1 x wire cable versus 2 x wires (half size of 1 x wire cable)

[eufemiano]

Hello Fellow mates,

I have a question, what are the pro's and cons of using 2 wires with half the size of a single wire ? I know it is uncommon, but is it possible? For example you need a wire with a size of 50mm2 but you don't have that and so, instead as an alternative you use 2x 22mm2 wires. would it be the same ?

thank you.

 

[electrofelon]

There are advantages to using parallel sets. One is that it reduces losses from skin effect if the single conductor would be large enough where skin effect becomes a factor (above 350 MCM. Second is the ampacity is quite a bit higher for two half sized conductors. The principal disadvantage is you would have more conductors so you would probably have to derate the ampacity, but still you can often come out ahead even after the derating.

 

[tom baker]

It would be 2 resistors in parallel, use ohms law and determine voltage drop for current
In the US use of parallel wires is common and only allowed for 1/0 and larger.
as current is not proportional to wire size

 

[LarryFine]

Smaller conductors can dissipate heat better, and are easier to handle and pull.

You can often run two smaller conduits easier and cheaper than one large one.

Parallel runs require more pulls, but again, they're easier pulls with less weight.

 

[drcampbell]

Extra-high and ultra-high voltage (220 kV and up) transmission lines often use multiple wires to form one conductor. While this practice does also improve heat dissipation, (the usual spacing is WAY more than thermal concerns dictate) the primary motivation is reducing inductive and corona losses. A widely-spaced bundle provides almost the performance of a single solid conductor of the same diameter.

 

[gar]

220330-0839 EDT

If one wire has a diameter of 0.1", and you parallel two 0.05" diameter wires, then the paralleled wires only have 1/2 the area of the single wire and thus resistance is 2 times that of the single wire per unit length. Thus, for the same current the power dissipation for the 0.1" diameter wire per unit length is 1/4 times the dissipation of the parallel wires. Big problem. For an equivalent resistance you need 4 of the half diameter wires.

.

 

[GoldDigger]

You also have to deal with the terminations of the paralleled wires if the end point terminals do not accept more than one wire.

 

[Arester]

As electrofelon pointed out – skin effect is a big factor and it grows with the wire gauge. Sometimes the benefits of using two runs maybe quite surprising. See the two calculations below;

Although two parallel runs shown require separate conduits spaced one diameter apart to get the ampacity required, the benefits may outweigh the cost of running the extra conduit. Not only you would be using one third of the copper, the voltage drop and cost of operating the feeder would be lower with the smaller wires. Installation cost may also be lower all things considered. (Unfortunately packing the parallel wires in one conduit would necessitate increasing the wire gauge from 3/0 to 250MCM)

 

[wwhitney]

As electrofelon pointed out – skin effect is a big factor and it grows with the wire gauge.

Skin effect may start to be a factor above 350MCM, but it can't be a very large factor at 500MCM.

The dominant factor that causes ampacity over cross-sectional area to decrease with increasing size is the worsening heat dissipation. With a circular cross-section, the perimeter is increasing slower than the area with increasing diameter. So the perimeter will reach the temperature limit of the insulation at a lower ampacity per unit area.

If we had wire of rectangular cross section of constant thickness, and only the width varies, then at least heat dissipation wise we'd expect to see fairly constant ampacity per unit area as the size increases.

Cheers, Wayne

 

[Arester]

Opps... That supposed to say " using one third less copper". Two runs of 3/0 use ~ two thirds the amount of copper a 500MCM wire is using.

 

[electrofelon]

Skin effect may start to be a factor above 350MCM, but it can't be a very large factor at 500MCM.

I'm not sure I would say that Wayne. For example I recently designed a 400 amp feeder, 165 feet, assume about 300A for VD purposes. Ianalyzed it with one set of 500 aluminum versus two sets of something. Turns out 2x3/0 have close to the same VD as the single 500, or two sizes down from half sized. I'll take it.

Edit: 1x500= 2.27 %, 2x250= 1.94%, 2x 3/0=2.71%. so it might not seem like much looking at the percentages but when you look at from the standpoint of two wire sizes different it seems more significant to me.

 

[wwhitney]

Edit: 1x500= 2.27 %, 2x250= 1.94%, 2x 3/0=2.71%. so it might not seem like much looking at the percentages but when you look at from the standpoint of two wire sizes different it seems more significant to me.

Except it's only one wire size difference. With the implied 240V supply, single phase, Southwire's calculator tells me 1 x 500 = 2.28% and 2 x 4/0 = 2.22%.

On the other hand, thanks for pointing out that a VD calculator tells you the impedances and hence skin effect. So we have:

(1) 250 kcmil Al, 75C ampacity = 205A, VD in your example = 3.38%
(2) 250 kcmil Al, 75C ampacity = 410A, VD in your example = 1.94%
(1) 500 kcmil Al, 75C ampacity = 310A, VD in your example = 2.27%

Now if both ampacity and conductance (1 / VD) scaled linearly with cross sectional area, then you'd expect (1) 500 kcmil to match (2) 250 kcmil. But instead we have a conductance ratio of 1.94 / 2.27 = 85%, and an ampacity ratio of 310 / 410 = 76%.

Those are closer than I expected and the conductance ratio is farther from 100% than I expected. But the ampacity ratio is still lower than the conductance ratio because of the heat dissipation effect. At first glance looks like skin effect and heat dissipation are about comparable effects, assuming they both contribute to reduce ampacity. I'd need to look at some smaller sizes to understand better the separate effects.

Cheers, Wayne

 

[electrofelon]

Except it's only one wire size difference. With the implied 240V supply, single phase, Southwire's calculator tells me 1 x 500 = 2.28% and 2 x 4/0 = 2.22%.

On the other hand, thanks for pointing out that a VD calculator tells you the impedances and hence skin effect. So we have:

(1) 250 kcmil Al, 75C ampacity = 205A, VD in your example = 3.38%
(2) 250 kcmil Al, 75C ampacity = 410A, VD in your example = 1.94%
(1) 500 kcmil Al, 75C ampacity = 310A, VD in your example = 2.27%

Now if both ampacity and conductance (1 / VD) scaled linearly with cross sectional area, then you'd expect (1) 500 kcmil to match (2) 250 kcmil. But instead we have a conductance ratio of 1.94 / 2.27 = 85%, and an ampacity ratio of 310 / 410 = 76%.

Those are closer than I expected and the conductance ratio is farther from 100% than I expected. But the ampacity ratio is still lower than the conductance ratio because of the heat dissipation effect. At first glance looks like skin effect and heat dissipation are about comparable effects, assuming they both contribute to reduce ampacity. I'd need to look at some smaller sizes to understand better the separate effects.

Cheers, Wayne

Yeah definitely the ampacity is the bigger factor, but all I am saying is, IMO, skin effect of a 500kcmil is not insignificant.

Btw my example was 208 three phase. And actually in this case, I did go with 2 parallel 250, even though 3/0 would have met my voltage drop goals. This is because this was MC cable and my supplier didn't have 3/0. They did have 4/0, but neither had the proper EGC size for a parallel installation. 250 does.

 

[Arester]

For those interested skin effect was already disussed @ https://forums.mikeholt.com/threads/ampacity.2461/

 

[wwhitney]

For comparison I compiled the table below from the (2017) NEC Tables. The area is from Chapter 9 Table 8 ; the 60 Hz AC Resistance is from Chapter 9 Table 9; and the 75C Ampacity is from Table 310.15(B)(16).

I chose resistance R rather than impedance Z because to my understanding that is the only component which generates a heating effect in the wire. (For voltage drop it would be appropriate to use Z instead). Then A * R would be constant if the current density were constant across the whole conductor cross section. I'm not sure why A*R is bouncing around so much between #8 and 400 kcmil; but the increase seen from 400 kcmil to 1000 kcmil is the skin effect.

Cheers, Wayne

Size​	A (mm^2)​	R (Ohms/km)​	75C Ampacity​	A * R​	Amps/mm^2​
8​	8.37​	2.56​	50​	21.4​	6.0​
6​	13.30​	1.61​	65​	21.4​	4.9​
4​	21.15​	1.02​	85​	21.6​	4.0​
2​	33.62​	0.62​	115​	20.8​	3.4​
1​	42.41​	0.49​	130​	20.8​	3.1​
1/0​	53.49​	0.39​	150​	20.9​	2.8​
2/0​	67.43​	0.33​	175​	22.3​	2.6​
3/0​	85.01​	0.253​	200​	21.5​	2.4​
4/0​	107.20​	0.203​	230​	21.8​	2.1​
250​	127.00​	0.171​	255​	21.7​	2.0​
300​	152.00​	0.144​	285​	21.9​	1.9​
350​	177.00​	0.125​	310​	22.1​	1.8​
400​	203.00​	0.108​	335​	21.9​	1.7​
500​	253.00​	0.089​	380​	22.5​	1.5​
600​	304.00​	0.075​	420​	22.8​	1.4​
750​	380.00​	0.062​	475​	23.6​	1.3​
1000​	507.00​	0.049​	545​	24.8​	1.1​

 

[wwhitney]

PS Empirically in the above table, ampacity is varying quite closely to diameter D to the 1.2 power. Rather than D to the 2nd power, i.e. area A.

Cheers, Wayne