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Nemysys

Electrics, cable, amps

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Currently going through a boat refit. Complete strip out and refit.

looking at a Dometic 12v Compressor fridge and a standard home domestic 240v combi microwave. At this stage we can easily provide a 2.5mm2 (or more) ring main style cable for both, about 3-4m in length, we can on board produce 240v 4KVA from shore or engine. Is this cable enough?

 

Also looking at the possibility of installing a macerator toilet but they need 14-25a plus for short periods, cable length here would be 10m or so at 12v, would a 6mm2 cable suffice, or should I go for 10mm2?

 

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2.5mm ring main cable sounds like domestic non stranded stuff, not allowed on boats, must be multi strand flex.

12V fridge, use nothing less than 10mm for a 6 to 8 m run else you will have volt drop problems Thicker is better. Same with the toilet 6mm is too thin for 20m run @ 25A 12v.

I would do both in 16mm, then you know it will be right for ever. And we are talking copper cross sectional area here not outside insulation diameter.

As for 240v mains power, as its only a 16A land line maximum, 2.5mm Arctic blue cable is the stuff to use. From Screwfix etc.

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When I was an apprentice 1976 to 1981 an old electrician told me "think of the future" and fit a socket in every room.

What do you think he would be telling you now?

 

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6 hours ago, AWETHEAYET said:

When I was an apprentice 1976 to 1981 an old electrician told me "think of the future" and fit a socket in every room.

What do you think he would be telling you now?

 

Well, if he was already old 40 years ago, probably not a lot... ;)

 

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7 hours ago, Nemysys said:

Currently going through a boat refit. Complete strip out and refit.

looking at a Dometic 12v Compressor fridge and a standard home domestic 240v combi microwave. At this stage we can easily provide a 2.5mm2 (or more) ring main style cable for both, about 3-4m in length, we can on board produce 240v 4KVA from shore or engine. Is this cable enough?

 

Also looking at the possibility of installing a macerator toilet but they need 14-25a plus for short periods, cable length here would be 10m or so at 12v, would a 6mm2 cable suffice, or should I go for 10mm2?

 

Use a volt drop calculator tool such as on this page.

 

https://www.12voltplanet.co.uk/cable-sizing-selection.html

 

eg for your 25A macerator, 6mm over 10m (each way) will drop nearly 1.5v, so you 12v becomes 10.5v. You’d probably want to choose bigger cable.

12v fridges have notoriously high start up currents and refuse to start if the voltage is too low. No way is 2.5mm anywhere close to being adequate. Usually the manufacturers give you a formula to calculate cable size in the installation instructions bu I would guess that even 6mm wouldn’t really be enough, more like 10mm required. Ultimately it depends on what state of charge you want the devices to carry on working down to. If you have thin cable, the devices will probably work ok when the batteries are well charged, and definitely when being charged (engine, battery charger etc) but once the SoC drops a bit, and hence the battery voltage drops a bit, they are likely to stop working.

Edited by nicknorman

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7 hours ago, Nemysys said:

At this stage we can easily provide a 2.5mm2 (or more) ring main style cable

It has already been said but …………..

 

If you are talking about the 'Flat Twin & Earth' (normally colour Grey) used in domestic wiring DON'T USE IT

 

Wiring should be multi-strand 'flexible' cable (similar to extension lead cable) and 2.5 square mm is the norm for your mains circuit.

 

For 'mains cables'  ISO 13297:2000 Small Craft Electrical systems  Alternating current installations 

is the one you need to follow :

 

10 System wiring
10.1 Conductors shall have a minimum rating of 300/500 V. Flexible cords shall have a minimum rating of
300/300 V.
10.2 Conductors and flexible cords shall be of multistrand copper, and of sizes no smaller than those determined
by reference to Table A.1.
NOTE A conductor used for equipment grounding is not considered to be a current-carrying conductor when making
reference to Table A.1.
10.3 The insulation-temperature rating of conductors and flexible cords outside engine spaces shall be at least
60 °C.
10.4 Conductors shall be at least 1 mm2 in area.

 

 

 

 

ISO 10133:2000 "Small Craft  Electrical systems  Extra-low-voltage d.c. installations"

 

Tells you all you need to know for the 12v wiring

 

7 Conductors
7.1 Electrical distribution shall use insulated stranded-copper conductors. See Table A.1. Conductor insulation
shall be of fire-retardant material, e.g. not supporting combustion in the absence of flame.
7.2 Conductors that are not sheathed shall be supported throughout their length in conduits, cable trunking, or
trays, or by individual supports at maximum intervals of 300 mm.
7.3 Sheathed conductors and battery conductors to the battery disconnect switch shall be supported at
maximum intervals of 300 mm, with the first support not more than 1 m from the terminal. Other sheathed
conductors shall be supported at maximum intervals of 450 mm.
Sheathed outboard-motor starter conductors constitute an exception to this requirement.
7.4 Conductors which may be exposed to physical damage shall be protected by sheaths, conduits or other
equivalent means. Conductors passing through bulkheads or structural members shall be protected against
damage to insulation by chafing.
7.5 Conductors shall have minimum dimensions in accordance with Table A.1, or the conductor manufacturer's
rated current-carrying capacity, based on the load to be supplied and allowable voltage drop for the load to be
carried. Conductors in voltage-critical circuits, such as starter motor circuits, navigation-light circuits and ventilationblower
circuits, whose output may vary with system voltage, shall be sized in compliance with the component
manufacturer's requirements. See 4.5 and 4.6.
7.6 Each conductor longer than 200 mm installed separately shall have an area of at least 1 mm2. Each
conductor in a multi-conductor sheath shall have an area of at least 0,75 mm2 and may extend out of the sheath a
distance not to exceeding 800 mm.
An exception may be made for conductors of minimum area 0,75 mm2 which may be used as internal wiring in
panel-boards.
7.7 A d.c circuit shall not be contained in the same wiring system as an a.c. circuit, unless one of the following
methods of separation is used.
a) For a multicore cable or cord, the cores of the d.c. circuit are separated from the cores of the a.c. circuit by an
earthed metal screen of equivalent current-carrying capacity to that of the largest core in either circuit.
b) The cables are insulated for their system voltage and installed in a separate compartment of a cable ducting or
trunking system.
c) The cables are installed on a tray or ladder where physical separation is provided by a partition.
d) A separate conduit, sheathing or trunking system is used.
e) The d.c and a.c. conductors are fixed directly to a surface and separated by at least 100 mm.
7.8 Each electrical conductor that is part of the electrical system shall have a means to identify its function in the
system, except for conductors integral with engines as supplied by their manufacturers.
7.8.1 All equipotential bonding conductors shall be identified by green, or green with a yellow stripe, insulation, or
may be uninsulated. Conductors with green, or green with a yellow stripe, insulation shall not be used for currentcarrying
conductors.
NOTE The protective conductor of the a.c. electrical system (see ISO 13297) also uses green, or green with a yellow
stripe, insulation and may be connected to the d.c. negative terminal of the craft engine.
7.8.2 Means of identification other than colour for d.c. positive conductors is permitted if properly identified on the
wiring diagram of the electrical system(s) of the craft.
7.8.3 All d.c. negative conductors shall be identified by black or yellow insulation. If the craft is equipped with an
a.c. electrical system (see ISO 13297) which may use black insulation for live conductors, yellow insulation shall be
used for d.c. negative conductors of the d.c. system. Black or yellow insulation shall not be used for d.c. positive
conductors.
NOTE 1 In conformance with IEC 60446, conductor insulation colours of the a.c. system are
live conductors: black or brown;
neutral conductors: white or light blue;
protective conductors: green or green with a yellow stripe.
NOTE 2 A colour stripe may be added to the conductor insulation for identification in the system.
Craft with a.c and d.c systems should avoid the use of a brown, white or light blue insulation colour in the d.c.
system unless clearly separated from the a.c. conductors and identified (see 7.7).
7.8.4 Insulation-temperature ratings of conductors in engine spaces shall be 70 °C minimum. The conductors
shall be rated oil resistant, or shall be protected by an insulating conduit or sleeving, and shall be derated in
allowable current-carrying capacity in accordance with clause A.1.
7.8.5 For additional conductor specifications, see ISO 6722-3 and ISO 6722-4.
7.8.6 Current-carrying conductors of the d.c. system shall be routed above anticipated levels of bilge water and
in other areas where water may accumulate, or at least 25 mm above the level at which the automatic bilge-pump
switch activates.
If conductors must be routed in the bilge area, the wiring and connections shall be in an IP 67 enclosure, in
accordance with IEC 60529, as a minimum, and there shall be no connection below the foreseeable water level.
7.8.7 Conductors shall be routed away from exhaust pipes and other heat sources which can damage the
insulation.
The minimum clearance of the conductors is 50 mm from water-cooled exhaust components and 250 mm from dry
exhaust components, unless an equivalent thermal barrier is provided.

 

 

Screenshot (48).png

Edited by Alan de Enfield

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7 hours ago, AWETHEAYET said:

When I was an apprentice 1976 to 1981 an old electrician told me "think of the future" and fit a socket in every room.

What do you think he would be telling you now?

 

When I rewired my present house in about 1986 my old dad (actually he was about 65) looked around in amazement. Every room had about 10 -12 outlets and the kitchen about about 16. Outlets not boxes. (3 seperate rings) . Some of the outlets now feed extension leads. Where the TV is the original demand was just that, the TV. Now it's TV, sky box, DVD player, landline phone extension, doorbell amplifier, table light, Amazon streamer thing. It also takes an extension lead when working on the patio. All low wattage items that need individual outlets. Times have changed.

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21 minutes ago, Slim said:

When I rewired my present house in about 1986 my old dad (actually he was about 65) looked around in amazement. Every room had about 10 -12 outlets and the kitchen about about 16. Outlets not boxes. (3 seperate rings) . Some of the outlets now feed extension leads. Where the TV is the original demand was just that, the TV. Now it's TV, sky box, DVD player, landline phone extension, doorbell amplifier, table light, Amazon streamer thing. It also takes an extension lead when working on the patio. All low wattage items that need individual outlets. Times have changed.

 

When I rewired my house back in 2001 I wad determined not to fall foul of the 'not enough sockets' effect, and I put 16 double socket outlets in the office/hobby room. Yes 16 doubles. 32 three-pin socket outlets in total. Ten years later we still ran out of available sockets in there with all the computers, printers, routers, knitting machines etc...

 

 

 

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Thanks all, lots of good info here.

yes I was looking to use thick twin and earth domestic cable. (Cooker point etc) Glad I asked now!

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We had a boater in last week who has ongoing problems with his 3 year old narrowboat. The taps cut off if the shower pump is used and his fridge only works intermittently. On closer inspection, most of his wiring is of maximum thickness 1.5mm2 and the whole wiring loom (240v and 12v) is all tethered together, hanging from the fuse boards unsupported until it enters the cabin side to pass along the boat. Shower pump/water pump and other 12v lighting is on the same circuit, many cables joined with chocblocks, no ferrules and cables unsupported , wires straining.

The RCD folder and cert has been signed off by the North Oxford boatyard that has thrown it together, and we have recommended taking it straight to Trading Standards as its not the only boat from this yard we have seen manufactured like this.

Edited by matty40s
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3 hours ago, matty40s said:

We had a boater in last week who has ongoing problems with his 3 year old narrowboat...

That’s a disgraceful story. Did you give him a ball-park quote for a rewire?

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1 hour ago, WotEver said:

That’s a disgraceful story. Did you give him a ball-park quote for a rewire?

No, the whole thing is wrong,  we arent getting involved with the boat any further and Trading Standards need to be involved. We are providing a report for the owner though.

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11 hours ago, WotEver said:

That’s a disgraceful story. Did you give him a ball-park quote for a rewire?

👹 maybe the NO electrician was BM qualified 😉

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I should have said North Oxford Canal.....problem is, this boatyard has been throwing lots of new boats out in the last 4 or 5 years.

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On 12/01/2020 at 09:21, Alan de Enfield said:


7.7 A d.c circuit shall not be contained in the same wiring system as an a.c. circuit, unless one of the following
methods of separation is used.
a) For a multicore cable or cord, the cores of the d.c. circuit are separated from the cores of the a.c. circuit by an
earthed metal screen of equivalent current-carrying capacity to that of the largest core in either circuit.
b) The cables are insulated for their system voltage and installed in a separate compartment of a cable ducting or
trunking system.

c) The cables are installed on a tray or ladder where physical separation is provided by a partition.
d) A separate conduit, sheathing or trunking system is used.
e) The d.c and a.c. conductors are fixed directly to a surface and separated by at least 100 mm.

I find Para 7.7 ambiguous.

What is meant by "same wiring system" above?

I have AC systems and DC systems. With a central walkway on the boat, I have DC distribution on the left side and AC distribution on the right. However there are DC and AC circuits on both sides of the boat. The cables for both run (as I'm sure they do on many boats) along the trunking space under the gunwales side-by-side. Am I in violation as I certainly don't have separate cable ducting or trunking (as mentioned in 7.7(b))?

My DC and AC cables also cross over from one side of the boat to the other cleated and bundled all together. There was only one convenient place to do this hence they are bundled together.

 

Is this something a BS inspector will pull me up on? 

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14 minutes ago, Stephen Jeavons said:

The cables for both run (as I'm sure they do on many boats) along the trunking space under the gunwales side-by-side. Am I in violation...

Yes, but only of the RCD. If your boat has an RCD then it shouldn’t have but nobody’s going to care. 
 

15 minutes ago, Stephen Jeavons said:

Is this something a BS inspector will pull me up on?

No, not at all. 

18 minutes ago, Stephen Jeavons said:

I certainly don't have separate cable ducting or trunking (as mentioned in 7.7(b))?

You do realise that those sub-sections are alternatives? So sub-clause (e) is quite common. 

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9 minutes ago, WotEver said:

Yes, but only of the RCD. If your boat has an RCD then it shouldn’t have but nobody’s going to care. 

Consort is awash with RCD's. Got four of 'em. One on the shore, one downstream of the IT, one on the distribution side after the Victron and yet another in the CH flow boiler circuit.

Most cabling is out of sight so unless an inspector tears the boat to pieces.....

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Just now, Stephen Jeavons said:

Consort is awash with RCD's. Got four of 'em. One on the shore, one downstream of the IT, one on the distribution side after the Victron and yet another in the CH flow boiler circuit.

Most cabling is out of sight so unless an inspector tears the boat to pieces.....

 Would we be mixing up Residual Current Device and Recreational Craft Directive? Its all in the context.

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21 minutes ago, Tony Brooks said:

 Would we be mixing up Residual Current Device and Recreational Craft Directive? Its all in the context.

Oh! okay. Silly me (I hate reusable acronyms). Thanks Tony 🙄 

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46 minutes ago, WotEver said:

Yes, but only of the RCD. If your boat has an RCD then it shouldn’t have but nobody’s going to care. 

Completely agree about the 'chance of being caught', but on the other hand, when you have a completely stripped out boat (and were intending to use domestic twin & earth), it is better to wire it correctly and in accordance with 'best practice' and the relevant specifications. It's a job you will only do once and doing it 'right' will cost little more than doing it 'wrong'.

 

Doing it wrong in the 1st place can lead to having to remove your boat linings to replace  / repair the fault.

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4 minutes ago, Stephen Jeavons said:

Oh! okay. Silly me (I hate reusable acronyms). Thanks Tony 🙄 

what - like BOGOF ?   In my experience it's an alternative for eff-off but it seems the "retail sector" considers it quite acceptable.

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1 hour ago, Stephen Jeavons said:

I find Para 7.7 ambiguous.

Could that be because you didn’t appreciate that the sub-clauses were options?

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28 minutes ago, WotEver said:

Could that be because you didn’t appreciate that the sub-clauses were options?

Sure, they are options assuming you don't comply to the main requirement i.e.

 

7.7 A d.c circuit shall not be contained in the same wiring system as an a.c. circuit, unless.......

 

Back to my original query: What is meant by "same wiring system"? That was the bit I found ambiguous. How do you read it?

 

Maybe, ocean-going gin palaces have the space and layout to have separate channels, conduits and spaces to allow DC and AC separation but how many common or garden narrowboats, even built with the requirement in mind, have the space to comply?

 

 

Edited by Stephen Jeavons

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2 minutes ago, Stephen Jeavons said:

Back to my original query: What is meant by "same wiring system"?

Lumped together. In the same place. Following the same path. Co-located. 


I’m sure you and others can think of more synonyms. 


Basically if you have 12V and 230V both travelling under the gunwale then they musn’t be in the same trunking or tied together etc. 

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