Solar and MCB 2 August 2022 at 10:37PM edited 2 August 2022 at 10:39PM in Green & ethical MoneySaving 6 replies 103 views

ispookie666

My curiosity was peaked following the recent discussion about RCD trips and went looking at my Consumer unit. 

My consumer unit is split into two 
1. Most of the house and Solar

2. Battery and EPS along with auto changeover contactor. 
I have a 6.6KWp solar install split over 4 roofs and have seen a peak generation of 5300KW, each micro inverter is able to output max of 290VA bringing this to 5800.  

The solar feeds into a Type C 20A MCB. The potential over current would be 4A max. 

 Is this MCB sufficient? Is this going to result in unwanted trips at peak generation?

On a side note, iBoost+ unit is knackered - 6m is pretty poor 

QrizB

I'm not an electrician but I'd expect a solar PV system with that capacity to be installed on a 32A circuit with 4sq.mm. conductors.

Heedtheadvice

The MCB is to protect the carrying capacity of the cables and provide a switch.

As per above by QrizB the circuit needs to be of sufficient capacity to carry the potential worst case current this will be the 6.6kW at the lowest voltage (216.2V). Not that 6.6kW is likely by the sounds of it on your system but play it safe.

That gives a max potential current of 30.5 A so a 32Acircuit and breaker ought to be used.

Now that is worst case, excluding faults, but looking at what you might normally expect as peak output you have witnessed;

5300kW could equate to 20.5A.

All this assumes current in phase with voltage so an allowance ought to be made for the enphase potential phase angle so maybe a nominal 10% extra  (I do not have figures to hand).

So even that witnessed power plus phase angle allowance rounded up to the next higher standard circuit carrying capacity would indicate 32A cable and MCB.

Now original sizing might take into account diversity (in that they all will not generate max at the same time it is assumed). 20A is still too low and the reduction in cost compared to a 32A system cannot really justify that down rating.....IMHO!

Upgrading to a 32A MCB, you need to confirm the cable is sized for that current (or higher) first before any change. The worst that will happen at present (assuming the cable can cope as is) will be a trip when you do not need it!

ispookie666

That was my thought.  
Will the MCB'S trip if the current carrying is just a few A above? 
I need to check the rest of the install for best practice 😳

NoobSolar

My installer replaced a fuse with a higher rating for me when the solar (6.4 kW and 6kw inverter) tripped it. I am not sure what the new fuse was, other than that it was higher!

ispookie666

Well, been to the installer and he insists 20A Type C MCB is appropriate.  Likely I'll end up in the same situation as @NoobSolar.  

Heedtheadvice

ispookie666 said:

Will the MCB'S trip if the current carrying is just a few A above? 

Not necessarily. Like most protection devices time is a significant factor and it come down to the time/current response curve.

1. Short circuit protection requires a virtually instantaneous cut off. Type C will cut off within a few milliseconds when a large overcurrent is detected i.e. five to ten times the rated current.

2. Over current protection time at rated current is about 100 seconds i.e. in your type C case 20A can flow for about 100secs before the breaker cuts. This is primarily to prevent damage to cables over heating. As the current rises the time follows a response curve getting shorter with increasing current until the short circuit instant cutoff mentioned above occurs..

I guess it can be considered borderline in your installation in some respects and 20A could be the correct value for overheating protection of the cable in use. I would consider it penny pinching not to use the cable and MCB ratings that QuizB mentioned and if there is a long run to consumer unit from the enphase inverters it could also prevent overvoltage tripping caused by the ever so slightly higher resistance of lighter cable, a known potential issue.

None of us is in a position, not being privvy to your installation detail, to say your installer is incorrect or for that matter correct!. If you get tripping, especially if it is too frequent, or overvoltage faults on the generation, then that might be the time to reconsider. Do not just go and change the MCB for a higher current rating!