DC Breaker Sizing

[SolarCurious1]

I am trying to size DC Breakers for a new system. I will have 36 panels on 4 strings. Each string will have 9 panels hooked up in series. The panels Isc/A is 13.93. Multiplied by 1.56, I get a rating of 21.765 amps, so I would upsize to a 25A breaker. For the voltage, each panel is a VOC/V of 49.75 volts. Multiplied by 9 panels and I get 447.75 volts. My inverters are Growatt's, which have a max voltage rating of 600V.

My question is related to selecting the breaker. Should I get a 2 pole, 25A 500V DC breaker or a 2 pole, 25A 1000V DC breaker? My understanding of the 2 pole configuration is that it splits the load to each pole on the breaker.

Thanks for any help on this.

Added questions for discussion:
1. I am looking at Chtaixi DC Breakers on Amazon. I plan on checking resistance through the breaker before installing them, but wanted feedback from anyone that may have used these breakers and if you recommend an alternate brand.

2. Probably a more difficult question but what size surge protector would I in the DC Junction box to protect this system if I chose to install one.

Thanks

 

[Brucey]

I am trying to size DC Breakers for a new system. I will have 36 panels on 4 strings. Each string will have 9 panels hooked up in series. The panels Isc/A is 13.93. Multiplied by 1.56, I get a rating of 21.765 amps, so I would upsize to a 25A breaker. For the voltage, each panel is a VOC/V of 49.75 volts. Multiplied by 9 panels and I get 447.75 volts. My inverters are Growatt's, which have a max voltage rating of 600V.

My question is related to selecting the breaker. Should I get a 2 pole, 25A 500V DC breaker or a 2 pole, 25A 1000V DC breaker? My understanding of the 2 pole configuration is that it splits the load to each pole on the breaker.

Thanks for any help on this.

Added questions for discussion:
1. I am looking at Chtaixi DC Breakers on Amazon. I plan on checking resistance through the breaker before installing them, but wanted feedback from anyone that may have used these breakers and if you recommend an alternate brand.

2. Probably a more difficult question but what size surge protector would I in the DC Junction box to protect this system if I chose to install one.

Thanks

You don't need overcurrent protection for a single or two parallel strings.

What I would suggest is an IMO disconnect which is specifically designed to interrupt DC repeatedly under load. Those chtaixi breakers should only be used as convenience disconnects (i..e don't operate them under load.)

I'd go with two of these dual string units, I've seen some use midnite spd600s mounted directly on these boxes. , one per string. Some jurisdictions won't accept these plastic enclosures and require metal just be aware.

IMO 600V DC Disconnect Rooftop Isolator Switch - 4 Pole (2 String)

Major Specifications Max Current Rating: 32A Max Voltage Rating: 700VDC Size: 7.08" x 3.85" x 4.2" Protection Rating: NEMA 4X Guaranteed arc suppression (3ms typical) Operator independent switching mechanism

www.currentconnected.com

 

[SolarCurious1]

You don't need overcurrent protection for a single or two parallel strings.

What I would suggest is an IMO disconnect which is specifically designed to interrupt DC repeatedly under load. Those chtaixi breakers should only be used as convenience disconnects (i..e don't operate them under load.)

I'd go with two of these dual string units, I've seen some use midnite spd600s mounted directly on these boxes. , one per string. Some jurisdictions won't accept these plastic enclosures and require metal just be aware.

IMO 600V DC Disconnect Rooftop Isolator Switch - 4 Pole (2 String)

Major Specifications Max Current Rating: 32A Max Voltage Rating: 700VDC Size: 7.08" x 3.85" x 4.2" Protection Rating: NEMA 4X Guaranteed arc suppression (3ms typical) Operator independent switching mechanism

www.currentconnected.com

Thanks Brucey - I did take a look at those. I did not know that the breakers should not be operated under load. The IMO disconnect seems like a very straightforward solution. Regarding the plastic enclosures, I have ordered a metal enclosure as the junction box, plan would be to connect both of these IMO's to the top of the box. Have you heard of jurisdictions not allowing the plastic box to be connected to the metal box (basically not wanting any plastic on the DC junction box). My system is ground mounted, not sure if that would make a difference. I know code said no plastic in structures for DC circuits, but believe you can use outdoors.

 

[SolarCurious1]

Brucey - IMO Offers this switch in a 32A and a 25A version. Based on my system, I would select the 25A switch is correct. With two strings being wired in a singe IMO switch, would the 25A rating be correct?

 

[Brucey]

Brucey - IMO Offers this switch in a 32A and a 25A version. Based on my system, I would select the 25A switch is correct. With two strings being wired in a singe IMO switch, would the 25A rating be correct?

Go with the 32A. That's a max limit and gives you flexibility for the future, it won't trip on overcurrent like a breaker but it's not possible to exceed with one or two strings

This one: SKU: SI32-PEL64R-4

 

[Brucey]

Thanks Brucey - I did take a look at those. I did not know that the breakers should not be operated under load. The IMO disconnect seems like a very straightforward solution. Regarding the plastic enclosures, I have ordered a metal enclosure as the junction box, plan would be to connect both of these IMO's to the top of the box. Have you heard of jurisdictions not allowing the plastic box to be connected to the metal box (basically not wanting any plastic on the DC junction box). My system is ground mounted, not sure if that would make a difference. I know code said no plastic in structures for DC circuits, but believe you can use outdoors.

Should be fine outdoors.

 

[SolarCurious1]

Should be fine outdoors.

Thanks for the feedback