24V + 10A = Fire?

I'm just curious, cause I ran into an issue with melted cords today.

Most of the time when I'm using 24V for switches, there are so few things that need it, it's only a 50W power supply and it's fused at 2A.

This machine I had an issue with was fused at 10A (For all the 24V controls... and there are a lot). It seems as though the switches should have been grouped together and fused seperately lower (But I didn't make the machine, so it wasn't my call).

I'm wondering if it's possible for 24V to create enough heat to melt cords going to switches (22 AWG, Inductive Proximity Switches). I've never seen it happen and was just curious if maybe we had something or someone playing around where they shouldn't.

yes it can easily generate enough heat also I've had fires where a 1 amp fast acting fuse does not get blown. So I've learned not to depend on Fuses to prevent fires which only seems to work if the fire causes a short to ground.

Don't have my reference with me, but 10 amps is pushing the limit for 22 AWG, plus I don't know the details. But I think it's possible.

It's not the volts that causes heat, it's the amps. If the amps are more than the wire size will allow, it will heat the wires. Doesn't matter if it's 24VDC or 115kV AC.

The wires after the fuse should be rated for the current the fuse is sized at.

So for 10A at least 1.5mm (or 16AWG).

OZEE said:

It's not the volts that causes heat, it's the amps. If the amps are more than the wire size will allow, it will heat the wires. Doesn't matter if it's 24VDC or 115kV AC.

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Technically, its the watts, V*A, that causes heat, specifically the amount of electrical energy that is converted to heat energy by the resistance of the wire. Ohm's and Kirchoff's laws apply not only to the load but to the conductors themselves. In the case of wire heating, the relevant voltage is not the voltage that the wire is carrying, its the voltage drop within the wire itself. This is where the wire size and amps comes into play. Higher amperage flowing in a wire means a larger voltage drop in the wire. The smaller the wire, the larger the voltage drop, hence a small wire cannot carry the same current as larger wire without getting hotter; Voltage drop is higher, therefore the V*A that is "burned" in the wire is higher. A wire carrying 10 amps connected to 24 volts or connected to 1000V has the same internal voltage drop per inch and thus the same internal heating, this is what leads to the saying that the current is what matters, but actual heating is stil from V*A.


22AWG copper wire is too small to carry 10A because the voltage drop in the wire is sufficient to cause the VA "burned" in the wire itself to be large enough to overheat the wire.

One thing to be careful of with DC power supplies is that many power supplies will limit the output current at the rated amperage by reducing the voltage output of the power supply. Thus a 10A power supply with a 10A fuse might not ever clear the fuse but continue to deliver 10A into a short.

A wire carrying 10 amps connected to 24 volts or connected to 1000V has the same internal voltage drop per inch and thus the same internal heating,...

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Careful here, Alaric. We know what you mean, but that last part "same internal heating" might be slightly misleading.

For example,assuming 24 VDC and 1000 VDC, and if a long wire has a resistance (the factor that causes voltage drop) of 1 Ohm, then the current due to the resistance would be I = E/R = 24/1 = 24 Amps, and the VA would be 24 x 24 = 576 VA, but for 1000 volts it is I = 1000/1 = 1000 Amps, and VA would be 1000 A x 1000 V = 1 million VA. I think 1 million VA would produce more internal heating than 576 VA.

Power Consumed is dependent on both volts and amps.

Just me, but I've always gone with the definition of watts, I^2*R. Smaller wire = higher R = higher watts.

Quick 2cents worth: While the prior math makes perfect sense, in practical use the ampacity tables I have seen only concern themselves with: wire material, wire gauge, and length of run. I have never actually seen them take into account VOLTS other than as it relates to the insulation.

The observation that the load should be divided among more fuses makes good sense to me. 10amps on 22 gauge wire is asking for trouble.

Stationmaster

Where I²R comes from:

(DC) Watts = E * I
E = IR
Substituting, Watts = IR * I = I²R

Also:
Watts = E * I
I = E/R
Substituting, Watts = E * E/R = E²/R

Lancie1 said:

Careful here, Alaric. We know what you mean, but that last part "same internal heating" might be slightly misleading.

For example,assuming 24 VDC and 1000 VDC, and if a long wire has a resistance (the factor that causes voltage drop) of 1 Ohm, then the current due to the resistance would be I = E/R = 24/1 = 24 Amps, and the VA would be 24 x 24 = 576 VA, but for 1000 volts it is I = 1000/1 = 1000 Amps, and VA would be 1000 A x 1000 V = 1 million VA. I think 1 million VA would produce more internal heating than 576 VA.

Power Consumed is dependent on both volts and amps.

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Thank you Lancie for bringing up a needed clarification.
If you will notice, I said "A wire carrying 10 amps connected to 24 volts or connected to 1000V has the same internal voltage drop per inch." I added the constraint of 10 amps (making the assumption that there is an appropriate circuit load) because we are talking about only the heating that occurs in the wire itself, therefore the voltage of concern in the VOLTAG DROP in the wire itself, not the voltage drop across the entire circuit. Thus if we constrain the current at 10 amps, in Lancie's wire with a resistance of one ohm there is a voltage drop of 10 volts, whether the circuit input voltage is 24V or 1000V (again, assuming an appropriate soruce/load at the ends of the wire for a complete 10A circuit) and a power dissipation in the wire of 100 watts. We must hope that Lancie's hypothetical wire is quite long or its going to glow.

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FWIW, I made a Q&D calculation on the OPs 22 guage wire - a 1 meter lenght will burn about 15 watts at 10 amps of current flow. It doesn't sound like much, but its more than enough to warm up that wire to an undesirable temperature, depending on how long its on and environmental conditions it could definitely cook the insulation.

Stationmaster said:

...in practical use the ampacity tables I have seen only concern themselves with: wire material, wire gauge, and length of run. I have never actually seen them take into account VOLTS other than as it relates to the insulation.

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That's because the interal voltage drop of the wire itself is accounted for when the tables are drawn up. The heating is caused by this voltage drop at a specific current and the tables are adjusted accordingly based on the ability of the wire to dissipate the heat. When you pack the wires in a conduit, you interfere with this heat dissipation, so the wires are derated. On the flip side, I use water cooled kickless cables on vacuum furnaces that are 400MCM yet are rated to carry 10,000 amps - its all about the heat removal and whatever voltage drop you are content to tolerate.

In many cases you may not automatically know the resistances involved but V and A you may know.

The thing is, why are the wires getting hot?

A 100 ft. of 22ga should be about 1.5 - 2 ohms; which using any formula offers up to 40 watts with 24v supply and 10amp.

The issue is with inputs the devices should limit the current into the ma range; which should keep the wattage below 1.

If the same wiring and supply is being used for outputs, these wires could have higher current flow and develop more wattage. The thing here is that if the wires are grouped tightly together the heat is cumulative and in time could melt the insulation.

There are other factors, temp rating of the wires may not be appropriate. It depends on temp and type of wire whether it can handle a 10A load, each wire should be rated to handle the current allowed by the fusing.

Then again, there could have just been a "short" that allowed full current to flow thru one wire, which melted the insulation and the insulation on wires close to it. Many times when things like this happen several things have to be replaced so it may not be obvious what caused the problem.

Tharon said:

I'm just curious, cause I ran into an issue with melted cords today.
...
I'm wondering if it's possible for 24V to create enough heat to melt cords going to switches (22 AWG, Inductive Proximity Switches). ...

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The short answer is NO. A switch is pretty much a switch and has minimal load (I did not say ZERO but it is close). I think what you are stating is a bunch of inputs on 24VDC all from the same power supply that is fused at 10 amps. The 22 AWG wire is the wire found on the field device itself. This in no way should be a problem. I have wireway with hundreds of 24VDC I/O (running on 16 AWG over 1000 feet) on 20A power supplies and have never had a meltdown. If you have only 1 wire going from the powersupply (and that is 22AWG) and it is carrying all the load to a field box, then you have a much bigger problem.

rsdoran said:

The thing is, why are the wires getting hot?

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Exactly, it is not the field device doing it.

rsdoran said:

The issue is with inputs the devices should limit the current into the ma range; which should keep the wattage below 1.

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Or even much less.

rsdoran said:

Then again, there could have just been a "short" that allowed full current to flow thru one wire, which melted the insulation and the insulation on wires close to it. Many times when things like this happen several things have to be replaced so it may not be obvious what caused the problem.

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My bet also.

Even though all the information may be technically correct, I don't think that is what you are asking. If I am off base, let me know, but I don't feel that the wire size to the field device is causing a problem.

I have seen prox cables burnt to a crisp before. Things were bad all over best I remember and a lot of stuff had to be replaced. Input card, cable, prox, power supply, etc. Its been a while but I know it was a suprise.

Control wire ampacity

As I consult the Canadian Electrical Code I see the following...

Assume single conductor, not a cable.
Insulation type TEW, SEW, REW. (common insulation types for control wire. 600v 90 Deg C.) Table 11.

No derating is required due to number of conductors in a raceway.

Rule 4-018 - use table 12 for MAX ampacity.

A wire size of #22 is rated for a max of 3 amps.
A 24 volt supply fused at 10 amps will need a #14 wire feeding a "buss" Table 12. The common buss or terminal block can then feed #22 wire. Rule 14-100 then says because you reduced the wire size, each #22 conductor must be fused individually at no larger than it's rated ampacity....3 amps. If you use an insulation type "lower" than mentioned, like TXF, then you have to derate the conductor ampacity mentioned in table 12. In this case the max is reduced to 2 amps.

This may be old, (18th edition), but it is still minimum standard for Canadian electricial installations. The NEC can not be far off this. Our codes are very close.

Bruce99 said:

ampacity.

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Is this a new word

Never heard of it before, just sounds funny to me