Breakers

    Here's the situation:

    I have a continuous load that draws 2800A. Normally, I would size the circuit for 1.25 x 2800A = 3500A. But if I use a 100% rated breaker sized at 3000A, does the wire still need to be sized for 1.25x2800A = 3500A? Or will a 3000A feeder still work? (8) sets of 500mcm CU are existing, and I'd like to not upgrade those.

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    apply this to a PV interconnection

    I asked that question to keep it simple. The REAL situation is a little more nuanced. I have a 3000A service with (8) sets of 500mcm, and I want to interconnect a PV system line-side to that service. The PV system output is 2800A, and I'd like to avoid upsizing those service feeders if possible. Article 705.12(A) says "the sum of the ratings of the overcurrent devices connected to power production sources shall not exceed the rating of the service."

    Is the answer any different in this scenario? Can the (8) sets of 500mcm still handle the 2800A of solar with a 3000A 100% rated breaker?

  3. #3
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    230.90(A) says service conductors can't be smaller than the protection.

    Now that you updated the info, I'll have to think about it more, but you could always do a line side tap with the PV.

    Although intuitively, they are likely concerned that the building be be drawing no load and all generation is backfed to the grid, and the service conductors would be smaller than the combined protection, which is a problem.
    Ron

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    Quote Originally Posted by ron View Post
    230.90(A) says service conductors can't be smaller than the protection.
    ...
    Perhaps whoever 'engineered' the service thought that 100% rated service disconnect breaker allowed them to use the 90°C ampacity of the conductors. That is incorrect. We still have to use the 75°C ampacity but are required to use 90°C-rated wire. The other difference is we don't have to pad the continuous load portion by 25%.

    Another question is whether a 100%-rated breaker is even permitted to take advantage of the 100% rating on its line side in a service application. There is no exception for service conductors as there is in the feeder and branch circuit articles.

    @Matt Donovan

    By chance, is that 3000A breaker adjustable and trimmed to not more than 2880A?

    500mcm is only rated 360A@75°C
    8 x 360A =2880A

    Are you certain the load on the service is 2800A continuous?
    Last edited by Smart $; 08-11-17 at 04:33 PM.
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by Smart $ View Post
    P

    By chance, is that 3000A breaker adjustable and trimmed to not more than 2880A?

    500mcm is only rated 360A@75°C
    8 x 360A =2880A

    Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

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    Quote Originally Posted by infinity View Post
    Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)
    Oops! Yes, they are.
    I will have achieved my life's goal if I die with a smile on my face.

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    Quote Originally Posted by infinity View Post
    Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)
    That's what I get.
    The more they overthink the plumbing, the easier it is to stop up the drain

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    Quote Originally Posted by dkidd View Post
    That's what I get.
    Still the questions of whether the 100% rating applies to service conductors (I think not), and if the load is truly 2800A continuous...?
    I will have achieved my life's goal if I die with a smile on my face.

  9. #9
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    lets ignore the service...

    In the case of feeders, I think I've found an answer in 215.12(A)(1) expection 1. Paraphrasing: where 100% rated breakers are used, the feeder may be sized for 100% of non-continuous load + 100% of continuous load.

    For services, I'm not sure if the rules are different. I can't find a comparable exception.

    @Smart$, this 3000A breaker does not yet exist, but for arguments sake, lets say its dialed up to 3000A. I'm trying to decide if I can spec it as 100% rated and get away with the existing conductors.

  10. #10
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    Quote Originally Posted by Matt Donovan View Post
    ...
    @Smart$, this 3000A breaker does not yet exist, but for arguments sake, lets say its dialed up to 3000A. I'm trying to decide if I can spec it as 100% rated and get away with the existing conductors.
    As I see it 2400A of PV is the most you can connect to a 3000A service.

    Exactly what breaker are we talking about: the PV system breaker or the service breaker, or both? Are you tapping into an existing service? Or just using the service conductors of a demo'd service? I just feel like we are not getting the complete picture...
    I will have achieved my life's goal if I die with a smile on my face.

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