Cable capcity

Well having pretty well rubbished the Lady in Beds link
Suggestions please for table(s) of max cable capacities in different situations.

IEE Wiring Regulations ??
 
VicS said:
Well having pretty well rubbished the Lady in Beds link
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It's appears to be easier to flame something than answer the question!
Nat's Post asked
What's the leeway in terms of current capacity on electrical cable?
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All the reply's regarding voltage drop did not answer that question.

I also look forward to PVC's further inputs :D
 
If what you are worrying about is the cable overheating, then there will be some leeway.
The copper of the cable has a known heat capacity, so if you look up the specific heat capacity of copper, work out the mass of copper in the cable, you are well on your way to working out how long it will take a particular overload to heat the cable from ambient to whatever the max temperature of the insulation is. This max temperature will vary from type to type of course.
There are of course other issues, like will the system work with the lower voltage delivered.

Beware some things like inverters can draw more current to make up for the lower voltage they are getting, they can be theoretically 'constant power loads' but the efficiency may fall causing the current to rise even more.
Short runs of cable can get away with more, because the heat can be conducted out the end of the cable. Equally it can be conducted in if the load gets hot!

Even fuses have short term overload capacity. Motors often draw a lot for a brief period as they start. It is hard to be specific when the question is quite general.

The real answer remains, 'fat cable is generally good'.
A lot of useful info can be found from www.rswww.com, in terms of manufacturers data for specific cables and many other items.
HTH.
PS Don't forget resistivity rises with temperature!
Is big cable looking easier yet?
 
The copper of the cable has a known heat capacity, so if you look up the specific heat capacity of copper, work out the mass of copper in the cable, you are well on your way to working out how long it will take a particular overload to heat the cable from ambient to whatever the max temperature of the insulation is.
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"Well on the way" maybe but I think you had better complete the calculation, putting in some likely/typical figures to show how it's done.
It's not something I can do without some further guidance. :o

I have 0.379 J/g/K as specific heat capacity for copper and 8933 kg/m³ as the density .........?
 
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VicS said:
"Well on the way" maybe but I think you had better complete the calculation, putting in some likely/typical figures to show how it's done.
It's not something I can do without some further guidance. :o

I have 0.379 J/g/K as specific heat capacity for copper and 8933 kg/m³ as the density .........?
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Thermal effects in conductors? Ooooh, I can do that!

Ohmic heating

W = I^2 R = I^2 x resistivity x length / area

Thermal balance for an insulated wire gives

W = specific heat capacity x density x length x area dT/dt

Putting them equal gives that

specific heat capacity x density x length x area dT/dt = I^2 x resistivity x length / area

so

dT/dt = (I / area)^2 x resistivity / (density x specific heat capacity)

Using resistivity = 1.6 x10^-8 Ohms/m, density = 8933 kg/m^3 and c = 379 J/kgK gives

Rate of temperature increase = current density squared x 4.73 x 10^-15

If we put the current density in Amps per mm^2 rather than Amps per m^2 and accept that to an engineer 4.73 = 5, that gives

kelvins per second = (amps per mm)^2 / 200

which is pleasantly memorable. So if, for example, you whacked 10A down a well thermally insulated 16 swg (1.3 mm^2) wire, the temperature would rise at about 0.3 K/s or 18 K/minute.

All off the top of my head, don't blame me if your boat explodes as a result of believing the above.
 
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Ubergeekian said:
All off the top of my head, don't blame me if your boat explodes as a result of believing the above.
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Ah, but resistance of the wire increases as its temperature increases, so you need to modify your calculations to take account of that.

Also, not everyone understand Kelvins, so could you give a conversion factor for degrees Centigrade please? ;)
 
pvb said:
Ah, but resistance of the wire increases as its temperature increases, so you need to modify your calculations to take account of that.

Also, not everyone understand Kelvins, so could you give a conversion factor for degrees Centigrade please? ;)
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Did you mean degrees Celsius :D
 
Kelvins are what we scientists use when we can't find the degree symbol on the typewriter.
Temperature in Kelvin is temperature in degC plus 273.15
A change of 1k is the same as a change of 1 degC.
Celsius is the new centigrade, since 1948!

Favourite useless temperature factoid -40 degC = -40degF = very roughly the freezing point of mercury!
 
VoltDropCalc.exe is the name of a programme which forum contributor SvenglishTommy, Tom Vernon, wrote and made freely available. I downloaded it in 2006 from Blueyonder I think.

It calculates cable capacity taking temperature effects into account. He called it ISO 10133 Wire Size Calculator. It works for 12 and 24 volts. I haven't managed to find it on the web today unfortunately.
 
Favourite useless temperature factoid -40 degC = -40degF
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Is a very useful fact.
It provides the basis for much better formulae for conversions between C and F, or vice versa, that the usual formulae where you can never remember whether to add or subtract 32 and whether you do at the beginning or the end.

C to F:- Add 40, times 9/5, subtract 40

F to C:- Add 40, times 5/9, subtract 40

Those I know! The add/subtract 32 formulae I honestly do not know.
 
I do this stuff for a living, and dont generally use anything below 2.5mm2 Tri Rated cable (Very flexible, and with better insulation than bog standard, which allows 30A max current) 4mm2 can cope with 41A, 6mm2 53A and 10mm2 75A. I believe Ancor tineed cable has similar or greater capacity, but you'll pay through the nose for it.

Fuse ratings should reflect the current carrying capacity of the cable, but ALL fuse or breaker types allow a limited time of overcurrent, depending on the type or rating of fuse or breaker (for instance, a domestic B type breaker allows an overcurrent of 5x the rated capacity for 5 seconds or so) As a general rule of thumb, I always use cable which has around 20% greater current carrying
capacity than the fuse or breaker fitted.

I don't recommend doubling up of cables, because it introduces a latent fault into the circuit. if you use 2 x 30A cables to run a load of 60A, then you would need a 60A fuse to protect the circuit. if one of the cables becomes detached or the terminal corrodes and introduces a high resistance, then only ONE cable remains, carrying 60A, while protected by 60A fuse. Result, a fire.

IEE regs only cover domestic and industrial installations, and dont have tables for the type of cables generally found in boats. I would use www.rswww.com for their large range of data sheets.
 
pvb said:
Ah, but resistance of the wire increases as its temperature increases, so you need to modify your calculations to take account of that.
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Not terribly significantly over the temperature ranges involved. 0.004 per Kelvin or thereabouts, so when the temperature has risen by 100K the resistivity has increased by a factor of (1 + 100*0.004) = 1.4 ... by that time heat loss will make the model inaccurate anyway.

Also, not everyone understand Kelvins, so could you give a conversion factor for degrees Centigrade please? ;)
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One. Same unit size, different zero.
 
pvb said:
My post really needed the irony icon, didn't it?
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Here you go, then.

120px-Fe2%2B.svg.png
 
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