[RE-wrenches] Undercurrent Protection?

[eric at harvesthesun.com]

 I realize this is potentially a stupid question, and one that is long overdue, but here goes:
 
Why in 690(B) are we setting a minimum threshold on OCPD when we're protecting against over current?
 
Shouldn't we be setting a maximum threshold...as is done in pretty much all other cases? The way I see it, we should take the array/inverter max output AC current and multiply by 125% to set our min ampacity on wire,
and then calc down to find the breaker trade size that lands between the max AC output current and max AC output current times 125%. That way, we avoid nuisance tripping but achieve the max overcurrent protection possible.
By following 690(B) we're actually decreasing OCP: Literally, 690(B) is stating that we can have a breaker 100x over the circuit rating, so long as it's not under the circuit rating times 1.25. To me this seems antilogical.
 
Mr. Brooks?
 
690.9(B) Overcurrent Device Ratings. Overcurrent device ratings
shall be not less than 125 percent of the maximum
currents calculated in 690.8(A).
 
455.7(B)...The overcurrent protection determined
from this section shall not exceed 125 percent of the
phase converter nameplate single-phase input amperes.
 
450.4(A)...Such overcurrent device
shall be rated or set at not more than 125 percent of
the rated full-load input current of the autotransformer.

 
450.5(A)(2) Overcurrent Protection. An overcurrent sensing device
shall be provided that will cause the main switch or commontrip
overcurrent protection referred to in 450.5(A)(1) to open if
the load on the autotransformer reaches or exceeds 125 percent
of its continuous current per-phase or neutral rating.
 
450.5(B)(2)(b) Ampere Rating. The overcurrent protection shall be
rated or set at a current not exceeding 125 percent of the 
autotransformer continuous per-phase current rating or
42 percent of the continuous-current rating of any seriesconnected
devices in the autotransformer neutral connection.




 
 
 
Eric Stikes
Founding Director & CEO
Good Sun Solar, A CA non-profit corp.
www.goodsun.life
(530) 559-5023
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