Is the bigger wire section always better from the electrical point of view?

Question

A bigger wire section is better for lowering power loss and voltage drop due to lower cable resistance.

Is there any case or application where using a wider cable section is worse?

(Exclude mechanical reasons)

It costs more. And depending on specific circumstances, there may be better options than increasing cable cross section. So in general, no, not always better. But what is your specific situation? — Justme, Jun 28 '22 at 18:50
@Justme no specific situation just curious about any case where would be (electrically) convenient to use a more narrow wire section — BamsBamx, Jun 28 '22 at 18:53
Did you have any idea what kind of cable or wire. There's alot of applications and parametrics= converting Big into MANY metrics, also differences between Big and Wide — Tony Stewart EE75, Jun 28 '22 at 22:15
Multi strands may be used for flexibility, and with AC due to skin effect. — Russell McMahon, Jun 29 '22 at 11:27
Sometimes you want power loss in the wire, e.g., for electric heating. — Andrew Morton, Jun 30 '22 at 17:14

score 17 · Accepted Answer · answered Jun 28 '22 at 18:51

17

Is there any case or application where using a wider cable section is worse?

Yes, it may create the wrong characteristic impedance. Here I'm thinking cable (as per your question) and coaxial cable that has a large diameter usually means it has a higher characteristic impedance that will not suit RF systems that (for instance) run at 50 ohms.

A bigger wire will have more capacitance to ground and this may not be good for high frequency power transmission systems.

Also, with high frequency power transmission, due to skin effect, a bigger diameter wire may seem preferable but it's a law of diminishing returns because HF currents tend to hog the output surface of the wire. Better t0 use a wire tube and save all that copper material in the main section around the middle.

An individual wire used to wind an inductor will likely have less inductance per metre length hence, require more turns to wind the inductor. I'm thinking here particularly of air-cored inductors.

answered Jun 28 '22 at 18:51

Andy aka

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Larger diameter conductor has higher AC losses for an inductor/transformer due to proximity effect. There is a balancing act between DC and AC losses when designing magnetics. – qrk Jun 29 '22 at 00:04
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Nobody would actually make "bigger coax" by scaling up the outer conductor without appropriately scaling the inner to get the desired impedance, though. If you want 50-ohm coax in 46mm diameter you can have it — it just has an inner conductor that's an 18mm copper tube! – hobbs Jun 29 '22 at 04:50
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But people will scale up coax to get a higher impedance and that is the point and, it's a point that the OP probably isn't aware of. – Andy aka Jun 29 '22 at 06:56
Regarding skin effect, for stranded wires and meshes, what is the point? Do they stop behaving like a single conductor and start working as separate wires? – jaskij Jun 29 '22 at 14:58
@jaskij That's a whole new question. See my brief comment on the original question - but its far too complex for a comment. Multi strand "litz wire" acts more like a skin-effect free single conductor at RF frequencies than a single conductor does. – Russell McMahon Jun 29 '22 at 22:27
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@hobbs Also worth noting that while you can scale up the entire coax to maintain the characteristic impedance, but you cannot scale it up indefinitely, eventually your signal will excite the waveguide modes (mostly TE11) instead of the TEM mode of the transmission line, and it's game over at this point. The larger the coax cable, the lower the waveguide cutoff frequency. While it's not a problem for a shortwave radio station at 30 MHz, for GHz microwave signals it's a real showstopper. – 比尔盖子 Jun 30 '22 at 10:36

score 9 · Answer 2 · answered Jun 28 '22 at 18:57

In high voltage AC applications, where the skin effect is significant, it is sometimes better to use multiple conductors over one larger one. A previous question related to this:

Why are the high voltage overhead power lines with the same potential being isolated?

score 6 · Answer 3 · answered Jun 28 '22 at 23:36

Is there any case or application where using a wider cable section is worse?

This can be generalized: is there any case or application where using a more resistive wire is preferred?

Obviously: yes! Heaters come to mind. But thermal cycling introduces fatigue due to cyclic thermal stresses, thus terminating thin heater wire reliably is a challenge. So it's a tradeoff between the material cost and reliability. Resistive wire in general is made in vastly smaller quantities than "regular" wire, thus it tends to cost more and have relatively fewer sources.

A less talked about common scenario is oscilloscope probe cables. High impedance (1Mohm) oscilloscope probes need quite a bit of distributed resistance in the cable to match the cable to the load impedance. Some probes have a hair-thin central conductor in the coax - to minimize capacitance and to increase the resistance per unit of length.

score 2 · Answer 4 · edited Jun 28 '22 at 20:52

Obviously, there are disadvantages such as cost and environmental impact. These alone call for always choosing the correct wiring dimension, which is generally defined as the smallest cross section that meets demand.

In some instances, a so called green engineering concept is applied, where you calculate the power loss of the cable and choose the most economical based on the load and duration. Often this is one size up from the necessary size, and the idea is that the decreased power loss eventually provides a saving that exceeds the higher purchase price.

One issue with bigger cables in AC

These factors aside, on the engineering side of things, there is one disadvantage (excluding mechanical aspects), when dealing with electrical circuits consisting of cables carrying single or multiple phase AC in particular.

Cables (conductors, really) have a rated resistance and capacitance, often stated in ohms per kilometer (when using SI units). These are combined to get the cable's impedance \$Z = \sqrt{(R^2 + X^2)}\$.

At lower cross sections the capacitance can safely be disregarded, as it's insignificant. This is the case with conductors smaller than or equal to 16 mm² (or 16 mm² equivalent). When larger, it should be taken into account.

As the cross section of the conductor increases, the resistance decreases (advantage) but the capacitance increases (disadvantage).

At some point, the numbers essentially cross over to a point where the capacitance is far more decisive than the resistance to the cable impedance, and obviously at this point, increasing the conductor cross section essentially comes with a penalty of increased impedance - but obviously with the gain of being able to conduct a higher current.

score 1 · Answer 5 · answered Jun 29 '22 at 18:20

There are a number of things that have already been discussed about transmission line effects and capacitance, but a MAJOR one occurs in ordinary mundane 'nearly DC' mains wiring.

If you make the conductors too big the PSC (Prospective short circuit current) can become very large which then makes circuit protection more difficult and expensive then it has to be, and raises the spectre of arc flash and blast.

Often the limits are needing enough current to flow in a fault condition to clear the fault quickly on the low end while not passing so much current that things get 'splodey on the high end.

Is the bigger wire section always better from the electrical point of view?

5 Answers5