Cable specification for ac / dc cables

[pegasus]

When a cable manufacturer quotes a current capacity for mains cable, would this figure also be suitable for DC current.

For example, 1 sq mm cable is normally quoted as having a current capacity of 15A. Would this cable be able to carry 15A at say 12 V dc?

I would have thought that the cable needs to be de rated for dc.

Further to this, is there a way of calculating volt drop for DC currents.

Are the volt drops similar for ac and dc for a given current?

For a given current, I would have thought that the volt drop for dc would be greater.

Any comments would be appreciated.

 

[ban-all-sheds]

When a cable manufacturer quotes a current capacity for mains cable, would this figure also be suitable for DC current.

For example, 1 sq mm cable is normally quoted as having a current capacity of 15A. Would this cable be able to carry 15A at say 12 V dc?

Can't think of a reason why not.

I would have thought that the cable needs to be de rated for dc.

Why?

Further to this, is there a way of calculating volt drop for DC currents.

Are the volt drops similar for ac and dc for a given current?

Yes - the whole point of RMS is to integrate the area under the curve of the waveform to give a value which is the "DC" equivalent.

For a given current, I would have thought that the volt drop for dc would be greater.

In theory it's very slightly lower, because the only component of the total impedance will be the resistance of the cable, whereas with an alternating voltage there will be a reactive component to consider. However, at low voltages and currents (i.e. the domestic supply), this component is so small that it can be ignored.

 

[IanDB]

For example, 1 sq mm cable is normally quoted as having a current capacity of 15A. Would this cable be able to carry 15A at say 12 V dc?

The power drop down a cable is due to the I(squared)R loss. You can pump more power down a cable at higher voltage hence if a cable is rated for 15A at 230v then it's more than adequate for the same current at 12v. However, the proportional voltage drop will be greater at 12v than 230v.

I think I've managed to confuse even myself at that point.

 

[ban-all-sheds]

Power drop, yes. Voltage drop is proportional to I, not I².

There are two basic properties which govern the "capacity" of a cable.

1) Thickness and type of insulation - this affects what voltage it is rated at.

2) Thickness of the conductors - this affects the current it is rated at.

If you had a power supply which could deliver 100's of amps at 1V, and you shorted the terminals with a bit of 1mm² T/E, then even though the cable is rated at 100's of times the voltage, the current flowing through it would melt it, or set fire to the insulation.

Conversely, if you connected 1MV across the 2 cores, then even with no load attached the voltage between the two conductors would (probably) result in destruction of the insulation.

 

[IanDB]

I know what I mean to say it's just that nobody else understands!

If the volt drop caused by say a 15A current flowing in a conductor of 0.1ohm is 1.5v then as a proportion, it has a greater impact when using 12v than 230v (12.5% vs 0.6%). Volt drop can be more critical in low voltage systems.

 

[plugwash]

i think the issues can be summed up as

AC/DC shouldn't make any difference to the current carrying capacity of a cable (hint: we mesure ac voltages and currents rms for a reason)

a cable that can take 15A at mains will happilly take 15A at 12V without damage

however the maximum run length of that cable at 15A with acceptable volt drop will be much smaller in a lower voltage system

 

[pegasus]

Many thanks on the replies.

I hadn't realised that when a cable manufacturer quotes a current capacity for mains installation cable it relates to the rms current value.

I had initially thought it related to the peak value which is why I thought the figure needed to be altered for DC operation.

 

[ban-all-sheds]

All AC numbers (230V supply, 32A breaker, 10.5kW shower etc etc) are RMS....

 

[plugwash]

no 10.5KW is NOT RMS

for a resistive load (which a shower pretty much is)

P(avg)=V(rms)*I(rms)

 

[AdamW]

I have to ask, what is this intended for? The only dc wiring I can think one would run in a "normal" house, would be LED lights of some description.

This is a big assumption, but 12V 15A dc sounds like you are planning on installing something in a car or van.

Whilst electrons are electrons are electrons, motor electrics are installed differently to domestic. With a car you usually (always?) use the metal bodywork as the negative/ground, and use a single conductor to carry power from the battery to the device to be powered.

 

[AdamW]

I knew 240V was an RMS figure (yes yes, 230V officially but we all know it is closer to 240V), but never really considered that we rate things for RMS currents! Funny how sometimes you have 2 and 2 but never get round to making 4

Another thought: I have never seen anyone mention capacitive loads on here, are there any common household devices that are capacitive loads? The only thing I can think of would be an ac/dc power supply.

 

[plugwash]

yeah but such powersupplys if of any nontrivial size tend to have decent power factor correction anyway

iirc normal domestic equipment is not allowed a power factor below 0.8