# circuit breaker and midnite combiner box mnpv6

Solar Expert Posts: 39✭✭
i have 4 panels [I'd like to add one more using a combiner box] tied in parallel using 3 sets of mc4y connectors. My (isc per panel is 8.57) so i am assuming the correct mathematics when it comes down to choosing the proper circuit breaker in my case would be 4 * 8.57 = 34.28 * 125% = 42.85 rounded 45.00 amps .

and for my newly added panel with the same isc, I'd simply multiple 8.57 * 125% = 10.71 rounded to a 12 amp circuit breaker .

Are my four panels currently tied in parallel considered a string and if so, do I leave it like that and just purchase a 45 amp circuit breaker? and why is that I can only find 2 pole 45 amp breakers, looks like it's twice the size of a regular circuit breaker.

• Solar Expert Posts: 1,032✭✭✭✭
Looks like you should be good with 12 amps. What is the wire good for ?

The combiner can usually be 15 amps per string which is "usually" the series fuse rating of the modules.

Two pole MidNite Solar breakers ?? Two poles in series ? If so, that's for 300V Voc (not 150 Voc) There are lots of
single pole breakers for 150 volt maximum string voltages.

Do you have a disconnect breaker downstairs ? That would be 45 or 50 or 60 amps and two pole or 300V if your strings are over 150V Voc.

boB
• Solar Expert Posts: 3,741✭✭✭✭
pyana1 wrote: »
Are my four panels currently tied in parallel considered a string and if so, do I leave it like that and just purchase a 45 amp circuit breaker?

A "string" is one or more panels in series, not parallel. Multiple strings can be put together (or 'combined') in parallel.

In your present system your string length is 1. Each panel is a string. You have 4 strings in parallel. The way you have them set up is not safe. When you combine 3 or more strings, each string must have its own circuit breaker. The way to combine your 4 strings is with a combiner box. Your panels should have a "series fuse rating". That is the size circuit breaker to use on each string in the combiner.

When you add a fifth panel, you will need a combiner box with room for 5 circuit breakers. The MNPV6 will do very nicely for that purpose.

The reason for a circuit breaker on each string is because if a panel shorts out, the current from the other strings could all pass through the shorted panel and cause a fire.

Once the panels are combined, the combined output will also need a circuit breaker. The breaker must be large enough that the panels can't produce enough current to trip the breaker... you don't want the breaker to trip just because it is very sunny out. The same breaker must be small enough to protect the wiring from melting if something goes wrong.

In your system the Isc per string is 8.57 amps. After you combine the 5 strings, the Isc is 42.85 amps. There are two 125% rules, and you may need to multiply by 125% twice... that depends on the particular circuit breaker.

The first multiplication by 125% is to make sure the breaker doesn't trip if the panels put out more than Isc for a few moments (42.85 amps X 125% = 53.56 amps)

Depending on the circuit breaker you use, you may need to multiply by 125% again. Some circuit breakers will trip if they carry their rated current for awhile. If your circuit breaker was of this type, you would multiply by 125% again... you don't want the breaker to trip if it carries 53.56 amps. Other breakers can carry their rated current indefinitely... in other words they can carry 53.56 amps forever without tripping. I believe that the DC breakers that Midnite sells can carry their rated load indefinitely, so any Midnite DC breaker that is 53.56 amps or larger will be correct for your system.

As a practical matter, you will round up to the next breaker size... 60 amps. With a 60 amp breaker protecting your wiring, you need wire between the combiner and the controller that can handle 60 amps without causing a fire... that would be #6 copper wire.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 39✭✭
vtmaps wrote: »

A "string" is one or more panels in series, not parallel. Multiple strings can be put together (or 'combined') in parallel.

In your present system your string length is 1. Each panel is a string. You have 4 strings in parallel. The way you have them set up is not safe. When you combine 3 or more strings, each string must have its own circuit breaker. The way to combine your 4 strings is with a combiner box. Your panels should have a "series fuse rating". That is the size circuit breaker to use on each string in the combiner.

When you add a fifth panel, you will need a combiner box with room for 5 circuit breakers. The MNPV6 will do very nicely for that purpose.

The reason for a circuit breaker on each string is because if a panel shorts out, the current from the other strings could all pass through the shorted panel and cause a fire.

Once the panels are combined, the combined output will also need a circuit breaker. The breaker must be large enough that the panels can't produce enough current to trip the breaker... you don't want the breaker to trip just because it is very sunny out. The same breaker must be small enough to protect the wiring from melting if something goes wrong.

In your system the Isc per string is 8.57 amps. After you combine the 5 strings, the Isc is 42.85 amps. There are two 125% rules, and you may need to multiply by 125% twice... that depends on the particular circuit breaker.

The first multiplication by 125% is to make sure the breaker doesn't trip if the panels put out more than Isc for a few moments (42.85 amps X 125% = 53.56 amps)

Depending on the circuit breaker you use, you may need to multiply by 125% again. Some circuit breakers will trip if they carry their rated current for awhile. If your circuit breaker was of this type, you would multiply by 125% again... you don't want the breaker to trip if it carries 53.56 amps. Other breakers can carry their rated current indefinitely... in other words they can carry 53.56 amps forever without tripping. I believe that the DC breakers that Midnite sells can carry their rated load indefinitely, so any Midnite DC breaker that is 53.56 amps or larger will be correct for your system.

As a practical matter, you will round up to the next breaker size... 60 amps. With a 60 amp breaker protecting your wiring, you need wire between the combiner and the controller that can handle 60 amps without causing a fire... that would be #6 copper wire.

--vtMaps

Thank you, I'm currently using a 60 amp mppt controller from morningstar . reply was on spot as needed . Thank you again 😎