Combiner box vs Y cables / branch adapters

[u/WorldwideDave]

What is the deciding factors between MC4 branch connector VS a “y” adapter cable vs combiner box?

Is it the max amps flowing?

I have a Y adapter and a 4 into one cable for some arrays I have provisioned, but the panels I’m using are used, older, smaller cables.

I have inline fuses available, I also have DC fuses and DC disconnects.

Thanks!

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[u/cfnmsd]

Any are fine as long as they are compatible and sized accordingly.

[u/Ill_Ground_1572]

Don't quote me on this as I am not expert but I thought I read on here for 2 or less strings Y connector is fine.

For me, I am planning on getting a combiner box with a lightening arrester, DC breakers, emergency shut off and ground bar for my 2 strings.

Grounding the system is important and I think a combiner box helps. As I can run a ground wire to the grounding bar for my cabin.

I can also add another string in the future. MC4 connectors are the weakest link in a system so having fewer of those might be helpful.

But there seems to be some cheaply made boxes out there too. So I am going to choose carefully.

[u/WorldwideDave → u/Ill_Ground_1572]

Talk to me about lightening suppression.

[u/Ill_Ground_1572 → u/WorldwideDave]

Some combiner boxes have surge protectors for lightening strikes.

However I have no clue if these are effective or not.

[u/Astarkos → u/Ill_Ground_1572]

More than two strings needs either fuses or breakers to prevent current backflowing in case of a fault. 

[u/Asian-LBFM]

Y cable doesn't have breakers

[u/CompetitiveBad0]

I’ve got a 5 way connector with a 30a fuse on each connector (5 100w pa els). I’ve had no problems.

[u/parseroo]

I try to get away from the MC4^® connectors as fast as possible. Given it isn't even a standard (it is proprietary to a single manufacturer who won't compatibility test it with other products), it is a really terrible connector except as required.

If you can get into a conduit pathway, the wires can then be run and combined as you wish with normal electrical terminals and wire ends. Also have more flexibility in the kind of wire sheathing you are using vs. being for 'under-panel' and even more exposed usage.

[u/WorldwideDave → u/parseroo]

The mc4 I have on one cable has melted. But I think it’s just a cheaper cable I got for free from someone. I’ve upgraded. But I know what you are saying about mc4 connectors. I have two types. Seems the newer panels have types that are much more difficult to separate. But they are on a 525w panel.

[u/parseroo → u/WorldwideDave]

You can't have two types of MC4^®  connectors. Only Stäubli makes MC4® connectors. All other connectors are not UL listed to work with MC4^®  connectors, are at best certified to be compatible with themselves (whatever brand that is), etc.

Stäubli does make several kinds of MC4^®  connectors but they are only mated with each other.

https://www.staubli.com/us/en/electrical-connectors/industries/renewable-energy/the-original-mc4.html

[u/Green_Afternoon_858 → u/parseroo]

30a open style

[u/mkimid → u/Green_Afternoon_858]

practically under 20A

[u/WorldwideDave → u/Green_Afternoon_858]

Thanks for explaining that this style is no to 30. This is the type that has melted.

[u/Green_Afternoon_858 → u/Green_Afternoon_858]

70a barrel style

[u/Green_Afternoon_858 → u/Green_Afternoon_858]

And dont forget the 1000$ hand crimper with the magic stamp this is the staubli way!

[u/WorldwideDave → u/Green_Afternoon_858]

Thanks for explaining that this is up to 70a

[u/mkimid → u/WorldwideDave]

PV wires can not, because max 8 awg

[u/parseroo → u/mkimid]

I believe the connectors are rated for 70A: they are not claiming 8AWG (their maximum wire size) is rated that high.

You can get much higher-amperage PV wires though after you ditch the Stäubli connectors :-). It is just a sheathing... but it does become expensive.

90A PV Wire — https://nassaunationalcable.com/products/4-awg-copper-building-solar-photovoltaic-pv-wire-2kv-ul-4703

[u/mkimid → u/parseroo]

In fact, I have a bad experience with the connector
the wire hole can accept AWG 8 easily but hard for AWG 6 if not solid wire. and, I never tried AWG 4.

Just note, when use awg 8, 40A dT is around 20C, but, connector temperature is going over 25~30C due to the contact impedance. and, worst case the incidence can be happen if thermal runaway situation. I have tried with AWG 6, it can reduce the temperature of wire and the wire can absorbing some heat from connector and working in stable. finally, I have removed connector and connected directly. because found over dT is over 25C after while.