Thread: One for the sparkies

Watts=Volts X Amps.

It doesn't relate to wire sizes.Wire sizes relate to Amps,or current.

There are charts and rules to work out wire sizes,depending on voltages,where the cables are run,etc,etc.

I am no sparkie,but i think that is correct.

Now where is Homestar when he is needed

really simply

the amps rating of the cable is the limit of the amps it can carry.
the voltage limit is set by the insulation.

watts is as stated volts by amps.

so 500v 10A cable can carry 5000w at 500V

but

500v 10A cable only being provided with 24V can only carry its 10A x the supply voltage (24v) so the wattage you get is the same formula 10Ax24V for 240 watts

Theres a few different ways the amps capability of a cable it caclutated most rely on some kind of temperature at a given number of amps others on a certain voltage drop over a given distance at a given number of amps. Be careful there are different standards for the limits.

elcheapo chineesium wire will have big thick insulation and very thin conductor so it meets the temperature test requirements but by the time your run more than a few feet of it the voltage drop at load is unacceptable.

The best method i believe is by the cross section of the conductor...

heres a quiky link (bout the 4th one on google images when I searched it)

Max Amps in Copper and Aluminum Wire

Originally Posted by Blknight.aus

really simply

the amps rating of the cable is the limit of the amps it can carry.
the voltage limit is set by the insulation.
...............................

elcheapo chineesium wire will have big thick insulation and very thin conductor so it meets the temperature test requirements but by the time your run more than a few feet of it the voltage drop at load is unacceptable.

The best method i believe is by the cross section of the conductor...

heres a quiky link (bout the 4th one on google images when I searched it)

Max Amps in Copper and Aluminum Wire

Until a Sparky illuminates our discussion (sorry, too good to pass up) keep in mind that if you run cable that only marginally carries the current you need (buckets of electrons per unit of time) then IF you have a short or gross overload, then a fuse / circuit breaker will take l o n g e r to fuse or trip.

Being generous with wire size in the planning will mean a quicker and safer response.

Amen to the comment on household wire "origins". - How much is your house worth ? The difference between elcheapo and quality ain't THAT great!

To the OP basically in the electrical industry AC amps = DC amps as generaly quoted. https://www.electronics-tutorials.ws...s-voltage.html

"Most multi-meters, either voltmeters or ammeters, measure RMS value assuming a pure sinusoidal waveform. For finding the RMS value of non-sinusoidal waveform a “True RMS Multimeter” is required.The RMS value of a sinusoidal waveform gives the same heating effect as a DC current of the same value. That is if a direct current, I passes through a resistance of R ohms, the DC power consumed by the resistor as heat will therefore be I²R watts. Then if an alternating current, i = I_max*sinθ flows through the same resistance, the AC power converted into heat will be: I²_rms*R watts.

Then when dealing with alternating voltages and currents, they should be treated as RMS values unless otherwise stated. Therefore an alternating current of 10 amperes will have the same heating effect as a direct current of 10 amperes and a maximum value of 14.14 amperes."

I'm not sure what your purpose is, but if you have some house wire lying around that you want to use for DC, here is a good table.

Choosing the Correct Cable Size for a DC Circuit - Enerdrive Independent Power Solutions

However a lot of house wire nowadays is solid strand i.e. one single conductor, this is brittle and not good for vibration, so I wouldn't use it in a vehicle.

Originally Posted by superquag

Until a Sparky illuminates our discussion (sorry, too good to pass up) keep in mind that if you run cable that only marginally carries the current you need (buckets of electrons per unit of time) then IF you have a short or gross overload, then a fuse / circuit breaker will take l o n g e r to fuse or trip.

Being generous with wire size in the planning will mean a quicker and safer response.

Amen to the comment on household wire "origins". - How much is your house worth ? The difference between elcheapo and quality ain't THAT great!

The first sentence is not really true as the fuse wire will already be "pre-heated" or cb "pre-magnetised" in the event of a fault, unless one has selected a larger size protection device to stop nuisance tripping, not a good idea.

I think you are alluding to the wire size which relates to earth fault loop impedance and PSC (Prospective Short Circuit) current.

Originally Posted by RANDLOVER

The first sentence is not really true as the fuse wire will already be "pre-heated" or cb "pre-magnetised" in the event of a fault, unless one has selected a larger size protection device to stop nuisance tripping, not a good idea.

I think you are alluding to the wire size which relates to earth fault loop impedance and PSC (Prospective Short Circuit) current.

not so, once you get to a certain point the wire itself becomes a current limiting device by turning into a heater, its entirely possible with cheap enough wiring to go full short and not blow out a standard automotive fuse, especially if the wire run is long enough.

Originally Posted by RANDLOVER

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However a lot of house wire nowadays is solid strand i.e. one single conductor, this is brittle and not good for vibration, so I wouldn't use it in a vehicle. [/FONT][/COLOR][/LEFT]

No,it is very rarely used anymore,at least in Qld.

After all these years some have actually worked out the small cost saving using the stuff is negated by the problems with using it.

Most,if not all building wiring is multi stranded,but still not suitable for automotive use.

Older places,single stranded is still found,and it is an absolute PITA to work with.

Originally Posted by BathurstTom

Hi all, my basic understanding is thar watts divided by volts will give you amps. How does this translate to wiring? I.e I can't see 240v 10amp housewire (2400w @ 240v) carriyng 200 amps @ 12v). Is there a formula?

Thanks,
Tom.

Hi Tom

You are correct and as Scarry mentioned, there is a relationship between current and conductor size
The higher the current value the higher the cross sectional area of the conductor
If you don’t increase the size of the conductor the lower the voltage will be at the “device” end
The lower the voltage the higher the current draw
As you stated 2400watts at 240 volts is 10 amps. If you allow a 10% volt drop in your conductor you will end up with 216 volts at your device
2400watts is still 2400watts so you will end up with 11.11 amps being drawn by your device (2400/216=11.11)

There are plenty of decent online calculators that can help you choose your cable size

Be aware that there are at least 3 different methods of measuring the conductor size depending on where you are and what you do in this world
See the attached link, they have a chart with 4 different ways of measuring the same conductor
Cable and wire size conversion

Australian standards require us as electricians to have a maximum voltage drop of 5% over the whole installation, consumer mains, sub mains and final sub circuits
When doing automotive wiring I work on the same principle (MAXIMUM of 5% volt drop)
I’m not sure of the requirements for auto electricians, but 5% of 240 volts is 12 volts and 5% of 12 volts is 0.6 volts
Maybe someone else can elaborate on the auto side of electrimagicians

That is the simples of it before we look at temperature derating of conductors
The hotter, the more resistance, the more voltage drop

With cable size BIGGER is BETTER

Gav