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#### Robby2022

Jan 19, 2022
10
Hi,

I need to calculate the temperature of a wire, due electric current passing through, and I have no idea how to do that. I need it for a device that I want assemble.

I'm using one single thread (strand) of speaker cable, like you can see in this picture:

Again, Just a single thread that I'm pulling out of the cable.

The length of the wire will be about 50 cm, and I'll connect it to a power supply of 12V DC, and 2A current.

(I think that it's copper, and I estimate that it's about 1-2mm thick)

What will be the temperature of the thread after about 1 minute?

Will it reach 60°C ?

I tried to look up for it at Google but the calculations are too complicated for me :-(

Please help me figure this out, if possible also show me the calculation so I can calculate it for other currents.

Thanks very much!

(It's not a homework question)

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,715
The temperature rise of a wire will depend on many factors:
• wire resistance (defined by length, diameter and material)
• current through the wire
• thermal resistance of the wire vs. ambient temperature (defined by the type and thickness of the insulation and the way the wire is mounted and possibly cooled)
Absolute temperature will then be the sum ob ambient temperature + temperature rise.

#### Robby2022

Jan 19, 2022
10
The temperature rise of a wire will depend on many factors:
• wire resistance (defined by length, diameter and material)
• current through the wire
• thermal resistance of the wire vs. ambient temperature (defined by the type and thickness of the insulation and the way the wire is mounted and possibly cooled)
Absolute temperature will then be the sum ob ambient temperature + temperature rise.

Thanks, but I know all that... I thought that using the data that I gave it will be possible to give me the actual temperature...

Again:

Single thread that pulled out of the speaker cable (like in the picture) I estimate that it's a 1-2mm thick copper, the length of the wire will be about 50 cm, and I'll connect it to a power supply of 12V DC, and 2A current, in free air of about 25°C.

Isn't that enough to calculate the temperature of the thread after 1 minute?

At least a rough estimation...

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,715
Thanks, but I know all that.
Then you should know that the info you give is insufficient.
Repeating the insufficient data doesn't increase the information content.

Here's a graph where you can read temperature rise vs. current for single insulated wire. Length is irrelevant. The longer the wire, the higher the resistance. Consequently more power is converted to heat provided the current stays the same (2 A in your case). But at the same time the available surface for getting rid of the thermal energy increases proportionally, therefore -> irrelevant. Your non-insulated wire's temperature may rise a bit less than the graph indicates as the insulation will reduce the flow of thermal energy from the wire to ambient air (thermal resistance).

#### Robby2022

Jan 19, 2022
10
Then you should know that the info you give is insufficient.

Repeating the insufficient data doesn't increase the information content.

Here's a graph where you can read temperature rise vs. current for single insulated wire. Length is irrelevant. The longer the wire, the higher the resistance. Consequently more power is converted to heat provided the current stays the same (2 A in your case). But at the same time the available surface for getting rid of the thermal energy increases proportionally, therefore -> irrelevant. Your non-insulated wire's temperature may rise a bit less than the graph indicates as the insulation will reduce the flow of thermal energy from the wire to ambient air (thermal resistance).

I looked at your link, sorry but it's like Chinese for me.

Why the info I gave is insufficient?

What data is missing to be able to calculate it?

I said that it's a 1-2mm thick copper thread, I gave you the current through the wire (2A), what else is missing?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,715
1- 2 mm will mean a factor 2 in resistance and therefore a factor 2 in power dissipation. Thus the temperature rise will also vary accordingly.

#### Robby2022

Jan 19, 2022
10
1- 2 mm will mean a factor 2 in resistance and therefore a factor 2 in power dissipation. Thus the temperature rise will also vary accordingly.

So... can you give me a number?

Can you say if (after 1 minute) the thread will heat to at least 60°C?

I want to know if I have to buy power supply of 12V (DC) 2A, or do I have to buy a 12V, 4A power supply?

I don't want to waste too much money on experiments and equipment.

#### Alec_t

Jul 7, 2015
3,241
50cm of 1mm diameter copper wire has a resistance of about 12mΩ. So a 12V source will try to drive a current of about 12V/12mΩ = 1000A through it. That would vapourise the wire virtually instantly.
You haven't said whether 2A is the rated current of the supply or whether it is a controlled current limit.

#### Robby2022

Jan 19, 2022
10
50cm of 1mm diameter copper wire has a resistance of about 12mΩ. So a 12V source will try to drive a current of about 12V/12mΩ = 1000A through it. That would vapourise the wire virtually instantly.
You haven't said whether 2A is the rated current of the supply or whether it is a controlled current limit.

I will limit the current to 2A (using resistors) in order to prevent damage to the power supply.

Giving this, can you tell me what will be the temperature of the thread after 1 minute?

Indoor, room temperature of 25°C.

#### Alec_t

Jul 7, 2015
3,241
The power dissipated in that piece of wire will be 2A x 2A x 12mΩ = 48mW. I doubt it will get noticeably warm to the touch.
If you're trying to make a heater, use resistance wire (e.g. nichrome) instead of copper

#### Robby2022

Jan 19, 2022
10
The power dissipated in that piece of wire will be 2A x 2A x 12mΩ = 48mW. I doubt it will get noticeably warm to the touch. If you're trying to make a heater, use resistance wire (e.g. Nichrome) instead of copper

Thanks very much!

I never heard of this, but now I read a little about it and it sounds like a piece of Nichrome thread will be perfect for what I need!

The only question is will current of 2A be enough for heating the wire to about 40-60°C ? which I estimate that it's exactly what I need.

I'll be happy if someone can calculate it and tell me (again, room temperature, indoor, about 25°C, and thread of about 0.1mm (or 0.2mm, or 0.5mm) and about 50cm length).

I really like this new direction, and I already started to check where can I buy this wire.

#### ChosunOne

Jun 20, 2010
462
The power dissipated in that piece of wire will be 2A x 2A x 12mΩ = 48mW. I doubt it will get noticeably warm to the touch.

You are correct, Alec.
Years ago, some of our field techs were careless about keeping their supply of fuses stocked for house calls. When they came across a blown 1A to 5A fuse for which they didn't have a replacement, they'd wrap one or more strands of wire around it (a single strand from a 22 AWG stranded wire, which had (IIRC) about 20 strands to the wire), which they swore a factory rep had assured them reliably melted at 1 (one) Amp, therefore was a slow-blow 1A fuse. Those strands were at least as thin, if not thinner than in the OP's picture.

I was the only one to set up a trial-and-error test and used stacked resistors to test what Amperage it would take to melt such a strand. It was a quick-and-dirty experiment but it established that the strands being used would carry about 4 Amps before melting (and slowly, by fuse standards). They didn't get noticeably warm at 3 A.

The "precision stranding" practice stopped. The guys either kept their fuses stocked, or made an extra trip to replace fuses they didn't have. There's a reason fuse wires aren't made of pure copper.

#### Robby2022

Jan 19, 2022
10
You are correct, Alec.
Years ago, some of our field techs were careless about keeping their supply of fuses stocked for house calls.... There's a reason fuse wires aren't made of pure copper.

Very interesting, thanks!

As I said in my previous post, I think that I will use a Nichrome strand.

#### Bluejets

Oct 5, 2014
5,985
I would have to doubt that one strand from a speaker wire flex would be between 1 and 2 mm thick........... more like around 0.2mm

#### Robby2022

Jan 19, 2022
10
Actually I'm very optimistic now, I found lots of stuff about this Nichrome wire and I think that I can heat it for what I need even with 5V 1A power supply... Here are only few of the things I found:

#### Robby2022

Jan 19, 2022
10
Actually I wanted the videos to start at certain timestamp so you will see how hot it gets, but it didn't work.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,715
@Robby2022 : It would have helped had you told us from the beginning what you want to achieve. The nichrome wire would have come up in the first or second reply.
As you repeatedly phrased the question I got the impression you were worried about temperature increase due to load current.

#### Robby2022

Jan 19, 2022
10
@Robby2022 : It would have helped had you told us from the beginning what you want to achieve. The nichrome wire would have come up in the first or second reply.

As you repeatedly phrased the question I got the impression you were worried about temperature increase due to load current.

The Nichrome wire came up on comment 11 and right away I replied and said that it's sound perfect for what I need

I prefer not to say what it is about because I may want to write a patent on this. But I tried to be very clear about what I want to achieve, I said that I want the wire to get warmed to at least 60°C in about one minute, indoor, in a room temperature of about 25°C.

Why isn't that enough?

Last edited:

#### Bluejets

Oct 5, 2014
5,985
I prefer not to say what it is about because I may want to write a patent on this.

Then sit back and watch as either it has been though of yonks ago or some kid from an underdeveloped country copies it and floods the market with 20 cent copies.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,715
I tried to be very clear about what I want to achieve, I said that I want the wire to get warmed to at least 60°C
Youi did - in post #7.
Right from the beginning there was no mentioning of your purpose at all.

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