DC circuit breaker selection question for PV application 1

[william879]

Hi All,

Here is a question coming from Ontario Canada.
I am trying to size the DC breaker and conductors for a Solar application.


The short circuit of the PV source circuit is calculated at 200A dc, 600Vdc

the ampere rating of the PV source circuit is 200Ax125%=250A dc

the continuous load of the PV source circuit is 200Ax125%=250A dc


The selected 600V DC circuit breakers carry standard 80% rating.

The frame size of the CB is rated at 400A.

Can I select a thermal-magnetic trip unit at 250A (=the continuous load), and select the conductor size greater than 250A?

Or I need to select a trip unit at 350A (which is the next plug size greater than 312.5A=250x125%), and select the conductor size greater than 350A?

Thank you very much for all the help!!!!!

Best....Jake

 

[jraef]

A standard breaker can technically CARRY 100% rated current, but per most rules you size the conductors at 125% of the FLC then you size the breaker to protect the conductors, so the breaker ENDS UP carrying only 80% of it's rated capacity. So when a breaker mfr says that the breaker is rated for 80% FLC, they are saying that the trip calibration and heat dissipation design is based on that expected use. You can order a breaker that is "100% rated" which generally just means different calibration of the trip unit and sometimes different lugs, but then there are other rules to follow when you do that and restrictions on the conductors you must use etc.

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