Cable design help

[Markc]

Hi All.

Because of recent events I am asking if anyone could also confirm the cable size for the following system?

2 x 4kWp systems, ground mount, side by side. One per phase (only two phases coming to the property and neighbouring farm buildings)

The array is 200mts away from the incomer. (150mts buried direct in ground 50mts clipped direct up outside wall and though loft space)

Also your thoughts on running either a AC cable this length and putting inverters outside or running DC cables this long and inverters inside or may be even half and half?

ALSO

Has anyone any advice on G59 applications? As the system above would fall into the G83-1 notification (I have already checked this out with UKPN) the potential client is interested in a possible larger system in the G59 area. We have not done one of these yet. What are the pit falls, possible costs and requirements. I am awaiting UKPN to get back to me on this also.

Thanks in advance

P.S if anyone wishes to PM me on any of this please do not hesitate.

 

[yellowvanman]

When comparing the ac/dc voltage drop need to know what sort of dc current you expect, dependant on module type. Obviously it would be better to run the long distance with whatever has the least current, but in saying that if you've got 4 dc strings and 2 ac supplies, then the number of cable cores will affect costs.

If you do the calcs for max cable operating temperature of 70C then you have an added safety margin because the cable won't ever rise in temperature to that value because the current carrying capacity is well above the expected current(s).

 

[Guest123]

Sounds like 230/460V split phase system rather than 2 seperate phases of a 3P 230/400V system. Dont think it makes a difference to your installation though but might catch you out if you weren't expecting it.

 

[Markc]

Sounds like 230/460V split phase system rather than 2 seperate phases of a 3P 230/400V system. Dont think it makes a difference to your installation though but might catch you out if you weren't expecting it.

Thanks Lenny. I am treating it as 2 x 230V supplies rather than 2 phase. The dwelling has peak and off peak phases but after consultation with UKPN they are happy to alter the metering arrangement to allow connection of one G83-1 at no cost.

 

[Markc]

When comparing the ac/dc voltage drop need to know what sort of dc current you expect, dependant on module type. Obviously it would be better to run the long distance with whatever has the least current, but in saying that if you've got 4 dc strings and 2 ac supplies, then the number of cable cores will affect costs.

If you do the calcs for max cable operating temperature of 70C then you have an added safety margin because the cable won't ever rise in temperature to that value because the current carrying capacity is well above the expected current(s).

Hi Yellowvanman,

I'm just a little paranoid at the moment after my problems with high voltage and VD on another job. Some think the 70C is to high but after problems I agree it factors in some movement. My cable calculation software suggests 70mm so I think a half way DC, half AC might be preferred.

 

[adroughl]

Check out the maximum distance run for the DC side as per inverter specs! If you check an SMA sunny boy inverter manual you will see that maximum mounting distance from array is usually no more than 25m (depending on cable size). Therefore to run the DC cables 150m before mounting the inverter, you would be drastically going against the manufacturers recommendations.

 

[Markc]

Check out the maximum distance run for the DC side as per inverter specs! If you check an SMA sunny boy inverter manual you will see that maximum mounting distance from array is usually no more than 25m (depending on cable size). Therefore to run the DC cables 150m before mounting the inverter, you would be drastically going against the manufacturers recommendations.

Correct on the SMA but others don't state this. Don't know why SMA does. The ones I have installed it was on the AC side only not DC.

 

[BruceB]

Check out the maximum distance run for the DC side as per inverter specs! If you check an SMA sunny boy inverter manual you will see that maximum mounting distance from array is usually no more than 25m (depending on cable size). Therefore to run the DC cables 150m before mounting the inverter, you would be drastically going against the manufacturers recommendations.

Can you give me a reference for that? I can find no mention of it in the 4000TL installation manual.
Regards
Bruce

 

[adroughl]

Hi Bruce

I've just checked and it is for the smaller SMA inverters. I guess that you could rune the DC side the length as long as you did all your cable calcs etc. Personally, I would look at housing the inverters locally and running the AC side back along the long run. Thats just my personal opinion with no calcs etc done.

Andy

 

[BruceB]

I have not had the problem of a really long ac run yet, but going by the voltage rise problems reported here over recent months, I would certainly consider running it in dc rather than ac.
Regards
Bruce

 

[Dcf]

draka do an iphone app for sizing cables that is quite handy. free as well.

 

[Markc]

draka do an iphone app for sizing cables that is quite handy. free as well.

Hi thanks, Yes I have that as well as PC cable cal software. Just needed some confirmation as to what or how others size their long run cables for PV. I do have a size just wanted to confirm it more than be told how to do it.

Thanks

 

[Worcester]

@BruceB

The only big challenge we find with cables runs is if using LV DC, (48V / 24 V for some off grid stuff like Wind turbines etc) the voltage drop can be horrendous.

With the Large PV strings and higher voltages >600V, sometimes we have to run a combined loop length including array of >200 m, because of the size of the roof. In that case we use 6mm2 or even 10mm2 PV cable.

On the AC side with 200m + cable runs, the voltage drop is a bigger problem than the current carrying capacity, so needs to be big enough to cope. That could be an expensive upgrade though.

With long runs even with a G59 certified inverter we are considering putting a G59 relay at the DNO Connection end, so the relay is detecting over / under voltage, feequency etc there, and running a control wire to a remote disconnector at the inverter end, then adjusting the inverter to run at higher voltages to cope with the losses, even at £1500 - £2000 for the relay plus install costs, that can be a more cost effective option if a cable is already laid. The G59 Relay is then the item that the DNO wants / needs to see is working correctly.

This also means that you can use non type approved inverters as the G59 relay is what the DNO wants. These inverters can then be sourced from variuos locations (US brands not generally marketed in the UK become a popular source)



The Sunny Design and

 

[BruceB]

useful thoughts, thanks

 

[adroughl]

Hey Guys

To take another point from the start of the thread, there is a comment about the system being ground mounted array. I am currently looking at a similar project and have heard issues about needed a fence around the array with minimum distance between fence and panels (2m??). This was general chat and if anyone has any info on requirements (I know- read DTI etc!) just to give me a bit of overview that would be great.
Andy