Weird electric question about solid bare wire

[GoldDigger]

For longer speaker wire runs (and in cheap lightweight short runs) the series impedance of the wire between the amp's feedback point and the speaker voice coil terminals will affect the control the amp output has on speaker motion. The ratio of nominal speaker impedance at audio frequencies plus the wire resistance over the effective output impedance of the amplifies, called the damping factor, is critical for good speaker performance. Where wire impedance is significant, minimizing it is as important as the current handling capacity of the wire.

At the short distance here, the resistance is unlikely to be a factor in conductor size choice.

 

[kec]

I had a decent stereo in my first mini-van, a Dodge Caravan, many years ago.

An in-dash CD/radio fed a 4-way active cross-over that fed:
A 2x35w amp running ribbon tweeters on top of the dash next to the OEM speaker openings,
A 2x40w amp running 5" mid-ranges in the OEM openings,
A 2x55w amp running a pair of 6.5" mid-woofers in the front doors,
And a 1x125w amp running a 12" sub-woofer in a large tuned-port box in the back of the van.

It had an excellent front sound-stage image.


But my best system is now my home theater.

Wow, I bet Pete Townsend and Roger Daltrey's [Rain on Me] must have rock you out of the van..

 

[RayM77]

Some speaker circuits are unbalanced with one terminal grounded (neutral). Others, typically for higher power. are balanced, with the center tap grounded but not run to the speakers. So in this particular case, a neutral running to the speaker is unlikely.

Honestly I’m not 100% sure, I just know I have 3 connections out, a +v, a 0v and a -v rails. But I came here because I have a friend that’s said this sight gives great advice.

 

[RayM77]

For longer speaker wire runs (and in cheap lightweight short runs) the series impedance of the wire between the amp's feedback point and the speaker voice coil terminals will affect the control the amp output has on speaker motion. The ratio of nominal speaker impedance at audio frequencies plus the wire resistance over the effective output impedance of the amplifies, called the damping factor, is critical for good speaker performance. Where wire impedance is significant, minimizing it is as important as the current handling capacity of the wire.

At the short distance here, the resistance is unlikely to be a factor in conductor size choice.

Mostly I just don’t want to look back and see glowing wires lol but this build needs to look good as well

 

[winnie]

Take a look at this old thread:

Busbar Ampacity

Where can I find the calculation for finding the ampacity of a busbar? I have saw it before, but cant remember where. Is it in the NEC?

forums.mikeholt.com

The ampacity of a round 0.4 diameter copper rod will be lower than that of a 1/4x1/2 bar. The two have essentially the same cross section area, but the rectangular bar has greater surface area for cooling. A 1/8x1/2 bar has the same surface area but greater cross section.

IMHO with 0.4 diameter copper bars, polished however you wish, at 150A you will have no problem with temperature rise. Your question is how much smaller you can go.

If you can get a copy of the calculation referenced in the linked thread, you could estimate the ampacity for any given bars.

Jonathan

 

[RayM77]

Take a look at this old thread:

Busbar Ampacity

Where can I find the calculation for finding the ampacity of a busbar? I have saw it before, but cant remember where. Is it in the NEC?

forums.mikeholt.com

The ampacity of a round 0.4 diameter copper rod will be lower than that of a 1/4x1/2 bar. The two have essentially the same cross section area, but the rectangular bar has greater surface area for cooling. A 1/8x1/2 bar has the same surface area but greater cross section.

IMHO with 0.4 diameter copper bars, polished however you wish, at 150A you will have no problem with temperature rise. Your question is how much smaller you can go.

If you can get a copy of the calculation referenced in the linked thread, you could estimate the ampacity for any given bars.

Jonathan

So I actually have it saved in my phone but most all talk about a specific voltage, I ask smaller as 4 gauge solid bare is fairly common online but some of the larger sizes don’t seem to be, also by the math there will be around 3k cfm blowing back and forth around the wire.

 

[winnie]

You have a copy of ANSI/IEEE C37.23? Can you share the bus bar ampacity calculation?

Heating of conductors depends only on current, not voltage. So the calculation should give the answer you seek.

Power dissipation in a conductor is equal to current squared time resistance. Copper

#4 wire has a diameter of 0.2043". It has a resistance of 0.253 milliohms per foot. Carrying 150A the wire has to dissipate 150*150*0.000253=5.7 watts per foot. This is about 4x the resistance and 4x the heat generation of a 0.4" diameter conductor.

You have 1/2 the surface area to get rid of that heat, but I don't know anything about the scaling of the heat dissipation to the air. Using the guess that 1/2 area means 2x thermal resistance, then I'd expect halving the diameter of the wire means 4x the heating and 8x the temperature rise.

Based on the bus bar tables I expect a 0.4" wire would work for you with less than a 30C temperature rise. So I'd expect #4 wire to work with less than a 240C temperature rise.

I think the #4 would work, but I wouldn't use it. It might get hot enough to sag in its supports or damage the plastic supports, and wire is supplied as soft temper rolls. You will never get it nice and straight. Copper rods will function as conductors and are available hard temper.

Jonathan

 

[winnie]

Also: if you want to go with wire, look at magnet wire suppliers. I know you can get #2 solid as magnet wire, and the 'varnish' type insulation might work as your end finish.

 

[ggunn]

Bare copper will allow noise on audio lines. It is ill advised to not have a insulator on the wire.

Insulation has no effect on RF noise entering a conductor; only grounded shielding will help, but any noise picked up by speaker conductors will be so much lower in amplitude than the audio signal, even at very low volume, that you will never hear it. Speaker wires are seldom if ever shielded.

 

[ggunn]

Honestly I’m not 100% sure, I just know I have 3 connections out, a +v, a 0v and a -v rails. But I came here because I have a friend that’s said this sight gives great advice.

If you say so, but I don't know of any speakers that have three terminals.

 

[wwhitney]

NEC Table 310.21 gives ampacities for bare conductors in free air with a 2ft/sec wind, 40C ambient, and 80C max conductor temperature, and it gives an ampacity of 155A for #4 Cu. I was going to just say "40C temperature rise" but I guess with the wind, the heat removal rate will depend on the air density which will depend on the actual air temperature. I think applying this to a lower ambient temperature is conservative, as the air will be denser and so should remove heat more quickly.

In contrast, NEC Table 310.17 gives ampacities for single insulated conductors in free air, and it gives an ampacity of 140A for #4 Cu with a 60C temperature rise. A bare conductor is going to dissipate heat better than that, as it will not have the thermal insulation effect of the electrical insulation, i.e. using these values is conservative. At 150A amps, the insulated #4 Cu conductor would have a temperature rise of 60C * (150/140)² = 69C.

At 155A, the insulated #4 Cu conductor would have a temperature rise of 60C * (155/140)² =74C. So I infer that the effect of removing the insulation and applying a 2 ft/sec wind is to approximately double the heat loss rate (74C vs 40C, but the ampacity values I expect are all round to the nearest 5A).

Cheers, Wayne

 

[drcampbell]

... A bare conductor is going to dissipate heat better than that, as it will not have the thermal insulation effect of the electrical insulation ...

Not necessarily. The limiting factor will be the heat transfer where the air touches the wire. Electrical insulation can improve that heat-transfer performance by increasing the available surface area. Even though the electrical insulation is a much worse thermal conductor than copper, it is a much better thermal conductor than air.

 

[Jraef]

Maximum space allows for a .4” conductor diameter.

Assuming inches:

.40 diameter is close to 000 (called 3/0) copper wire, which is .410 diameter and is good for 165A.

If the .40 is a hard limit, then the next size down is 00 (2/0), which is .365 and good for 145A.

I don't know of anyone who makes "solid" wire at those sizes however, it is all stranded.

You could have someone fashion you some "bus bar", but determining the size is a little more complicated.

 

[wwhitney]

.40 diameter is close to 000 (called 3/0) copper wire, which is .410 diameter and is good for 165A.

That figure of 165A does not seem applicable to the OP's use. It applies when you want to limit the conductor temperature rise to 30C, and the conductor is in a conduit or cable with up to 3 other CCCs. The OP's description sounds like it will shed heat more easily than that, and may tolerate well above a 30C temperature rise.

Cheers, Wayne

 

[Speedskater]

Highly-Conductive 101 Copper Rods are available from 1/8 to 1/2 inch in 1/16 steps.

This reminds me of dB drag racing or SPL level in vans contests. The would fill the doors & panels with concrete and 3 inch thick plastic windows all to prevent vibrations. Up to a dozen alternators to supply electricity. Up to 185 dB SPL.

 

[wwhitney]

Not necessarily. The limiting factor will be the heat transfer where the air touches the wire. Electrical insulation can improve that heat-transfer performance by increasing the available surface area. Even though the electrical insulation is a much worse thermal conductor than copper, it is a much better thermal conductor than air.

Good point, although on the last point, because of convection, that's not immediately obvious to me. I wonder how it works out in practice?

Call r the copper radius, a*r the insulation thickness, and D the conductor temperature rise in the uninsulated case for some fixed current. Then for the insulated case, the insulation surface temperature rise will be D/(1+a). The temperature rise across the insulation will be C * ln(1+a) for some constant of proportionality C, which is something like 2 * pi * k, where k is the appropriate thermal conductivity.

So the ratio of the conductor temperature rise in the insulated case to that in the uninsulated case is 1/(1+a) + (C/D)*ln(1+a). For large values of a, this is clearly bigger than 1. But for small values of a, whether this ratio exceeds 1 will depend on whether the ratio C/D is bigger than 1, as the power series expansion of the above starts off 1 + (C/D - 1) * a + higher order terms.

Cheers, Wayne

 

[4x4dually]

I'm just here for the pictures.....so where are they?

This build sounds amazing. Me and my 12" Kicker subs will be over here in the gutter waiting on your project completion to wow us.

 

[ggunn]

Good point, although on the last point, because of convection, that's not immediately obvious to me. I wonder how it works out in practice?

Call r the copper radius, a*r the insulation thickness, and D the conductor temperature rise in the uninsulated case for some fixed current. Then for the insulated case, the insulation surface temperature rise will be D/(1+a). The temperature rise across the insulation will be C * ln(1+a) for some constant of proportionality C, which is something like 2 * pi * k, where k is the appropriate thermal conductivity.

So the ratio of the conductor temperature rise in the insulated case to that in the uninsulated case is 1/(1+a) + (C/D)*ln(1+a). For large values of a, this is clearly bigger than 1. But for small values of a, whether this ratio exceeds 1 will depend on whether the ratio C/D is bigger than 1, as the power series expansion of the above starts off 1 + (C/D - 1) * a + higher order terms.

Cheers, Wayne

As someone else has pointed out, since there would be no insulation to endanger with the heat, why would we care how hot the bare wire gets? It's unlikely to get anywhere near hot enough to significantly compromise the metal itself.

 

[jim dungar]

As someone else has pointed out, since there would be no insulation to endanger with the heat, why would we care how hot the bare wire gets? It's unlikely to get anywhere near hot enough to significantly compromise the metal itself.

But it could affect the 'acrylic' it is being mounted into.

 

[zbang]

Again, do the math to figure out the resistive loss and then decide if that's acceptable; all else follows from that.