If you;'re thinking of 551.7.2  then (my interpretation) is that when a generator is installed on a circuit shared with loads and there's no control of the layout, then it is possible for both the current flowing into the circuit (e.g. from the grid) In plus the output from the generator (ig) to be flowing in the same direction at the same time towards the load, and so if there aren't any compensatory measures yes Iz would then have to be ≥ In + Ig.

To apply the same logic to a circuit that supplies a single item of equipment which can be either a load (e.g. when charging) or a generator (when discharging) but can never be both at the same time, I think is illogical (but could be implied from the wording of the regulation).

  - Andy.

There is more to this than meets the eye, as it depends where between the inverter and the incoming mains the loads connect.

perhaps the inverter generates 16A max, and,  if in battery charging mode the current never exceeds16A then - so far so good,a cable for 16A ( or the larger of the two) is suitable.

But perhaps that connects to a dis board or CU  on a sub main  with another load attached - so the supply cable and fuse feeding into that could see that load + or - 16A, and needs to be rated for the sum.

Except if it is a house CU then the input fuse is 100A, the inverter may add another 16 - should the internals of the CU be rated for 116A ? perhaps !
Mike

Leccylee said:

But regarding EESS / Battery systems. where the current capacity of the cable and assemblies must be equal to all loads, for example 16A MCB plus 16A inverter means 32A Iz.

Maybe im being dumb, but why? Surely if inverter is running at full whack of 16A, current cant flow in both directions?

A student asked me this week and I was a little stumped, which as a trainer is somewhat embarrassing!

Until very recently (i.e. if the course and exam is based on the 1st and 2nd Editions of the IET CoP for EESS) the thought process was that, as the battery is both a load and a source, 551.7.2 (i) to (iv) applied. It's very 'black and white' as you say, but that's what BS 7671 says.

If the course exam is based on 2nd Ed of the CoP, that is what you must teach in order that the learners can pass the exams. This has the unfortunate side-effect that the cable often has to have at least twice the current-carrying capacity it needs to, as you've observed.

HOWEVER, In the last few weeks, the 3rd Edition of the IET CoP for EESS has come out. This has a different view of 551.7.2 (i) to (iv), and says that provided there are no other loads on the circuit, you size to I_Z ≥ {largest of I_n + I_g} where

I_Z is the current-carrying capacity of the cable in the installation conditions

I_n is the nominal rating of the overcurrent protective device in the CU into which the generator circuit connects

I_g is the nominal output current rating of the generator (inverter).

The guidance on what is now 551.7.2 (v) regarding the current rating of consumer units has also changed a little from 2nd to 3rd Ed of the CoP.. The 3rd Ed now contains 3 different approaches as to how to meet the requirement of 551.7.2 (v).

Why stop at that? 16 A from the roof, 16 A from the battery and 100 A from the mains?

Once again, it depends upon where the supplies join.

132 A for a house seems a lot. What is in there - a cannabis factory?

I appreciate that CoPs and standards must change with time, but the laws of physics do not.

Indeed my confusion, they cant be going in both direction at the same time.

Hi Graham, I was hoping you'd reply.
Totally appreciate the new addition and take all that onboard, thank you.

But can you explain a little more WHY in versions 1 and 2, it needs to be double the size etc. As I see it a battery system is either "charging" or "discharging", so how can it do both at same time and need to account for both?

My only answer thus far to students it must be due to "heating effect", but hate sounding so unsure to class as to "why"

Leccylee said:

But can you explain a little more WHY in versions 1 and 2, it needs to be double the size etc. As I see it a battery system is either "charging" or "discharging", so how can it do both at same time and need to account for both?

My only answer thus far to students it must be due to "heating effect", but hate sounding so unsure to class as to "why"

The indents (i) to (iv) in 551.7.2 are actually intended if someone wanted to connect a generator in a circuit that does have other loads, and is really not intended to consider the case that a battery storage system is used.

There is really nothing more to explain than the technical committee responsible for the IET CoP for EESS took the the stance that BS 7671 had to be complied with verbatim for the first two Editions, and some technical discussions took a more pragmatic view of Regulation 551.7.2 for the 3rd Edition - it's  that simple.

Understood, thank you Graham.