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Old 17-03-2020, 19:52   #1
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Breaker and solar wire run questions

Understanding (if I'm understanding correctly) that a breaker between the solar panel and the MPPT is there as a convenient "disconnect", what size a breaker?
I'm thinking about running 10/2 wire from each of my LG 370 watt panels approx 25 feet to Victron 100/30 Smart MPPTs ( one for each panel) with a Midnite Solar BabyBox about half way between. The BabyBox would be placed in a convenient and protected space. The MPPTs would be within 10 feet of the main positive and negative bus bars, thus reducing the required wire size. Is there a better way to wire this setup?? I'm going to have approx 35 feet wire run when all said and done. Two 370 watt and two 250 watt panels, each with its own controller (Victron 100/30 and 100/20, respectively).
Thanks for your input.
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Old 17-03-2020, 20:27   #2
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Re: Breaker and solar wire run questions

LG 370 panels appear to be 36V and 10A at MPP.

10AWG/25’ (50’ round trip) gives you well under 3% voltage drop, so fine.

If you want a breaker as a disconnect go 15A. 10A is just too close and is under Isc.

Overall what you have described is pretty much how I’d do it. Would probably omit the BabyBox, don’t like extra connections, but understand sometimes it is desirable for convenience.

Remember on the battery side each 100/30 will be putting out 30A (or close) at full power, I’d probably run #8 at 10’.
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Old 17-03-2020, 20:29   #3
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Re: Breaker and solar wire run questions

Thanks for the confirmation and input, Dsanduril!
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Old 17-03-2020, 21:03   #4
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Re: Breaker and solar wire run questions

To be completely legit on the solar panel side use a two pole breaker/ switch ( break all ungrounded conductors) and watch out for the DC voltage rating of whatever you select.

On Cbreeze three solar circuits each with 20A 48 VDC 2 pole breakers feeding Victron controllers, Outputs parallel to a 50 amp single pole breaker feeding the main bus bar (50 amp fuse on bus bar end). I also added a Blue Sea shunt ammeter in the negative lead reading the sum of the solar charging currents.


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Old 18-03-2020, 09:32   #5
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Re: Breaker and solar wire run questions

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Originally Posted by Dsanduril View Post
LG 370 panels appear to be 36V and 10A at MPP.

10AWG/25 (50 round trip) gives you well under 3% voltage drop, so fine.

If you want a breaker as a disconnect go 15A. 10A is just too close and is under Isc.

Overall what you have described is pretty much how Id do it. Would probably omit the BabyBox, dont like extra connections, but understand sometimes it is desirable for convenience.

Remember on the battery side each 100/30 will be putting out 30A (or close) at full power, Id probably run #8 at 10.
I would use at least #6 to the batteries
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Old 18-03-2020, 09:34   #6
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Re: Breaker and solar wire run questions

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Originally Posted by Frankly View Post
To be completely legit on the solar panel side use a two pole breaker/ switch ( break all ungrounded conductors) and watch out for the DC voltage rating of whatever you select.

On Cbreeze three solar circuits each with 20A 48 VDC 2 pole breakers feeding Victron controllers, Outputs parallel to a 50 amp single pole breaker feeding the main bus bar (50 amp fuse on bus bar end). I also added a Blue Sea shunt ammeter in the negative lead reading the sum of the solar charging currents.


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with a two pole breaker aren't you interrupting both the ground and power?
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Old 18-03-2020, 12:03   #7
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Re: Breaker and solar wire run questions

On the typical solar side of things there is no ground, just positive and negative connections. On the 12 volt output there is positive and negative with the negative grounded, so only single pole breaker/ switch required.


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Old 18-03-2020, 13:00   #8
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Re: Breaker and solar wire run questions

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Originally Posted by Frankly View Post
On the typical solar side of things there is no ground, just positive and negative connections. On the 12 volt output there is positive and negative with the negative grounded, so only single pole breaker/ switch required.


Frankly
I agree on using a two-pole breaker for other reasons, and on grid-tied solar there is frequently no connection, but with DC-DC MPPT charging it is cheaper (and therefore relatively common) to have the negative side of the controller pass through from battery to panels. The OP's controller manuals don't have a schematic, but do say this:

Quote:
WARNING: DC (PV) INPUT NOT ISOLATED FROM BATTERY CIRCUIT.
Their website FAQ says:

Quote:
Q5: What about grounding one of the PV array poles?

The MPPT is non isolated, meaning that the minus of the PV input is at the same potential as the minus of the battery output.
Grounding is always done at one point only, to avoid ground currents. For a battery system, grounding must be done at the battery.
Which would lead me to believe that at least one side is common all the way from battery to PV modules. If the battery negative is earthed then so are the panels and a single-pole breaker may work as an overcurrent or disconnect device. But since the panels are current limited, and the controller prevents reverse current, there isn't much point in a breaker for a single panel to a single controller. A two-pole toggle switch is just as useful, I use breakers because they are sometimes easier to come by and have better terminals.
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Old 18-03-2020, 13:25   #9
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Re: Breaker and solar wire run questions

Breakers are there to protect the wire.
Use 30 amp breaker to protect 10 gauge wire.

Breakers are sized by the wires they protect.
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Old 18-03-2020, 14:34   #10
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Re: Breaker and solar wire run questions

Solar panels are a little different as they operate as current sources not the more typical voltage source that makes up most electrical systems. As long as the wire between the panels and the controller is capable of handling the short circuit current rating of the panel (s), then no overcurrent protection is required/ needed. When you start looking under rocks for a say 48VDC rated switch, you are often led to circuit breakers as the most cost effective solution.

If I remember correctly my Victron controller has the negative solar connection common with the 12 V negative output. So in theory a single pole switch would suffice and will certainly interrupt the current flow from the panels. But now we have a wiring configuration that is controller dependent. Next controller might not be configured that way.

Been messing with this stuff for over 60 years. Cutting corners has a way of coming back to bit me in the butt.


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Old 18-03-2020, 14:41   #11
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Re: Breaker and solar wire run questions

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Breakers are there to protect the wire.
Use 30 amp breaker to protect 10 gauge wire.

Breakers are sized by the wires they protect.
If you are subject to the NEC then the calculation is actually the other way round, you determine the load/current, from that you select the required breaker size, and then from that you select the required minimum wire size. Wire larger than the minimum is quite acceptable - as long as you don't go so ridiculously large that the breaker terminals aren't rated for the wire size you select.

30A is the maximum size you could use, doesn't mean it is the size you should use. Wire can be sized for things other than ampacity, such as voltage drop, and there is no reason to use a breaker larger than the expected current. In fact, it defeats the purpose of a breaker - clearing faults - if the breaker is sized too large for the potential fault.

All of which is moot in the OP's configuration, he has a current-limited device on the PV side and thus a breaker in entirely useless (as a breaker), it only serves as a disconnect. Things could be different if there were multiple panels in parallel behind the breaker, but there aren't (in this particular case).

The one place where wire protection is needed is at the main bus. There should be a fuse or a breaker on that connection so that a fault anywhere between the bus and solar panel can be cleared.
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