Guage mm wiring for dc outlets

[Adam1991]

How can I work out the gauge of mm for dc wiring to cigar light sockets (split across 2 circuits) and usb sockets (again across 2 sockets)? I can’t get my head around the volt drop when there are multiple outlets! Thanks!

[BEngo]

Volt drop is total current multiplied by wire resistance (total wire length there and back multiplied by wire resistance per metre).

 

E.g.  for 2 x 2A 12V outlets calculate the drop for 4A.    For USB outlets at 5V  you want the total outlet current  at 5V,   reduced by 50 % to allow for the voltage conversion at the sockets.

 

N

[Adam1991]

Hi bEngo, thank you for your advice! Do you by any chance know the average amps of 12v cigar lighter for the calc? Really struggling to find any info online

 

thanks 

Adam 

[Tracy D'arth]

Cigar lighters in cars are usually fused at 10A or 15A, lots of things that get plugged into those sockets on boats draw 10A. Not the best sockets for DC supplies.

[Adam1991]

Hi Tracy, what socket would you recommend instead? Thanks! 

[Tracy D'arth]

5A three pin sockets to BS 546 as were used for house wiring in the 1950s. They will happily cope with far more than 5A DC. In fact many use the 2A equivalents and have no problems with current carrying capacity. Still made by many accessory makers. You need 3 pins to ensure polarity.

Unfortunately the cheap cigar plug has become universal, lots burn out or melt.

Edited January 23, 2021 by Tracy D'arth
added

[Adam1991]

Hi Tracy, thanks for that info, so difficult to get a clear idea online about which dc socket should be fitted. For calculating volt drop woul you calculate for these at 5amp each? Thanks 

[frangar]

What are you thinking of powering from the DC outlets? Although popular for phone chargers etc a few years ago Unless you had a specific item in mind Id be looking at installing USB outlets now. 

[Tracy D'arth]

13 minutes ago, Adam Mc Gowan said:

Hi Tracy, thanks for that info, so difficult to get a clear idea online about which dc socket should be fitted. For calculating volt drop woul you calculate for these at 5amp each? Thanks 

No, calculate at 10A, the difference in cable size will be insignificant, you will probably need 4mm cable depending on the length of the run at least. You may have trouble getting thicker cable into the terminals anyway.

[Tony Brooks]

12 minutes ago, Adam Mc Gowan said:

Hi Tracy, thanks for that info, so difficult to get a clear idea online about which dc socket should be fitted. For calculating volt drop woul you calculate for these at 5amp each? Thanks 

 

No. The plug and socket has nothing whatsoever to do with cable or fuse size. Its what you plug into them that counts. So list everything you may plug in at any one time (to all sockets) plus their current draw or wattage and we can tell you the cable size.

 

I note that you don't mentions the fuse. the fuse protects the cable and the cable is sized to carry the maximum current from all connected equipment whilst allowing an acceptable volt drop.  The maximum fuse size should be no greater than the cable's current rating but with volt drop you will be drawing less current.

 

 

 

[wakey_wake]

On 23/01/2021 at 14:19, Tony Brooks said:

[...] but with volt drop you will be drawing less current.

 

This is true for resistive-ish loads such as motors, mug boilers and tungsten lamps, but it's not the whole picture.

 

For anything powering IT kit (laptops, phones) or most other voltage converters including LED lighting units, they're trying to draw the power they need. When the voltage dips, perhaps because something else is sucking on the same cable, then they will also suck harder.

 

In numbers: suppose a laptop wants 90 watts as 4.7 amps at 19 volts, and you're using a DC:DC converter with 85% efficiency. That thing needs 106 watts input, so at 12.6 volts it will draw 8.4 amps but if the voltage dips to 10 volts (assuming it doesn't shut down) it will at least try to draw 10.6 amps to deliver the same amount of power. Beware also that this isn't enough to blow a 10 amp fuse for a good while, if at all.

 

See also "We almost had a fire on our boat! Smoking wires" (youtub). Nobody hurt, it sounds like they learned something.

[Tony Brooks]

5 hours ago, wakey_wake said:

 

This is true for resistive-ish loads such as motors, mug boilers and tungsten lamps, but it's not the whole picture.

 

For anything powering IT kit (laptops, phones) or most other voltage converters including LED lighting units, they're trying to draw the power they need. When the voltage dips, perhaps because something else is sucking on the same cable, then they will also suck harder.

 

In numbers: suppose a laptop wants 90 watts as 4.7 amps at 19 volts, and you're using a DC:DC converter with 85% efficiency. That thing needs 106 watts input, so at 12.6 volts it will draw 8.4 amps but if the voltage dips to 10 volts (assuming it doesn't shut down) it will at least try to draw 10.6 amps to deliver the same amount of power. Beware also that this isn't enough to blow a 10 amp fuse for a good while, if at all.

 

See also "We almost had a fire on our boat! Smoking wires" (youtub). Nobody hurt, it sounds like they learned something.

 

 

You are talking about the "apparent resistance" of the load. I was talking about the voltdrop along the cables and what you say will make that worse, much like water pumps. As the voltage at the motor is reduced the current rises so any votdrop along the cables rises and makes matters worse until something overheats or burns out. In motor cases its usually the motor but could  be the cable.

 

As for "sucks harder" that "sucks" as they say in the USA. It's the voltage that pushes, there is no sucking involved. Now I will accept that as stabilised power supply or regulator may well reduce its apparent resistance as the voltage falls so current at that reduced voltage remains the same or even increases but talking about "sucking" only serves to make things more difficult for those with less electrical knowledge to get to understand better.

 

If a power supply needs protecting then it should have its own inbuilt protection like the fuse in some cigarette lighter plugs or an under supply voltage shutdown but I am sure many Far Eastern designs do not. That is another matter. The basic rules are cables are sized for the maximum acceptable voltdrop when all the appliances are drawing their stated load and fuses are used to protect the cable.

 

If a power supply/regulator manufacturer dos not protect from under voltage or supplies incorrect current data then that is another thing all together. Without an accurate quoted maximum draw it's not possible to calculate a sensible cable size.

 

 

Edited March 13, 2021 by Tony Brooks

[wakey_wake]

16 hours ago, Tony Brooks said:

You are talking about the "apparent resistance" of the load. I was talking about the voltdrop along the cables and what you say will make that worse, much like water pumps.

 

I am, and they interact in ways that (I think) most people don't understand. Hell if I'm not thinking clearly, I can misunderstand it.

 

So what's constructive, in the context of electrical planning? Having seen the power budget with voltage mismatch errors recently, I wonder if it would help to invite people to

• budget in watt hours rather than amp hours
• budget on each cable run in watts rather than amp
• then convert back into amp hours / amps at a suitably pessimistic voltage, discounting at each voltage conversion step and along each cable run

Then the question stops being "how many amps do my lights / fridge / laptop use?" and becomes (essentially) "how many joules per day?" plus "what percent is lost in transfer and conversion?"

 

16 hours ago, Tony Brooks said:

As the voltage at the motor is reduced the current rises so any votdrop along the cables rises and makes matters worse until something overheats [...]

Thanks, it had not occurred to me that undervoltage can cause stalling, which can cause overcurrent.

Consumer-grade motors I've seen don't have the stall current on the spec plate. ?

 

At least when DC:DC converters go undervoltage without shutting down they usually just struggle, and fail to deliver the output voltage; they can't reduce their apparent resistance below a certain point so the power transfer drops. This tends to make them locally safe / self-limiting, but what we're discussing is the effect on a wider system. Throwing them in as a magic gizmo to fix a voltage problem is the problem, because they're not magic.

 

16 hours ago, Tony Brooks said:

As for "sucks harder" that "sucks" as they say in the USA. It's the voltage that pushes, there is no sucking involved. [...] talking about "sucking" only serves to make things more difficult for those with less electrical knowledge to get to understand better.

Fair comment. Actually I'm hard pressed to find any example real of "sucking". Vacuum cleaners, vacuum pumps and lungs don't do it. Heat pumps... maybe? Black holes do suck by some interpretations but not others.

 

Analogies can help build an intuitive understanding, but it has to be clear what's real and what's not. Intuitive understandings can be misleading!

 

16 hours ago, Tony Brooks said:

The basic rules are cables are sized for the maximum acceptable voltdrop when all the appliances are drawing their stated load and fuses are used to protect the cable. [...] Without an accurate quoted maximum draw it's not possible to calculate a sensible cable size.

I found the thread on (lead acid) battery charging. I guess there is one for cable gauge? Should there be something in that or nearby for voltage converters, since they're increasingly useful and remarkably cheap? e.g. "Why don't my new LED lights dim when the water pump fires? The old halogen lights always did."

[Tony Brooks]

8 hours ago, wakey_wake said:

 

I found the thread on (lead acid) battery charging. I guess there is one for cable gauge? Should there be something in that or nearby for voltage converters, since they're increasingly useful and remarkably cheap? e.g. "Why don't my new LED lights dim when the water pump fires? The old halogen lights always did."

 

Every cable drum has the current the cable can safely carry written on it and also in the catalogues but the most important word there is the CAN. On low voltage DC boat systems the major problem is volt drop along the run so anything to do with cable size will have to include run length, maximum current flow and acceptable volt drop. I think someone on here has published a spreadsheet and there are numerous  topics showing the calculations. 

 

Most boaters have no problems as long as they have done the volt drop calculations because the cables normally end up far larger than the cable's current capacity on its own indicates.  Like wise bundling is not such a problem because the cables are normally running well under capacity so the heating is minimised.

 

Also the BSS and ISOs for new builds specify  a minimum number of strands so for low current circuits such as LEDs the smallest cable allowed my well be far larger than that strictly needed volt drop wise.

 

I think the think about LEDs not dimming is just a lack of understanding by the user. If they really were 12V LEDs they would tend to dim but the ones most use in boats are not, they are usually 10 to 35 volts with inbuilt stabiliser so they won't dim unless the voltage drops below 10V.

Edited March 14, 2021 by Tony Brooks

[Alan de Enfield]

52 minutes ago, Tony Brooks said:

Also the BSS and ISOs for new builds specify  a minimum number of strands so for low current circuits such as LEDs the smallest cable allowed my well be far larger than that strictly needed volt drop wise.

 

 

Indeed they do,

CSA, Current rating and minimum No of strands per core.

 

0.75mm2 would normally be 24/0.20mm

1.0mm2 would normally be 32/0.20mm

1.5mm2 would normally be 30/0.25mm

4.0mm2 would normally be 56/0.20mm

etc etc

 

Typical automotive wiring would be 

1.0mm2 = 14/0.30mm (so would fail the RCD requirement)

2.0mm2 = 28/0.30mm (so would fail the RCD type B requirement)

etc etc

 

RCD requirements from ISO 10133 "Small craft — Electrical systems — Extra-low-voltage d.c. installations"

Edited March 14, 2021 by Alan de Enfield

[Richard10002]

3 hours ago, Tony Brooks said:

On low voltage DC boat systems the major problem is volt drop along the run so anything to do with cable size will have to include run length, maximum current flow and acceptable volt drop.

 

Most boaters have no problems as long as they have done the volt drop calculations because the cables normally end up far larger than the cable's current capacity on its own indicates.  Like wise bundling is not such a problem because the cables are normally running well under capacity so the heating is minimised.

 

 

Our boat was built in 2000 and we bought it in 2011. As I added more and more electrical stuff in the lounge, I was always concerned that the cables might not be up to it, and there was definitely a significant voltage drop.

 

In addition, as time went by, the number of cables to "things" grew into a real mess.

 

So... I resolved to created a conduit for all these wires/cables and fit a new pair of thicker cables from batteries to lounge. I spent ages procrastinating with how to feed the cables, how to create the conduit, what thickness of cable.

 

I tend to use the 12V Planet voltage drop calculator: https://www.12voltplanet.co.uk/cable-sizing-selection.html

 

This suggested that, if 50A were to flow along my 8m one way run, I would have a voltage drop of just over 5%, or 0.616V, (39A would be a drop of 4%).

 

In the end, rather than wondering what was the minimum I could get away with, I decided to fit 25mm2 cable, (50A circuit breaker close to the start), with a branch, (joint box), at the back of the lounge, and ending at the front of the lounge. Based on distance, likely max current and bundling in the conduit, this was almost certainly more than necessary, both from a current carrying capacity, and voltage drop - particularly as Li batteries means the voltage starts at over 13V most of the time. Adding everything up and exaggerating a bit, I generally got an estimate of a max of 30A flowing.

 

It might have cost a bit more than it could have, but it's a one off job which "ought" to have been done 20 years ago and, in reality, I might have saved a few tens of pounds with thinner cables. In addition, I now have a nice shelf, (conduit), running the length of the lounge which is handy for putting "more stuff" on.

 

I suppose what I am trying to say is that, if you are trying to decide between 10mm2 cable and 6mm2 cable, (based on current and volt drop), you wont go wrong if you fit 16mm2.

[Tony Brooks]

The only problem with what you wrote and I accept that may not be what you did is that if the cables leaving the joint box were significantly thinner than 25sq mm CCSA you should have them fused at the joint box to protect the thinner cables. I think electrically it's a good system except you are likely to end op with "fused spurs" all over the place or fuse boards in maybe  three locations along the boat length. That is fine while you have the boat because you know where they are but a future buyer may have problems finding them.

[Richard10002]

7 minutes ago, Tony Brooks said:

The only problem with what you wrote and I accept that may not be what you did is that if the cables leaving the joint box were significantly thinner than 25sq mm CCSA you should have them fused at the joint box to protect the thinner cables. I think electrically it's a good system except you are likely to end op with "fused spurs" all over the place or fuse boards in maybe  three locations along the boat length. That is fine while you have the boat because you know where they are but a future buyer may have problems finding them.

 

Yes... every cable that leaves the jointing boxes has an appropriate fuse - I told my story to highlight the fact that we can be a bit too pernickety, when it comes to sizing things to a minimum.

 

I did consider having a fused switch panel at each jointing box but, having procrastinated for years, It was time for "a good plan today", rather than "a perfect plan tomorrow" 

 

I may add a couple of panels at some time, if I get a round tuit.

 

In fact, Alan, (de Enfield), linked to some that I liked the look of a while ago... which was extremely helpful. 

[Mad Harold]

9 minutes ago, Tony Brooks said:

The only problem with what you wrote and I accept that may not be what you did is that if the cables leaving the joint box were significantly thinner than 25sq mm CCSA you should have them fused at the joint box to protect the thinner cables. I think electrically it's a good system except you are likely to end op with "fused spurs" all over the place or fuse boards in maybe  three locations along the boat length. That is fine while you have the boat because you know where they are but a future buyer may have problems finding them.

You are right about the fused spurs Tony.

I have added a tv and a diesel heater to my boat,and fitted both circuits with line fuses.

I know where the line fuses are,(one in the battery compartment,and one in the toilet compartment) and I take your point about a new owner not knowing where they are.

[Idle Days]

57 minutes ago, Mad Harold said:

You are right about the fused spurs Tony.

I have added a tv and a diesel heater to my boat,and fitted both circuits with line fuses.

I know where the line fuses are,(one in the battery compartment,and one in the toilet compartment) and I take your point about a new owner not knowing where they are.

 

That's you (we) should be keeping your owners manual up to date regarding such changes.