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MCS & System Design

Discussion in 'Solar PV Forum' started by VMan, Jun 23, 2011.

Discuss MCS & System Design in the Solar PV Forum area at

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  1. VMan

    VMan Guest

    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??


  2. MichaelM

    MichaelM Guest

    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:

    Last edited by a moderator: Jun 23, 2011
  3. VMan

    VMan Guest

    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!
  4. MichaelM

    MichaelM Guest

    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..
  5. VMan

    VMan Guest

    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 XXX( 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 ???
  6. VMan

    VMan Guest

    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.

  7. Solar_Powell

    Solar_Powell Guest

    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
  8. VMan

    VMan Guest

    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??

  9. Solar_Powell

    Solar_Powell Guest

    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...
  10. VMan

    VMan Guest

    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?
  11. solarsavings

    solarsavings Electrician's Arms

    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.
  12. Worcester

    Worcester Forum Mentor

    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!!!)
    Last edited: Jun 25, 2011
  13. solarsavings

    solarsavings Electrician's Arms

    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.

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

    Great results?


  14. VMan

    VMan Guest

    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

  15. BruceB

    BruceB Electrician's Arms

    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.
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