Cable size

DonalOF

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

2011

https://www.boards.ie/discussion/2058246403/cable-size

What size is the load?

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

DonalOF

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

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.

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?

2011

https://www.boards.ie/discussion/comment/119085404#Comment_119085404

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).

hayse

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

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.

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.

2011

https://www.boards.ie/discussion/comment/119088169#Comment_119088169

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.

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).

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

DonalOF

Thanks Kirk.

2011

https://www.boards.ie/discussion/comment/119088372#Comment_119088372

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

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.

https://www.southwire.com/calculator-vdrop

8 AWG is about 10mm.

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

Play around as you need.

hayse

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

DonalOF

Thanks all. I’m going with 10.

2011

https://www.boards.ie/discussion/comment/119088841#Comment_119088841

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.

hayse

https://www.boards.ie/discussion/comment/119088863#Comment_119088863

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

2011

https://www.boards.ie/discussion/comment/119088863#Comment_119088863

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

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.

10-10-20

https://www.boards.ie/discussion/comment/119089561#Comment_119089561

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

DonalOF

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

2011

https://www.boards.ie/discussion/comment/119089782#Comment_119089782

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.

Bruthal.

https://www.boards.ie/discussion/comment/119088282#Comment_119088282

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.

Bruthal.

https://www.boards.ie/discussion/comment/119089782#Comment_119089782

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.

2011

https://www.boards.ie/discussion/comment/119091000#Comment_119091000

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

Bruthal.

Ok

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.

2011

50A??

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

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.

hayse

My last post was based on a post you have edited you composed 2011. It’s difficult to have a discussion if you are changing your posts.

2011

The OP’s electrician has seen this and has concluded that an MCB with a 20A rating is suitable. I would think that he/ she is best placed to make that call.

A minimum of 50A is based on absolutely nothing.

kirk.

https://www.boards.ie/discussion/comment/119093154#Comment_119093154

There's only 1 operator

You don't need to double the capacity when you add an appliance or a machine here as it's only a domestic shed

There probably isn't 50amp spare capacity for the shed anyway because the house is already part of the load on a likely 12kva ESBN supply

You seem to be a bluffer anyway making up numbers and arguments

hayse

https://www.boards.ie/discussion/comment/119095725#Comment_119095725

Live in the real world now, you have to allow for expansion, winter time the op may introduce a heater, a kettle may be brought. He might have someone who wants to use their own tools if they are working on a project together.

50 amps may be over the top but they would want 40 anyway.

2011

https://www.boards.ie/discussion/comment/119096576#Comment_119096576

The real world is none of us have seen this therefore we only have the OP's information to guide us:

1) The OP described the load as "A couple of socket outlets. Light and Small Tools only. Max 15amp. Haven’t installed the cb yet."

2) The OP's electrican has slected a protective device rated at 20A.

3) This has been described as a "garage/shed" by the OP, not a commercail workshop where people bring their own tools.

If this is a 50A laod as you have advised then it requries a dedicated supply from the ESB. I suspect that this is nto a road that the OP wants to go down.

hayse

You’re right.

DonalOF

Thanks everyone for comments. Time to let it go now. Going with my Electrician’s recommendations.

kirk.

https://www.boards.ie/discussion/comment/119096576#Comment_119096576

I know you're both a logician and a mathematician as you claim so I respect that

You do know you don't add up the appliances to get the power requirement ya ? Especially as it's just a domestic shed with 1 user, not to mention the additional cost of 100m of cabling and the fact that this 40-50amp of capacity may not even exist at the supply end

hayse

Kirk you are correct I’m wrong.

DonalOF

My first time posting on Boards.ie😂

2011

The OP’s question has been answered and the OP has taken the sensible approach of hiring a Registered Electrical Contractor.

Everyone that has contributed is on the same page now.

I think we can leave it at that.