Advertisement
If you have a new account but are having problems posting or verifying your account, please email us on hello@boards.ie for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact hello@boards.ie

Cable size

Options
  • 18-05-2022 7:24pm
    #1
    Registered Users Posts: 10


    100m run from Consumer mcb Unit to Garage. Is 6mm sq cable OK?

    Post edited by 2011 on


«1

Answers

  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    What size is the load?

    What rating Is the MCB? Is it C or B type?



  • Registered Users Posts: 10 DonalOF


    A couple of socket outlets. Light and Small Tools only. Max 15amp. Haven’t installed the cb yet. Probably 32A ‘c’ type.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Why select a 32A MCB if the max load is only 15A ?

    If you selected an MCB of the same type rated at 20A then a smaller cable size could be selected than if a 32A was selected.



  • Registered Users Posts: 10 DonalOF


    I am wiring to a 3-way garage/shed consumer unit in garage (w/2 X16A and 1 X6A cb’s) and want to maintain the voltage high as possible from house consumer unit in case I invest in a bench saw in the future?



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    So what load are you sizing for??? Determining this is your first step. Next step is to select the rating of the protective device (MCB).

    Post edited by 2011 on


  • Advertisement
  • Registered Users Posts: 247 ✭✭hayse


    Op you need to do calculations. This will determine your cable size. 100 metres is quiet a run.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    The volt drop on this cable will be determined by two factors:

    1) The current drawn

    2) The resistance of the cable.

    The protective device will need to operate within the required disconnection time. A higher rating will drive a larger cable size. This is why it is inadvisable to use a 32A MCB for a long cable run if it will draw no more than 15A. You will drive the cost through the roof unnecessarily.

    I’m not sure what bench saw you have in mind but I wouldn’t expect it to draw a whole lot. I suggest you check the nameplate.

    Normally a diversity factor would be applied to a cable such as this.



  • Registered Users Posts: 209 ✭✭Bruthal.


    Motorized machinery may have it's start current last longer on longer runs of cable when undersized, possibly causing noticable dips in lighting etc on the shed end, and more chance of trips of devices supplying the cable.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Not so much an issue with LED,s.

    The one I have (Makita) has a built in soft starter which reduces the starting current right down.



  • Registered Users Posts: 10 DonalOF


    Thanks 2012, Hayse, and Bruthal. I have not decided on the make/size of Bench saw yet - and may not even put one in, but if I do - it will have it’s own dedicated cb and cable run within the Garage (thus thus the 3-way Garage shed Consumer Unit)…Tools I will be using (one at a time) are a mix of Carpenters Work Tools - 110V and 220V - all low load rating 5A/900W to 13A/1780W (some nameplates scratched etc).



  • Advertisement
  • Registered Users Posts: 4,957 ✭✭✭kirk.


    You'd take a couple of readings at the supply end, volt drop and fault loop

    You can calculate the volt drop and Zl at the shed end then using the resistances of the swa and socket circuit cables

    6sq doesn't leave any capacity beyond a couple of kilowatts depending on startup and Zl ,as said

    Should be looking at 10sq



  • Registered Users Posts: 10 DonalOF


    Thanks Kirk.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    I would be surprised if a 10 sq. would be sufficient if supplied from a 32A MCB. This is a very long cable run!



  • Registered Users Posts: 7,171 ✭✭✭10-10-20


    Roughly speaking:

    @235v AC, with a 15A load and a min of 3% V-drop (7.8v) over a 100m run, an 8 AWG cable is recommended here as a minimum conductor size.

    8 AWG is about 10mm.

    6 AWG (16mm) would give a V-drop of <2%.

    Play around as you need.



  • Registered Users Posts: 247 ✭✭hayse


    I would run a 16 square it’s a no brainier. Without doing any maths it was obvious 6 square wouldn’t do.



  • Registered Users Posts: 10 DonalOF


    Thanks all. I’m going with 10.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    As I am sure you know volt drop will occur on the final circuit and on the cabling to the main distribution board, neither of which have been considered in the above. In other words this is only a portion of the total volt drop. Additionally this calculation is based on a 15A design current and the OP is clearly unable to confirm that this value is correct.

    But volt drop is only part of what has to be considered, the OP must also ensure that the earth fault loop impedance is sufficently low to ensure disconnection sufficiently low.



  • Registered Users Posts: 247 ✭✭hayse


    There wouldn’t be much difference in 10s and 16s. Make the investment right now.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Clearly you have not assessed this sufficiently. It is time to get the assistance of a qualified electrcian.



  • Registered Users Posts: 10 DonalOF


    Clearly you have….daaawwwww…,you have all the information and could not give me a definitive cable size. BTW my Electrician told me 6sq off a 20A breaker in the Consumer Unit would be fine.



  • Advertisement
  • Registered Users Posts: 7,171 ✭✭✭10-10-20


    Yes. Free information at face-value aren't a replacement for a qualified REC and certification. 👍️



  • Registered Users Posts: 10 DonalOF


    Thanks everyone for all the Comments. No more now please.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    No, I don’t have all the information. Not even close! You still have not answered the first question I asked which is what is the load? You kind of answered by saying it was 15A but then you reversed out of this.

    Secondly more information is required such as length of final circuits, length of tails to main DB.

    Anyway it is good to hear you have an electrician and he seems to be taking a more sensible approach in terms of the reduced MCB rating (now 20A). No doubt he has done the calculations and 6 sq. will be sufficient.



  • Registered Users Posts: 209 ✭✭Bruthal.


    Well, it was more about the affect on voltage then just the lights themselves. They might be leds, they might not. Machinery might have soft starting, or maybe not.



  • Registered Users Posts: 209 ✭✭Bruthal.


    Sounds like a 6sq is good for all, sort of evaluation. Too small for that length on a 20A breaker would be my limited opinion.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Yup, and all of that should be assessed so that the cable can be sized correctly.



  • Registered Users Posts: 209 ✭✭Bruthal.


    Ok



  • Registered Users Posts: 247 ✭✭hayse


    If it’s a workshop aim to have 50 amps minimum available. Work back from there. This would be having a standard 12kva supply.



  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    50A??

    That is a random value! What is that based on???



  • Advertisement
  • Registered Users Posts: 247 ✭✭hayse


    I read it and have contributed with factual correct information. I wasn’t advocating to use 6 square at all.

    Its a workshop, in 6 months time a new machine might come on stream and what good is a limited supply then. It’s a no brainier and baffling why you are disagreeing to logic.



This discussion has been closed.
Advertisement