MCS & System Design

[VMan]

Ok guys,
On the system design side of things, what is required for the MCS assessment?
Our install is to be 2.25 kWp system, with inverter mounted approx 15 meters from array,and Ac cable to CU about 5 meters away.
i have done some basic cable calcs for DC cables, (to confirm Vd < 3%) and got a schematic for the install, what else would be required??

Thanks

V

 

[MichaelM]

You'll also need to calculate wind loadings:

F = q[SUB]s[/SUB] C[SUB]p,net [/SUB]C[SUB]a[/SUB] A[SUB]ref

For example:

Dynamic wind pressure qs = 1243 Pa (from Table 1 for a building 10m high in zone 2 at an altitude of ≤ 100m)
Cp,net = -1.39 (uplift) 1.00 (pressure) (using recommended values)
Ca= 1.00
Aref= 2m x 2m = 4m2 …
F = 1243 x -1.3 x 1.0 x 4.0 = -6464 N (upwards acting)
F = 1243 x 1.0 x 1.0 x 4.0 = 4972 N (downards acting)


The PV module and its fixings should be designed to withstand an uplift force of 6464 N and a downward acting force of 4972 N. If the module is supported by (say) 4 hooks, each hook, its fixings and the supporting roof structure should be designed to resist a quarter of the wind force.

This is pain but a requirement under the MCS. I can't for the life of me remeber the MCS document number on wind loadings though...

You can find a wind map for the zones here:


[/SUB]

 

[VMan]

pants, this is the 1st i have heard of this, on our PV curse we were told to asses the roof visually and if unsure call in a structural engineer for some calcs!

 

[MichaelM]

We got pulled up on this on our first MCS audit (nearly 4 years ago now) so I assume they are still looking for it now. You'll also have to calculate the estimated output of the system using SAP and go through the system voltages/amperage etc hope this helps..

 

[VMan]

PV array is to be mounted <200mm above the roof surface in the central available roof area
Module Width 994 mm
Module length 1652 mm
No. Of Modules 9
PV Array Area 14.8 m2 (Aref)
The site is located in ---( Zone 3 Figure 1)
Building Ridge Height 10 m
Site is in hilly area (Use Table 2)
Site altitude is approximatly 160 m above sea level

Dynamic wind pressure 2954 Pa (qs) (Use Tabel 2)

Wind Uplift -1.3 (Cp,net) (Mounted >100mm <300mm from edge of roof)
Wind Downward pressure 1 (Cp,net)

Wind Force F= qs Cp net Ca Aref
Upward Force = 2954 x -1.3 x 1 x 14.8 = -56754 N
Downward Force 2954 x 1 x 1 x 14.8 = 43657 N

Ok so is this correct, now how would i go about determining if the fixings will withstand this sort of pressure??? this seems like one hell of a force ???

 

[VMan]

Thanks Michael, the SAP not a problem, the system output not a problem either, its just how to get it all down to show the assessor, for the system voltage / amps would something like this be sufficient you think?

Modules: 9 x Sharp Nu-250
Array Configuration 1 String
Peak Power 2.25 kWp
V oc 37.9 V x 9 = 341.1 V (Voltage Open Circuit)
I sc 8.76 A x 1 = 8.76 A (Short Circuit Current)
V mp 31 V x 9 279 V (Maximum Power Voltage)

DC Voltage & Current Rating for Cables
Voltage: 279 x 1.15 = 320.85 V
Current: 8.76 X 1.25 = 10.95 A
Voltage Drop Max 3% = 9.6255 V

any advise would be great, thank you.

V

 

[Solar_Powell]

Good point MichaelM

VMan - another factor that will need to be taken into consideration is Snow. For a quick reference on wind and snow loads for your area (UK) check out the Kingspan website (resource centre). if you've got deep pockets you could buy the British Standard Books.

Wind Loading – BS6399: Part 2: 1997 incorporating amendment No.1
Snow Loading - BS6399: Part 3: 1988

i hope this helps

 

[VMan]

I know this a stupid question now, but what do i do with the wind load calcs now? do i find manufacture rating of fixings and see if they are suitable / decide how many fixings i will need?
I cant find data sheets for Click-fit, which i intend to use?
Again any help would be great, or perhaps if i could possible send a pdf with my system design and calcs on it to a couple of you guys and you could tell me whats missing??

Cheers.

 

[Solar_Powell]

Click Fit :whatchutalkingabout, personally i'd run away from using it - the thought of the array weight+wind+snow loads on a roof hook placed over a batton does not sit with me. if however you feel that it's the right mounting system for you then they have a excel calculator on thier website. An alternative fixing system would be IBC Solar, Hilti, Schuco...

 

[VMan]

Your right there about the fixings actually, but on the other and drilling through the rafters to fix a coach bolt doesn't thrill me much either tho?!? especially when you cant gain access to loft space!

Any chance of a quick bullet pointers into what should go into the design side of things?

So Far:
* SAP Calcs.
* Schematic
* DC Cable Calcs inc Voc, isc Ect.
* Ac Cable Calcs
* Wind/Snow Loading

Would i need centres of mounting brackets to be written down or would i need a CAD design?

 

[solarsavings]

Click Fit :whatchutalkingabout, personally i'd run away from using it - the thought of the array weight+wind+snow loads on a roof hook placed over a batton does not sit with me. if however you feel that it's the right mounting system for you then they have a excel calculator on thier website. An alternative fixing system would be IBC Solar, Hilti, Schuco...

So I'm not the only one to worry about the cf system.

I guess if you lift every tile where you intend to fit a roof hook to check the condition of the batten and it's fixings and it's ok, then you can sleep soundly at night for the next few years.

 

[Worcester]

Ironically, we use this regulalry, I've got copies of the BRE windload and uplift test results and reports and they gave me the confidence, this methodology is also used by Worcester Bosch for their 48kg solar Thermal panels.

Installed properly (extra brackets if needed, don't just stretch the spacing) at 400 mm spacing the average load / bracket for panels and mountings is less than 3.5kg + snow and wind

Most of you know my background, and I tend to be ultra cautious, especially where loading is concerned, we have had great results with this and will continue to use it where suitable.

I sleep very soundly with this sytem:yes:, have nightmares just thinking about hanger bolts on slate!:38: (still can't get any test reports for those!!!)

 

[solarsavings]

Ironically, we use this regulalry, I've got copies of the BRE windload and uplift test results and reports and they gave me the confidence, this methodology is also used by Worcester Bosch for their 48kg solar Thermal panels.

I know a little about WB thermal kit chunky fixings, still replies on the integrity of the batten and tiles it's clamped to doesn't it.

Installed properly (extra brackets if needed, don't just stretch the spacing) at 400 mm spacing the average load / bracket for panels and mountings is less than 3.5kg + snow and wind

Counts for nowt if as I said above.........laying paper over cracks.

Most of you know my background, and I tend to be ultra cautious, especially where loading is concerned, we have had great results with this and will continue to use it where suitable.

Great results?

I sleep very soundly with this sytem:yes:

2.45am:nono:

have nightmares just thinking about hanger bolts on slate!:38: (still can't get any test reports for those!!!)

:juggle2:

 

[VMan]

Don't mean to be rude, but back on topic, in what way do i need to show these wind calculations with respect to the system design for the assesmen??
I have looked at a couple of mounting system data sheets and cannot find the forces / weights these systems can take.
I'm assuming i take the wind forces i calculated, and the find the forces the brackets can withstand and calculate the minimum number of bracket i need to secure the array?? this correct?

any idea how i can get the necessary data for the mounting systems??

Thank You

V

 

[BruceB]

Try this.
Schuco publish a whole load of manuals. I get them from the wholesalers site, but you can probably find them on the Schuco site.
Regards
Bruce

 

[VMan]

Hey Guys,

so my calcs area as follows:

PV array is to be mounted <200mm above the roof surface in the central available roof area
Module Width 994 mm
Module length 1652 mm
No. Of Modules 9
PV Array Area 14.8 m2 (Aref)
The site is located in ---( Zone 3 Figure 1)
Building Ridge Height 10 m
Site is in hilly area (Use Table 2)
Site altitude is approximatly 160 m above sea level

Dynamic wind pressure 2954 Pa (qs) (Use Tabel 2)

Wind Uplift -1.3 (Cp,net) (Mounted >100mm <300mm from edge of roof)
Wind Downward pressure 1 (Cp,net)

Wind Force F= qs Cp net Ca Aref
Upward Force = 2954 x -1.3 x 1 x 14.8 = -56754 N = -56.8KN
Downward Force 2954 x 1 x 1 x 14.8 = 43657 N = 43.7 KN

So now i need kN/m2??

-56.8/14.8 = -3.84 kN/m2
43.7/14.8 = 2.95 kN/m2

Say i use 4 brackets per m2, so i take 2.95/4 = 0.73kN (each bracket must withstand 730N right??)

how do i find out if the brackets can take 730N???Click-fit just gave me there calculator, but the spreadsheet only allows upto 1038Nm2 where i have 2950Nm2!!!

Argh!!! any ideas where to go from here??

Do i need to calculate how many brackets are needed for MCS or would calculating wind & snow lods be sufficient then passing these onto the supplier to supply necessary brackets ect?

Thanks

 

[Bryan McGrory]

Hi Guys,

I am the director of BMG Roof Surveys and we specialise in providing Desk Top Roof Loading Reports.

These reports have been accepted by both Building Control and MCS.

You can find out more information at www.bmgroofsurveys-solar.com

A Residential Property Report is priced at £70 and Flat Roof (including Ballast Calc) and commercial properties are priced at £90.

We carry PI Insurance and by having BMG carry out the calcs by qualified engineer and it gives all parties the piece of mind that they are carried out correctly.

If you have any questions please do not hesitate to contact me.

Many Thanks,

Bryan McGrory
Company Director
[email protected]
07738 470 160

 

[moggy1968]

I've never been asked for wind load calcs on my assessments. I've been asked about it so had to show I have considered it, but never been asked for calcs. I'm with ELECSA

 

[Bryan McGrory]

Hi Moggy,

In order to comply with the MCS regs you have to carry them out and MCS always look for sight of the Roof Loading Report for every install. To be honest I know very little about ELECSA however on reviewing thier documentation (Part A - Structure) it most definatley refers to establishing the static and wind loadings.

I find it mad that most Installers are not carrying out these checks. I have loads of roofs fail the check and the panels have already been installed. I would imagine its only a matter of time before we all hear about a major fail. Heres hoping im wrong!!!

We are here if you require ourservices and I am going to contact ELECSA to find out if they are aware of the MCS reg for this.

Many Thanks,

Bryan

 

[moggy1968]

I know that round this way 1m or less spacing on brackets is ok for uplift. We provide additional support for 35mm joists or joists with greater than 80cm spacing.
They seemed happy with that.
I'm sure they are aware of the regs and they were satisfied that we had considered it but didn't ask for detailed calculations.
when you say you have loads of roofs fail, can you quantify that. What sort of roofs and what is the cause of failure?

 

[Bryan McGrory]

There have been varying reasons for roofs failing. By failing I dont mean the roof has collapsed or anything, only failing when checking the loadings.

The most common factor is properties which are exposed and on high gruond. I would estimate that around 10-15% of the reports we have had fail.
Another common factor is snow loading. I woudl estimate that a good 15-20% of roofs fail due to this.
Costal areas and countryside areas are the most common areas where roofs fail.

We have now carired out over 1000 reports since our company forming in November last year so that gives you an idea of number of roof fails.

 

[Bryan McGrory]

I have just stumbled accross the ELECSA stance on wind loading. Elecsa - Adding Loadings to a Roof

It basically says "Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading". I would say they should be carried out on every install.

 

[Helio]

I have just stumbled accross the ELECSA stance on wind loading. Elecsa - Adding Loadings to a Roof

It basically says "Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading". I would say they should be carried out on every install.

In full it says (my bold font):

'Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading.

Both of these issues will need to be considered by contractors installing solar photovoltaic panels. In many cases this may involve structural checks and/or calculations being undertaken. Decisions involving these issues will form part of Microgeneration Certification Scheme (MCS) assessment so must be documented. '
​

Now, while I agree that it is no small matter, ELECSA (who we are currently using) do not say that you must undertake calculations; only that you have to justify your decision. I have asked ELECSA to clarify this in relation to MIS 3002, issue 2.1, which says in section 4.4.4 (again my bold text):

'The contractor shall ensure that the roof structure is capable of withstanding the
loads (static and wind loads) that will be imposed by the PV modules and their
mounting arrangements. If there is any doubt, a structural engineer must be
consulted. Guidance on the mechanical installation and wind loads are given in
BRE Digests 495 and 489 or BS 6399.'

So, if I read things correctly, currently, I could say that I have considered the roof's integrity and decided that my mounting system and modules are acceptable and that would be enough to satisfy the MCS inspector (at least for ELECSA) as long as it's documented. This seems vague and should be clarified. Surely, it should be that we either do need to have calcs done or not and the smart money, as Bryan would no doubt agree, would be on having them done.

We too were told to use common sense on our 2399 course and inform domestic customers if we thought a survey should be done, taking care not to offer any advice on structural matters which we are unqualified to do. The additional £250.00 for a structural survey cuts into margins but if we all have to do it, there isn't an unfair price advantage for those who do not have them done every time.

Scottish Borders Council Building Control, for example, require a survey with all PV installations.

It does feel though, that more and more burdens are being placed on installers all the time. EPCs, FiT chaos, Green Deal etc........

 

[jason121]

Were do you stand on PV solar kits, do they allow for wind load loading ?

 

[Bryan McGrory]

At least they have provided a bit of clarification on the matter.

The only issue I can see is can everyone work out wind loadings (including uplift) on every install. If not then how can you be certain your installation is safe?

Our reports confirm the static loads, the imposed wind loads and the uplift wind loads which from reading the MIS 3002 4.4.4 is what every intallation requires.

Our reports are priced at £70 for a Residential Pitched Roof Property and £90 for Both Commercial and Flat Roof (including ballast calc) properties. If £250 is being quoted walk away. Thats way too much for what you require.

 

[jason121]

I have been told you cannot calculated wind loading on a pitch roof all calcs are based on 90 degrees not 25 to 45
Any answers

 

[Bryan McGrory]

You can caluclate the wind loading on any angle, you just need to know how to do it. Whoever has provided you with this information has also been misinformed.

 

[jason121]

Not much to choose from post 70's , either fix at 600 centres or 1200mm.
We use one size rail on every job and mainly fix at 1200mm centres will noggins

 

[Bryan McGrory]

You have to refer to BS6399 as this will confirm the requirements for wind loadings.