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Assistance on Axpert invert output CB


IrshadS
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Can you please advise on the amp for the invertor output breaker. 

I currently have 2 Axpert 5KVA (5KW) inverters connected in parallel, Generally in winter when i do have additional items running, then the Inverter 1 output breaker keeps tripping.

They both seem to be 20A CB. Can i increase this to 25A

AC to Inverter has a 40 A CB

 

IMG-20210627-WA0001.thumb.jpg.0692715345b794a51831ec2996c1bebb.jpg 

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You'd have to check what thickness the wires are from the AC Output from the inverter (possibly AC input as well if running Hybrid) to determine what circuit breaker you can use.

A circuit breaker is installed to protect the wires, nothing else.

At full tilt, each inverter would be pushing out 22.7A (5000w / 220v = 22.7).
At your current setup, each inverter is theoretically limited to 4400w (220v x 20A = 4400w)

Once you have confirmed the wires are thick enough and what current they can carry safely, then you can install a circuit breaker with a lower rating than the wires (to protect the wires).

I strongly recommend getting a qualified electrician to do this. If not done properly, you could potentially burn your house down or kill yourself.

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I would be cautious of looking at the suggestions and connections in the axpert manuals ... when you see a breaker on the neutral wire as per the manual ... eeesih.

This is an interesting topic ... circuit breaker protection for inverters ... 1 unit  alone ... easy ... you can fit a 20 curve D curve or curve 1 breaker .. .some people use a 32 amp breaker with 4 or 6 mm* wire ... to ease the inrush current.

When you start connecting them in parallel ... this is where I am interested to hear how others would do it.

I am busy preparing a 3 x 5 KVA axpert inverters ... supplied directly from a 63 amp circuit breaker in the meter .. then out to the main DB rated at 63 amps.

I saw an installation the other day ... it had 3 x single phase 63 amp single pole breakers connected to the inverter with 10 mm* wire and a 63 amp single pole breaker on the output. 

Would using a 32 amp triple pole breaker ... bridged at the top across the 3 poles ... out the bottom of the breaker connected to each  inverter using 6 mm* wire (the same length) ... then 6 mm* out the each inverter  ( maybe even 10 mm* all  the same length wire) to a bussbar at the top of a 63 amp single phase breaker or

a 6 mm* wire from each inverter to the top of a triple pole breaker ... with a bussbar at  the bottom .. this doesnt sound right ... I did send a email to CBI to query this ... but never got a response.

Circulating current to come to mind ... I am interested to hear how other do it. 

 

 

 

 

 

 

 

 

 

 

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6 hours ago, IrshadS said:

Can i increase this to 25A

Changing to a 25A breaker on the output side might let the inbuilt breaker on the inverter trip as I think that might be a 22A -25A breaker rated to protect the inverter. You are running the inverter load almost at maximum. Can you not rather move load over to inverter 2?

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Something you need to understand about circuit breaker ... the tripping characteristics. 

IT is critical that you use the correct breaker designed for the application.

Curve 1 or D curve breakers for example are used for motor startup (just an example) 

Then you get a breaker which is designed for an application like transformer loads ... an example is a magnetic table on a CNC lathe ... or the supply (400/230 V) for CNC machines ... this breaker I assume would be more suitable for big transfomer type inverters. 

Then you get the standard breaker ... the most common used in domestic applications. 

Increases the current rating of a circuit might not be the best option ... rahte rbe more specific with the type you install.

An example - I have always used D curve/curve 1 breakers for aircon ... to allow for the compressor startup ... however this is not longer a good idea on the new inverter/rotary compressor type aircons ... they tend to hum ... using a standard breaker works better.  

 

 

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87 - the only concern I have with taking one inverter out a parallel system ... the load capacity ... how would you control the load while one unit is not connected ... that means you would have to isolate the output ... which means you would need 3 output breakers ... if you have a 63 ammp breaker you would have to reduce it to a 40 amp while the 1 unit is not connected ? 

I am learning as I go ... so any advice or solutions would be appreciated. 

 

 

 

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23 hours ago, IrshadS said:

They both seem to be 20A CB. Can i increase this to 25A

Yes. 5000 VA / 230 V = 21.7 A, so they should be rated at 25 A each. [ Edit: this is a blunder! See the end of this post. ] The cable to the breaker should be rated for at least 25 A, and the cable to the load for at least 40 A. I have 4 mm^2 from each inverter, commoned at the DB (so ~30 A each, 60 A total).

But first check for loose connections. At around 20 A, slightly loose connections can get quite hot, which can cause the breaker to trip from overtemperature, even if the current is well within its rating.

Edit: Sigh. Although 25 A is adequate when the inverter is in battery mode, when in bypass mode, it can carry up to 40 A from the AC-in port. So the output cables and hence breakers have to be rated for 40 A.

Edited by Coulomb
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21 minutes ago, isetech said:

how would you control the load while one unit is not connected ... that means you would have to isolate the output ... which means you would need 3 output breakers ...

That's getting too fancy, I think.

If running temporarily with one inverter, just be careful with simultaneous loads. If you screw up, you'll trip a breaker, or more likely the inverter will stop with a fault code. For the hopefully rare times when this happens, you can afford that minor headache, and avoid a very complex setup.

I have one breaker on each output (don't skimp on that), but only one isolator for the AC-in. Occasionally I wish I had two input breakers.

But also, I have an automatic contactor that runs the house off utility when I "mash the mushroom" (operate the emergency stop button on my inverter system). With that, and one breaker in the switchbox (DB), I have a completely dead system to work on safely. But I get that not everyone wants to get that fancy.

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3 hours ago, Coulomb said:

That's getting too fancy, I think.

If running temporarily with one inverter, just be careful with simultaneous loads. If you screw up, you'll trip a breaker, or more likely the inverter will stop with a fault code. For the hopefully rare times when this happens, you can afford that minor headache, and avoid a very complex setup.

I have one breaker on each output (don't skimp on that), but only one isolator for the AC-in. Occasionally I wish I had two input breakers.

But also, I have an automatic contactor that runs the house off utility when I "mash the mushroom" (operate the emergency stop button on my inverter system). With that, and one breaker in the switchbox (DB), I have a completely dead system to work on safely. But I get that not everyone wants to get that fancy.

Interesting topic. I have something similar. Two parallel inverters. Breaker on the supply side of each and breaker on the output side of each. Breakers on the supply side are 40A each and on the output side are 50A each. Single pole (common neutral) on the supply and double pole on the output. I currently have one isolated and removed (long story) and the other is operating seemingly well. I tried to match wire lengths on the input and on the output. My mistake: I think I oversized the wires on the output because I had a future upgrade in mind. My wire size is 16mm2. I'm fairly convinced that even though I attempted to match the wires as best as I could I think with wires that size small differences can make a big difference. Opinions?

I can say that in most other respects the system seems to work well and has worked well for quite a long time (barring a lightning strike). 

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1 hour ago, Nonlinear said:

Breakers on the supply side are 40A each and on the output side are 50A each.

Generally, the cables and therefore the breakers on the input side would be larger, because the output can carry all the input current (when in bypass, the highest current case for the output), except for utility charging current (which flows through the input cables, but not the output cables).

1 hour ago, Nonlinear said:

My mistake: I think I oversized the wires on the output because I had a future upgrade in mind. My wire size is 16mm2.

Wow. You must have had trouble getting the copper into the AC-out terminals on the inverter. Generally, this is a sign of too-thick cables.

1 hour ago, Nonlinear said:

I'm fairly convinced that even though I attempted to match the wires as best as I could I think with wires that size small differences can make a big difference. Opinions?

I don't follow. Surely it's the difference in resistance of the two paths that matters. The thicker the cables, the lower the resistance, so the lower the difference between two paths with a given mismatch in length.

Duh. I've just realised I made a blunder with the AC output breaker sizes for 5 kVA inverters. The AC-out cables carry the up-to-5 kVA from the inverter in battery mode, but in bypass mode, they can carry up to (typically) 40 A from AC-in. More than 40 A continuous will be prevented by the AC-in 40 A breaker built into these models.

Utility charging in these inverters is typically limited to 3 kW at near unity power factor, so with inefficiency about 3.1 kVA, or 14 A @ 220 V. So strictly speaking, the input breakers should be at least 54 A, but I think that 50 A would be adequate. So I would use 50 A breakers on the input, and 40 A breakers on the output. Perhaps that's what @Nonlinear meant?

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  • 2 weeks later...
On 2021/06/30 at 2:29 PM, Coulomb said:

Wow. You must have had trouble getting the copper into the AC-out terminals on the inverter. Generally, this is a sign of too-thick cables.

It was a struggle (to put it mildly). For bonus points I can confirm I used the regulation SANS cable too which was even harder to do.

On 2021/06/30 at 2:29 PM, Coulomb said:

I don't follow. Surely it's the difference in resistance of the two paths that matters. The thicker the cables, the lower the resistance, so the lower the difference between two paths with a given mismatch in length.

Correct. The problem though is small differences can have a large impact when resistance is already low. So, for example, with a very large (say 16mm2) cable the resistance on each could be very low, about 0.02Ohm. Now a small difference due to terminations could make the one cable 0.025 and the other 0.015. Now you can see that the resistance differential is potentially a problem. If you use smaller cable (say 6mm2, which would still be legal) then the nominal resistance would be about 0.1Ohm, now a small change in due to terminations would only change that from 0.105 and 0.095. So now a small resistance change is not a problem. 

On my two inverters I have noticed that despite my best effort at getting the cable lengths exactly the same, and terminating exactly the same way, the one inverter always runs with more load. There may be other reasons for this that I am unaware of.

On 2021/06/30 at 2:29 PM, Coulomb said:

So I would use 50 A breakers on the input, and 40 A breakers on the output. Perhaps that's what @Nonlinear meant?

Indeed it is.

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4 hours ago, Nonlinear said:

The problem though is small differences can have a large impact when resistance is already low.

Yes, you're right. Higher resistance, despite the higher losses, would make it easier to share the loads. My bad.

Certainly, 16 mm² is overkill for that application.

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