Combiner Box > Why fuse the negative?

[Kleazy]

Background:​

I am building a combiner box. The install is in a Central American country and there won't ever be any inspection by any authority hence I build as it makes sense more so than what is required.

2 PV strings that will be combined in parallel inside the box.
Currently doing design research online here and on YouTube.

Many designs use a fuse for each string and then a double circuit breaker for positive and negative. It seems excessive.
And of course a SPD which I am a believer in.

Then I look at the Midnight Solar "minimalistic" approach of only fusing or circuit breaking the positive while the negative is permanently wired towards the charge controller.

Questions:​

1. What does US code require for PV combiner boxes?
2. What is the point of using fuses AND circuit breakers for the same circuit in series?
3. What is the advantage/disadvantage of protecting the negative end of the circuit with a breaker?

 

[timselectric]

1. What does US code require for PV combiner boxes?

There is no code specific to combiner boxes.
But there are codes for the PV circuits.
OCP (fuse or breaker) is required on parallel strings, if the available current exceeds a single strings rating. (Usually takes 3 or more strings to reach the requirement)
The breaker in these combiner boxes double as a disconnect.
Code requires that all ungrounded conductors of a circuit be disconnected (opened) simultaneously.
The two pole breaker is a disconnect requirement.

2. What is the point of using fuses AND circuit breakers for the same circuit in series?

None, other than redundancy.

3. What is the advantage/disadvantage of protecting the negative end of the circuit with a breaker?

None, other than compliance with the disconnect requirement.

 

[Brucey]

Background:​

I am building a combiner box. The install is in a Central American country and there won't ever be any inspection by any authority hence I build as it makes sense more so than what is required.

2 PV strings that will be combined in parallel inside the box.
Currently doing design research online here and on YouTube.

Many designs use a fuse for each string and then a double circuit breaker for positive and negative. It seems excessive.
And of course a SPD which I am a believer in.

Then I look at the Midnight Solar "minimalistic" approach of only fusing or circuit breaking the positive while the negative is permanently wired towards the charge controller.

Questions:​

1. What does US code require for PV combiner boxes?
2. What is the point of using fuses AND circuit breakers for the same circuit in series?
3. What is the advantage/disadvantage of protecting the negative end of the circuit with a breaker?

So with the Midnites they tend to have three voltage levels for their combiner box fuses

150V DC breakers, 3/4"
300V DC breakers, double wide
600V DC fuses.

The breakers can be opened under load and used as a disconnect. The fuses cannot be.

So depending on if your charge controller is a 150/250/high voltage model you would use appropriate breaker or fuse. The Midnite mnpv3 and 6 are UL listed and the negatives go to a common bus per their diagram.

Now if you use the mnpv6 as a split bus unit with three breakers on either side going to separate controllers you may need to keep the negatives seperated but for a single charge that's not a concern

Here's my mnpv6 cobbled together with branch connectors

 

[Brucey]

Oh so NEC code I believe requires that when DC cables enter a dwelling it has to be in metal conduit. So if you had the mnpv3 on the side of your house, you'd have the solar cables coming up to it (mine just lay on the ground). Then from the bus I'll have 4 AWG coming into the house, in metal conduit to keep things "legit". Now I'm not sure if nec requires the disconnect to be within a certain distance from the meter/service entrance, in my case this will be feeding a portable system and not connected to the grid for export.

 

[Kleazy]

So with the Midnites they tend to have three voltage levels for their combiner box fuses

150V DC breakers, 3/4"
300V DC breakers, double wide
600V DC fuses.

The breakers can be opened under load and used as a disconnect. The fuses cannot be.

So depending on if your charge controller is a 150/250/high voltage model you would use appropriate breaker or fuse. The Midnite mnpv3 and 6 are UL listed and the negatives go to a common bus per their diagram.

Now if you use the mnpv6 as a split bus unit with three breakers on either side going to separate controllers you may need to keep the negatives seperated but for a single charge that's not a concern

Here's my mnpv6 cobbled together with branch connectors

View attachment 200900

I am using an Outback FM-100 charge controller that supports up to 300V PV input.
The Pv's are "BlueSun" BSM50M10-72HPH
The NMOT Open Circuit Voltage is: 48.18V while the STC Voc is: 49.3V

I can safely series string 5 panels and max out around 250V.

My plan was to circuit breaker each string to 15Amps on the positive side only and then afterwards combine.
Leave the negative permanently wired between CC and PV.
Use one SPD for the combined string.

Do everything in DinRail including "bus-bar" via DIN Blocks with bridge taps.

 

[timselectric]

Oh so NEC code I believe requires that when DC cables enter a dwelling it has to be in metal conduit. So if you had the mnpv3 on the side of your house, you'd have the solar cables coming up to it (mine just lay on the ground). Then from the bus I'll have 4 AWG coming into the house, in metal conduit to keep things "legit". Now I'm not sure if nec requires the disconnect to be within a certain distance from the meter/service entrance, in my case this will be feeding a portable system and not connected to the grid for export.

It can either be next to the meter. Or a sign next to the meter can identify its location.

 

[Kleazy]

Oh so NEC code I believe requires that when DC cables enter a dwelling it has to be in metal conduit. So if you had the mnpv3 on the side of your house, you'd have the solar cables coming up to it (mine just lay on the ground). Then from the bus I'll have 4 AWG coming into the house, in metal conduit to keep things "legit". Now I'm not sure if nec requires the disconnect to be within a certain distance from the meter/service entrance, in my case this will be feeding a portable system and not connected to the grid for export.

Yes my system uses metal conducts for the PV to combiner box. Combiner box to Charge controller will be with 6x6 wire way and metal conducts.

There will be no external disconnect as I don't like the idea of people messing with my stuff. This is on-grid with potential selling. Let's find out if my power-authority requires an external disconnect switch. If they actually inspect I either incentivize them not to worry about it or wire something up. This is in Costa Rica.

I might start a build in progress thread in Show and Tell.

 

[Brucey]

I think you need to tap the negative and positive when installing an spd300DC

 

[Kleazy]

I think you need to tap the negative and positive when installing an spd300DC
View attachment 200910

I plan to combine with a DIN Rail block.
I think this will make for a lean clean install.

And yes the SPD will be one of those "ports".

 

[mciholas]

3. What is the advantage/disadvantage of protecting the negative end of the circuit with a breaker?

In theory, none. Each string is its own circuit so breaking one lead breaks the current.

In practice, if panel A negative rubs into panel B positive (say through the long term action of wind or through damage by animals), then panel B positive bypasses panel B breaker to panel B negative. On a bright sunny day, the short circuit current of the panel will flow in the negative lead and there is no breaker to interrupt that if there is none in the negative leads.

The short circuit current isn't all the much more than the MPPT operating point current, so really that shouldn't trip the breaker, but it is a cross circuit condition and might be good to protect it just because.

Breakers in the negative leads seems like a breaker in the AC neutral, seems kind of weird. The above was the only thing I could fabricate that made any half way sense.

Mike C.

 

[Kleazy]

In theory, none. Each string is its own circuit so breaking one lead breaks the current.

In practice, if panel A negative rubs into panel B positive (say through the long term action of wind or through damage by animals), then panel B positive bypasses panel B breaker to panel B negative. On a bright sunny day, the short circuit current of the panel will flow in the negative lead and there is no breaker to interrupt that if there is none in the negative leads.

The short circuit current isn't all the much more than the MPPT operating point current, so really that shouldn't trip the breaker, but it is a cross circuit condition and might be good to protect it just because.

Breakers in the negative leads seems like a breaker in the AC neutral, seems kind of weird. The above was the only thing I could fabricate that made any half way sense.

Mike C.

Good explanation and I feel the same about an AC circuit needing an OCP on neutral.

Anyways...I am reconsidering and will now designing it with one circuit breaker per each PV positive and negative (2 per string) with a double DIN-Rail breaker. Then combine the strings after that point. This way I can enable/disable each string with one flip.

And my combiner points will be done by Schneider Comb-bus bars so the entire form factor remains DIN-Rail. I have not seen anybody build that in a PV combiner box yet. Has 100A rating.

 

[Sennen]

If you have any smaller gauge wire on the bus bars, it's best to fuse those as the 300a fuse on the positive will happily melt a 16 gauge power wire without blowing

 

[DIYrich]

Breakers in the negative leads seems like a breaker in the AC neutral, seems kind of weird. The above was the only thing I could fabricate that made any half way sense.

Mike C.

in Tim's explanation, he said ungrounded conductors need a breaker. In AC the neutral is attached to ground. In panels, the negative is not necessarily attached to ground. Power can flow from the negative wire to ground.

 

[timselectric]

For clarification:
A breaker used as OCP, does not need to protect positive and negative.
But
A breaker used for a disconnect is required to open all ungrounded conductors of a circuit simultaneously.

If a breaker is used to cover both OCP and a disconnect. It's required to meet the more stringent requirements of the disconnect.

 

[Kleazy]

For clarification:
A breaker used as OCP, does not need to protect positive and negative.
But
A breaker used for a disconnect is required to open all ungrounded conductors of a circuit simultaneously.

If a breaker is used to cover both OCP and a disconnect. It's required to meet the more stringent requirements of the disconnect.

Yes. Makes a lot of sense now.
Thanks for the clarification.

My goal is OCP and disconnect hence will use double breakers.