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What is the logic for solar sub boards avoiding overload of main board bus bar

Alan B

Just looking at an installation now where there's a single phase 100 amp supply, high load and solar.

Initially I thought guidance says if I put solar on a seperate board I avoid going over 100 amps for the main switch and busbar.
But then thinking about it if I put the solar on a sperate board and it's feeding the house rather than the grid all I achieve is focusing the extra current flow on the main switch and one end of the bus bar. I guess whoever came up with guidance assumed the solar would be feeding the grid or there was more than one board taking the load, seems like a poor assumption.
It makes more sense if the limitation is heat dissipated by devices, 15W or so per RCBO or MCB at full load, which from memory at least is different to the guidance i have watched or seen in the past. I can also put the solar at the far end of the bus bar and distribute the devices running closer to max load.

If it's to do with heat dissipation I am down to working out how many devices are running at a significant proportion of max load, heat dissipated and max thermal load on the enclosure, data that's probably not available. 


To me it's more important to distribute the load over more than one board. 

I know there are a few boards that are rated at 125A, so far from what I have looked at a cheap make I don't trust or an expensive makes that tend to over inflate costs of doing things like upgrading to type A RCD's and I avoid in principle because of this.My hope is that Fusebox say it's ok with their 125A main switch,they don't have any instructions or data sheet available online

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  • I been thinking about it a bit more and reading a few manuals and think I have a few options.

    I have looked at the few 125A consumer units available and the costs are very high, schneider want about £68 for an RCBO and that's before I start looking for bi directional and switched neutral. I suspect the others are similar as they all derived from industrial / commercial units units. About £600 on materials for a domestic consumer unit isn't going to work. Size is also going to cause an issue.

    I have also found that the customers inverter probably supports a feature that can protect the consumer unit from currents exceeding 100A by dialing back generation after import exceeds a threshold. But this involves adding a CT etc, not sure I want to start playing inside a solar inverter.

    Then the best and easiest option for me is to move a couple of the big loads (around 40A) on to a small sub board, I can then be fairly certain the total load on the main DB will not exceed 100A.

    My main observation is that I have seen quite a few people in the past, probably on facebook say that it's acceptable to just use a second consumer unit for solar supply, but clearly not having read or understood that the solution is only acceptable with a fuse or other protective device protecting the consumer unit. 551.7.2 has to be read very carefully to be understood properly.



  • in reply to Alan B

    Hi,

    I haven't seen inverters with the feature you describe.  Often inverters have a means of limiting generation so as to limit maximum export to a set value and use a current clamp on the incoming tails to achieve this (often according to ENA G100) - that is different to what you are looking for.  In theory the feature could be abused to achieve your goal by putting the current clamp the appropriate way round on the tails feeding the CU and setting the export limit to 84A but if the inverter is only rated at 16A it may not permit an export limit of 84A to be configured.

    Also note that 536.4.202 doesn't permit you to use diversity in your calculations when working out how to protect the CU from overload so when you are moving the loads around you will need to make sure that the sum of the ratings of the remaining MCBs / RCBOs is less than 100A.  There is a BEAMA guidance document detailing this which basically gives you the choice of either making sure that the sum of the MCBs is less than the main switch (and any RCDs covering multiple circuits), or the protection of the supply to the CU is rated at less than the main switch (and any RCDs covering multiple circuits).  For many domestic CUs, protecting the CU by limiting the size / number of breakers is a problem as you will have 76A in the standard two rings and two lighting circuits alone, before you start looking at electric cooking, immersion heaters, etc.

    [ Edited to change 100A in the first paragraph to 84A on the assumption that the inverter is connected to the main CU, if it is connected to a separate CU then 100A was the right number. ]

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  • in reply to Alan B

    Hi,

    I haven't seen inverters with the feature you describe.  Often inverters have a means of limiting generation so as to limit maximum export to a set value and use a current clamp on the incoming tails to achieve this (often according to ENA G100) - that is different to what you are looking for.  In theory the feature could be abused to achieve your goal by putting the current clamp the appropriate way round on the tails feeding the CU and setting the export limit to 84A but if the inverter is only rated at 16A it may not permit an export limit of 84A to be configured.

    Also note that 536.4.202 doesn't permit you to use diversity in your calculations when working out how to protect the CU from overload so when you are moving the loads around you will need to make sure that the sum of the ratings of the remaining MCBs / RCBOs is less than 100A.  There is a BEAMA guidance document detailing this which basically gives you the choice of either making sure that the sum of the MCBs is less than the main switch (and any RCDs covering multiple circuits), or the protection of the supply to the CU is rated at less than the main switch (and any RCDs covering multiple circuits).  For many domestic CUs, protecting the CU by limiting the size / number of breakers is a problem as you will have 76A in the standard two rings and two lighting circuits alone, before you start looking at electric cooking, immersion heaters, etc.

    [ Edited to change 100A in the first paragraph to 84A on the assumption that the inverter is connected to the main CU, if it is connected to a separate CU then 100A was the right number. ]

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