# Breaker Sizing

Solar Expert Posts: 29
Hey everyone,

I have done some searching, and cannot find the answer to my question.
My knowledge of electrical does not extend past solar system design.

I am trying to find out how to size breakers and fuses for my system.

Starting from the top, I will have 3 strings of 4 panels in series, going to a combiner box. They are 210 watt evergreen's 12v 11.23A.
Does this mean I should have a 20A fuse in the combiner box for each string.

How about farther down the line before the charge controller?

Is there a general rule of thumb, or could you point me in the direction of some good information?

Thanks all,

Island Mon

• Super Moderators, Administrators Posts: 33,313 admin
Re: Breaker Sizing

Assuming that the panels are Evergreeen ES-A-210, then looking at the spec. shee (PDF):

Isc = 12.11 Amps
Series Fuse Rating = 20 Amp
So, each string should have a series fuse or breaker rated at 20 amps.

The cable between the combiner and the charge controller should be able to handle the entire string's Isc maximum current times a couple of safety factors (this part I am not sure of--related to NEC code--somebody else who knows more can confirm/fix):

Isc*1.25 (sizing of branch wiring) * 1.25 (solar panel safety factor) = 12.11 Amps * 1.25 * 1.25 =18.9 amps per string

If the wire is sized correctly (larger that # of strings * 18.9 amp)--then you should not need a another breaker between the panels and the solar charge controller.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Breaker Sizing

The circuit breaker sizing depends on NEC 690.8 and the CB's duty rating.

NEC 690.8, Circuit Sizing and Current, states that maximum current is Isc x 125% and shall be considered continuous. An additional 125% safety multiplier is required for overcurrent protection devices (OCPD's) that are not rated for 100% continuous-duty (i.e, fuses).

An exception to 690.8 allows for OCPD's rated for 100% continuous-duty to be spec'd without the additional 125% safety factor.

If your planning on using continous-duty DC-rated CB's from Outback or other suppliers, the correct maximum current for each series string is 12.2 A Isc x 125% = 15.25 A. The next higher standard size CB is 20 A.

Three strings in parallel would be 12.2 A x 3 x 125% = 45.75 A. The next standard size for a single DC-rated 100% continuous-duty CB for the entire array is 50 A.

Finally, note that the 2008 NEC generally requires ground fault protection (690.5). However, an exception (690.5 x2) is made for uninhabited dwellings when equipment is grounded in accordance with 690.45.

HTH,
Jim / crewzer