Confusion over the 1% volt drop on AC side

[andyemms]

Could someone please advise, is this to be calculated from the inverter to the main C/U, or could this be to a sub board?

 

[BruceB]

In principle it should be back to the origin of the installation, just like any other circuit. However on large installations that could produce an uneconomic solution and by considering actual voltages and voltage drops you may, with some engineering judgement, come to a more reasonable solution.

Regards
Bruce

 

[andyemms]

I'm looking at a 4kw install connecting AC to an outbuilding sub board, sited 100m from main C/U supplied by a 10mm2 swa 3 core. Your thoughts?

 

[BruceB]

Without doing any sums, I recall a similar installation being discussed before on here and concluding that it would be most unlikely to meet the 1% voltage drop requirements. Run the numbers and see what it looks like on a sunny day based on actual supply voltages measured on an equivalent day.

 

[andyemms]

Thanks for your help BruceB.

 

[Graeme Harrold]

Using the data above, 16A on 10mm is going to give you a VD of 7.04V(cable mate). Not sure on the 1% drop as it was not mentioned on the course I just completed, but willing to know otherwise. As far as I knew it was the standard 5%. Teach me otherwise.........

 

[yellowvanman]

Remember the volt drop calaculations are with the copper @ 70C, in reality it won't be at that temperature unless you are running it at it full current carrying capacity. 10mm will have a CCC of >50A (depending on install conditions), so with 16A running through it, it will never get to 70C.

Thats only taking into consideration the PV current out, you would also need to look at the 'other' current(s) flowing.

You need to keep the volt drop down because the inverter needs to geenrate ac at greater than the mains, if the mains is up towards its upper limit (256V) then the inverter could struggle to generate anything higher.

 

[Graeme Harrold]

Just downloaded the SMA info, and fount the 1% info, Fitting the 4000TL and it states a minimum of 6mm cable must be used. Could have been a costly and head scratching time if Id gone with the 4mm as planned.

 

[andyemms]

Not sure on the 1% drop as it was not mentioned on the course I just completed, but willing to know otherwise. As far as I knew it was the standard 5%. Teach me otherwise.........

3% on the DC side, 1% on the AC side. Download the Photovoltaics in Buildings, Guide to the installation of PV systems and make it a regular read. Loads of info in there for what must be done and for what is good practice.

 

[andyemms]

Thats only taking into consideration the PV current out, you would also need to look at the 'other' current(s) flowing.

How would you calculate 'other' currents flowing? Confused.com

 

[yellowvanman]

How would you calculate 'other' currents flowing? Confused.com

By looking at the loads on the DB you've connected the PV system to, you should be able to work out the 'nett' current flow, which will either be flowing from the remote building to the main building or vice versa.

 

[andyemms]

Thanks for that, I was struggling to see what you meant by it, but the fog has now lifted!

 

[fiardor]

Hey guys having a bit of a nightmare with my MCS accredited electrician who i work with who insists that the volt drop is 5% and that a 2.5mm twin and earth cable is sufficient in all eventualities. He insists that the MCS told him its 5% and produced a course folder showing this and then tries to show me the calculations. But the DTI guide and everything i have read on here says 1%.

So my question is who is the master of all volt drop? BS 7671? DTI? MCS?

Oh and when i produced the SMA installation guidelines for a 4000TL and it showed 6mm for the a.c he just laughed and said it was overkill even though i pointed out that this was their manufacturing guidelines and they should be adhered to.

We are due an MCS visit soon and i am wondering if i should do something before that visit.

We have done atleast 200 instals with 2.5 T+E or 2.5mm FP200 (most 4kw and 4000TL)

Also we are using chipboard to mount these systems but i am sure they should be fire rated.

Many thanks

Fiardor

 

[Graeme Harrold]

BS 7671 first, DTI then over rides that, then Manufacturers instructions come top. If he cant get his head round that, find another electrician.

 

[Worcester]

At our very first MCS inspection he picked up on what it was mounted on and wanted to see the receipt
1/2 Hour flame retarded MDF is expensive and dificult to get hold of.
Instead we use ordinary MDF and a 1/2 hour fire resistant board mounted behind the inverter.


Here's SMA' recommendations:

• Long-term Heat Influences on the Wooden Ground
The mounting ground at the rear of the inverter is heated when the inverter is in operation. This happens for hours of every day, over a period of 20 years and longer. If the mounting is on wood the constant heating dries the wood out, making self-combustion a possibility.
SMA Solar Technology AG has had this influence examined and the results published in an external expert report. This report was based on timber typically used for building in Europe, and on the assumption of a vertical mounting of the inverter. The mounting ground was a solid wood wall, at least 10 mm thick. The different species of wood included: spruce, pine, beech, oak, larch, Douglas fir, Sipo, Meranti, linden and birch. The test was conducted exclusively for the SMA inverters Sunny Boy and Sunny Mini Central. The results also apply for other devices using the same housings (e.g. corresponding Windy Boy, Sunny Island models). Self-combustion was ruled out by the expert report. It is advisable, however, to ensure air exchange between the wooden wall and the housing, which provides additional safety. Thus, in the case of new systems with inverters with flat rear panels (e.g. Sunny Mini Central), washers should be used to establish a clearance of at least 0.5 mm between the metal and the wood.
There are instances where recommendation has been made to install sheet metal on the wooden wall, because this is noncombustible. However, this would not permit air exchange between the wooden wall and the sheet metal which is heated by the inverter. The prevention of air exchange creates the risk of selfcombustion. The above recommendation is therefore invalid and a mounting of this sort is not permissible. A statement concerning other wood products cannot be made (e.g. chipboard, fiberboard, etc.). Questions of suitability should be addressed to the manufacturer of the wood product. The assumed highest temperature which can be reached on a sustained basis during operation is 80 °C.

The 1% drop is in the DTi guide (see sticky at the top) http://www.electriciansforums.net/photovoltaic-solar-panels-green-energy-forum/23511-pv-guide.html

The a.c. cable connecting the inverter(s) to the consumer unit should be oversized to minimise voltage drop. A 1% drop or less is recommended. However in larger installations this may not be practicable or economic due to the very large size of cable resulting. In this case the designer should minimise voltage drop as far as possible and must remain within voltage drop limits as prescribed by BS 7671.

SMA make it clear 6mm2.

You'll need to go back and remount and rewire all your inveters.

Who signed them off as compliant ?