Capacity of wire.

Bikerman2019

I am just wondering about the principle of a wires capacity and appropriateness for a job.


For example, a data sheet I am reading says AWG 18 is good for 15A. Is there any needed consideration for voltage? I am curious about this. I was thinking about adding a PIR to an ordinary light over a front door. Assuming the light is getting 230V and the bulb is an LED 10W. I figure that is about 50mA current.


Could you in theory, use much smaller wire or is does the voltage have to be taken into account and how


Thanks

punisher5112

.75 flex or 1.5mm cable is a must.

Bikerman2019

punisher5112 wrote: »

.75 flex or 1.5mm cable is a must.

Thanks for that, but I am trying to understand the principles to take into account. i.e. Is voltage a consideration when choosing a wire and why?

punisher5112

Bikerman2019 wrote: »

Thanks for that, but I am trying to understand the principles to take into account. i.e. Is voltage a consideration when choosing a wire and why?

Regs would determine it as it wouldn't be suitable using anything else.

If someone were to change the fitting or add more to it it could over heat and melt the sheath.

salmocab

Well generally we don’t take voltage into account as we use standardised voltage. In theory higher voltage lowers current as they are inversely proportional. We don’t use AWG anyway.

DublinDilbert

Bikerman2019 wrote: »

Thanks for that, but I am trying to understand the principles to take into account. i.e. Is voltage a consideration when choosing a wire and why?

No voltage won’t impact the cross sectional are of the cable, but it will determine the insulation required.

For mains, most cables are typically oversized, it also allows for them to be de-rated when covered with insulation. Sometimes larger cables are also used for mechanical strength, example is 10sq earth cable.

The cable size will be mainly determined by the load current and the protective device feeding the cable.

Xertz

One of the most important considerations is how the circuit is protected from overcurrent and whether the wire you're using is adequately sized to be safe with that level of protection.

A lighting circuit for example might be protected by a 10amp MCB, so any wiring used along the route is rated accordingly.

Also I would probably consult something other than an AWG table as American Wire Gauge does not apply to types of cable that you'd use in Ireland or Europe and the steps in amperage ratings are different too.

However, if you use a thinner cable than what is safe with the rating of the circuit (not the lamp) and there's a fault like a short-circuit, then you are at risk of having the cable melt and burn before the protective device i.e. the circuit breaker trips.

You can't just use thinner cable based solely on the calculation of the load, otherwise you'd need local fusing to eliminate fire hazards.

Bikerman2019

DublinDilbert wrote: »

No voltage won’t impact the cross sectional are of the cable, but it will determine the insulation required.

For mains, most cables are typically oversized, it also allows for them to be de-rated when covered with insulation. Sometimes larger cables are also used for mechanical strength, example is 10sq earth cable.

The cable size will be mainly determined by the load current and the protective device feeding the cable.

mmmm Thanks folks. Going to digest that. I am shortly completing a course in Electronic Engineering and use quite fine wire. This is something we have not covered but am curious about.


Cheers folks.

BbSean

If you are dealing with mains the the regs will state what cables you should be using.

As for considering voltage, it really is the amperage going over the cables.

More about it here:

https://production-technology.org/power-losses-in-cables/

Basically the lower the amperage the higher the voltage you can run over the cable.

That’s why the high voltage network runs at very low current,- minimal loss over the cable ( heat).

Hope this makes sense

Bruthal

salmocab wrote: »

Well generally we don’t take voltage into account as we use standardised voltage. In theory higher voltage lowers current as they are inversely proportional. We don’t use AWG anyway.

Well it's not that simple in reality of course. Sure, 1kw at 230v is 4.5 amps and would be 9 amps at 115v.
But only because the load designed for 230v will have double the impedance of the 115v load.

Connect the 1kw 115v load to 230v and it will now be 4kw.

1 million volt supply at 10 amps still only needs 1.5 cable, but is carrying 10MW.

When its stepped down through transformers, the V x I remains contant, so in that case, the voltage reduces, and current increases in proportion.

brightspark

Another factor is the cable insulation, e.g. PVC, Teflon, or even mineral insulated (MICC). (The reason is that different materials degrade at different temperatures limiting the current specification reduces how much the cable heats up)

Also how the cables are installed (buried with other cables without much heat dissipation or in free air).

Voltage drop on longer cable runs is also a factor.

LenWoods

Also worth checking if there are specific PIR sensors for LED lighting prior to purchasing any PIR

As I fitted a TimeGuard segment switch which is originally designed for an immersion heater: I fitted one to some 20W osram LED floodlights in my attic and I ended up removing it; just fitted a normal switch because there was a trickle of electricity passing through which saw the floodlights operating in a very dim mode instead of being completely off.

I didn't fit a PIR in my attic as in the event of temporarily accommodating a rodent the movement would increase my electrical bill, so I don't know if there are LED specific PIR but its something worth checking prior to purchase and installing.