Cable sizing

[BoatyMacBoatface]

12 hours ago, Murflynn said:

what on earth have the IEE wiring regs got to do with a 12v boat system?

Wiring Regs apply to all installations, not just in buildings

[WotEver]

15 minutes ago, BoatyMacBoatface said:

Wiring Regs apply to all installations, not just in buildings

I'm afraid you're wrong on that. Besides, we were talking about 12V DC, not low voltage AC. 

[BoatyMacBoatface]

On 29 May 2017 at 19:34, WotEver said:

16mm is indeed fine, as established earlier in the thread it will give <0.5V drop. 8 metres is the one-way distance, so 16m for the calculations as in post #2. 

What figures did you enter into that calculator that you linked? I can't find any that work. 

The current is indeed travelling 16m. Unless I was taught wrong the length of the cable run is the value to be used for calculations, not the total length of cable. Your ready reckoner gives a reasonably accurate answer although if you were selecting the next size up from 7.2mm² you'd probably find it easier to get hold of 10mm²

I used twin and earth, conduit in an insulated wall, power 0.3 kW, 12V DC, length of run 8 metres, ambient temperature 30 degrees C

I haven't taken into account the cable and conduit factors.

11 minutes ago, WotEver said:

I'm afraid you're wrong on that. Besides, we were talking about 12V DC, not low voltage AC. 

That's true, BS 8450:2006 Code of Practice for Installation of Electrical and Electronic Equipment in Ships applies.

And what makes you think DC isn't covered by the Regs?

Edited May 30, 2017 by BoatyMacBoatface

[Alan de Enfield]

2 minutes ago, BoatyMacBoatface said:

I used twin and earth

Do you mean the 'domestic' solid wire T&E ?

[BoatyMacBoatface]

8 minutes ago, Alan de Enfield said:

Do you mean the 'domestic' solid wire T&E ?

My reason for using twin and earth for the calculations is that it has the same electrical characteristics as any other copper cable of the same csa with the same insulation and the online calculator doesn't offer a complete range of cable types.

Twin and earth is only solid up to 2.5mm^2, above that size it's stranded and also available stranded at 2.5mm^2.

[Murflynn]

35 minutes ago, BoatyMacBoatface said:

 

That's true, BS 8450:2006 Code of Practice for Installation of Electrical and Electronic Equipment in Ships applies.

And what makes you think DC isn't covered by the Regs?

no it doesn't.  

the relevant standard for SMALL CRAFT is BS EN ISO 10133:2001 (and any revisions) ‘Small craft. Electrical systems. Extra-low-voltage d.c. installations’.

small craft are not SHIPS.  

and the IEE Regs are not applicable to 12VDC installations in small craft.  

The IEE Regs are not recognised by the RCD.  They are not BS, EN, or ISO standards.  They have been adopted as part of the Building Regs, which is irrelevant.  If you are in any doubt please refer to 

https://books.google.co.uk/books?id=aQR6kWS6VeUC&pg=PA253&lpg=PA253&dq=IEE+Wiring+Regulations+scope+and+applicability&source=bl&ots=pwWwhFlsIm&sig=FyV_v7E5XZUudwpUZq80g32NSfQ&hl=en&sa=X&ved=0ahUKEwigrIubw5jUAhUhAcAKHbmZAXkQ6AEITTAH#v=onepage&q=IEE Wiring Regulations scope and applicability&f=false 

where pleasure craft are specifically excluded.

Edited May 30, 2017 by Murflynn

[WotEver]

1 hour ago, BoatyMacBoatface said:

Unless I was taught wrong the length of the cable run is the value to be used for calculations, not the total length of cable.

You were taught wrong. 

[WotEver]

1 hour ago, BoatyMacBoatface said:

Your ready reckoner gives a reasonably accurate answer

My simple equation gives a nearasdammit spot on answer for copper conductors. It is due to the fact that the resistance in ohms of copper cable per kilometre is pretty close to 18 divided by its cross-sectional area. I didn't make that up, it's just a fact.

[WotEver]

1 hour ago, BoatyMacBoatface said:

I haven't taken into account the cable and conduit factors

You don't need to for the majority of 12V circuits on a boat. Voltage drop calculations take over way before you even begin to approach the current capacity of the chosen cable size. 

[WotEver]

1 hour ago, BoatyMacBoatface said:

if you were selecting the next size up from 7.2mm² you'd probably find it easier to get hold of 10mm²

8.5mm² is widely available. 

1 hour ago, BoatyMacBoatface said:

BS 8450:2006 Code of Practice for Installation of Electrical and Electronic Equipment in Ships applies.

No it doesn't. But that's already been explained in detail. 

[smileypete]

2 hours ago, BoatyMacBoatface said:

I haven't taken into account the cable and conduit factors.

The 'amps divided by three' rule of thumb (to get mm2) will largely take care of that.

For most normal circuits on a 12V boat, A/3 will allow for up to 5m separation between equipment and batts, above that just increase the mm2 pro rata.

[jddevel]

On 5/29/2017 at 18:10, BoatyMacBoatface said:

Out of interest, what size are the tails?

Not sure how to measure them. Do I just use a vernier?

[BoatyMacBoatface]

Oh

shuts up

WotEver, Murflynn, et al, I bow to your superior knowledge. I now have a whole new area of learning to investigate.

jddevel, a vernier won't give you an accurate answer but it should put you in the right ball park. Using a micrometer on standard cable sizes often shows quite large variations in the diameter of each strand

[Alan de Enfield]

9 hours ago, BoatyMacBoatface said:

My reason for using twin and earth for the calculations is that it has the same electrical characteristics as any other copper cable of the same csa with the same insulation and the online calculator doesn't offer a complete range of cable types.

Twin and earth is only solid up to 2.5mm^2, above that size it's stranded and also available stranded at 2.5mm^2.

Twin & Earth (solid or stranded) should not be used on 'small boats'.

I'll let you explain the reasons why not.

[Murflynn]

2 hours ago, BoatyMacBoatface said:

 

WotEver, Murflynn, et al, I bow to your superior knowledge. I now have a whole new area of learning to investigate.

 

nothing to do with 'superior knowledge' ...............  just the capacity to understand the need to research technical issues thoroughly before committing to an unproved statement. ............ which is a large part of what being an engineer is about.  

engineering is more and more about compliance with codes and standards; it is essential to know which ones apply.

[WotEver]

2 hours ago, Alan de Enfield said:

Twin & Earth (solid or stranded) should not be used on 'small boats'.

I'll let you explain the reasons why not.

Nevertheless, as BMBF pointed out, using it in that nasty TLC calculator will give a cable size result. The biggest problem with that particular calculator is that it doesn't permit you to enter your own target voltage drop for a circuit so the result might be too large or too small for its intended application.