Breaker size

[solareclipse]

I have a tracer MPPT controller that can use 150 volts solar. I have 2 190 watt panels rated 45 volt open circuit they will be charging 24 volt battery bank, if I connect the panels in series which would be 90 volt opv how do I size the breaker? thanks for any help

[Sunking]

I have a tracer MPPT controller that can use 150 volts solar. I have 2 190 watt panels rated 45 volt open circuit they will be charging 24 volt battery bank, if I connect the panels in series which would be 90 volt opv how do I size the breaker? thanks for any help

If you are talking between the panels and controller, no breaker is needed or required if you wire them in series and size the wire properly. If you want to install a fuse or breaker it is sized by a minimum 156% of the panels Isc value.

[solareclipse]

If you are talking between the panels and controller, no breaker is needed or required if you wire them in series and size the wire properly. If you want to install a fuse or breaker it is sized by a minimum 156% of the panels Isc value.

Thanks, We have #10 solar Pv wire with about a 15' run from panel to controller.

[mapmaker]

Welcome to the forum, solareclipse,

Sunking's advice is correct, from a safety standpoint... The solar panel can't produce enough current to exceed the ampacity of its own wire.

However, there is a very good reason to install a breaker (make sure it is DC rated). The reason is that a breaker makes a good switch.

Most charge controllers require that the battery be connected to the controller BEFORE the solar panels are connected. When you want to turn off the controller, you must FIRST disconnect the solar panel, and then the battery.

It is much more convenient to flip a breaker than to disconnect the wiring from the solar panel to accomplish the above. Also, there are only a limited number of times you can make and break a wiring connection before it fails.

Of course, you must have a breaker (for safety, as well as convenience) between the battery and the controller.

--mapmaker

[ILFE]

Does this apply if you wire more than two panels in series as well? That is, no breaker / fuse being required between the panels and the controller?

[inetdog]

Does this apply if you wire more than two panels in series as well? That is, no breaker / fuse being required between the panels and the controller?

For a series circuit, one fuse or circuit breaker will open the entire circuit.
In some cases (for an ungrounded system) you may need to have a fuse or CB in both the + and - end conductors of the string, but the number of panels in the string has no effect on that requirement.

[Sunking]

Does this apply if you wire more than two panels in series as well? That is, no breaker / fuse being required between the panels and the controller?

Correct if there is only one string of panels regardless of how many are in series, no over current protection device is required. Two strings, even just two panels in parallel require over current protection devices.

[ILFE]

For a series circuit, one fuse or circuit breaker will open the entire circuit.
In some cases (for an ungrounded system) you may need to have a fuse or CB in both the + and - end conductors of the string, but the number of panels in the string has no effect on that requirement.

I understand one breaker would be needed, of appropriate size equal to the Isc of one panel, times 1.56?

My system is grounded.

Correct if there is only one string of panels regardless of how many are in series, no over current protection device is required. Two strings, even just two panels in parallel require over current protection devices.

This has me confused. If I have four 250 watt panels, all in series, Isc of 8.28 Amps, should I not have a fuse or breaker equal to 13 Amperes?

What happens if one (or more) panel ends up having a problem, causing it to short? Or, do the diodes serve as protection in series wiring of panels?

[mapmaker]

This has me confused. If I have four 250 watt panels, all in series, Isc of 8.28 Amps, should I not have a fuse or breaker equal to 13 Amperes?

What happens if one (or more) panel ends up having a problem, causing it to short? Or, do the diodes serve as protection in series wiring of panels?

The current in the circuit can never exceed 8.28 amps no matter what. The circuit breaker will never trip. As I mentioned earlier, the breaker is used as a switch, not for over current protection.

Two strings, even just two panels in parallel require over current protection devices.

Sunking, shouldn't that be MORE than two strings requires OCP in each string? With only two strings, if one shorts out the other string cannot exceed the current rating of the shorted string. (this all assumes, of course, that the two strings have the same series fuse rating).

--mapmaker

[ILFE]

The current in the circuit can never exceed 8.28 amps no matter what. The circuit breaker will never trip. As I mentioned earlier, the breaker is used as a switch, not for over current protection.

Okay. Sorry about that. Sometimes I am slow. I see exactly what you mean, now. Just wire it with the proper sized cable from the modules to the combiner box.

[Sunking]

This has me confused. If I have four 250 watt panels, all in series, Isc of 8.28 Amps, should I not have a fuse or breaker equal to 13 Amperes?

NONE REQUIRED. Minimum size wire is a 12 AWG copper. Most likely larger to overcome voltage losses. 12 AWG wire can safely conduct 16 amps to eternity. The over current protection device is there to protect the wire from over currents. A series string with an Isc of 8 amps cannot do any damage to a 12 AWG conductor

What happens if one (or more) panel ends up having a problem, causing it to short? Or, do the diodes serve as protection in series wiring of panels?

You still only have at most 8 amps on a 12 AWG wire. Again 12 AWG wire can handle 16 amps until eternity.

A panel is a current source, not a voltage source like a battery. Short a panel out and at most it can only deliver the Isc rating. In your case 8 amps. Short out say a small 12 volt 50 AH battery and you will get about 1000 amps enough to vaporize a 12 AWG wire and start a fire.

[ILFE]

Fair enough. Thanks guys.

Fuses just for the batteries and loads, then.

[inetdog]

Fair enough. Thanks guys.

Fuses just for the batteries and loads, then.

Unlike fuses, which generally are not safe to remove while current is flowing, a properly rated DC circuit breaker can be used as a disconnect switch if the CC (or inverter/charger) does not contain one.

[Sunking]

Unlike fuses, which generally are not safe to remove while current is flowing, a properly rated DC circuit breaker can be used as a disconnect switch if the CC (or inverter/charger) does not contain one.

Yeah but that is a bad idea Dave. Breakers are not deigned to be used as a switch. Using it as such weakens it.

[inetdog]

Yeah but that is a bad idea Dave. Breakers are not deigned to be used as a switch. Using it as such weakens it.

True for some DC CBs but not all. Have to look closely at the specs.
MidniteSolar offers two different lines of DC breakers at this time, panel mount and DIN rail mount. The DIN models are much more limited in capability.


And, of course, if the breakers are not properly DC rated and properly installed if polarity sensitive, the first time you open them under load may be the last.

As far as mechanical weakening of the springs and mechanism, there are families of CBs that are specifically rated for switch use (as in at least two operations per day for years), and even if not so rated, the number of times you would flip the breakers in a DC combiner would be small compared to the testing they are put through.