How much current

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Ponchik

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CA
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Electronologist
Where can i find out how much current a piece of non insulated metal can handle. For example, a #4 solid copper, 3/8" piece of copper rod, a 1/8" piece of steel plate, 1/8" aluminum bus bar......

The ampacity capability of different metals.

Thanks
 

Ponchik

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CA
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Electronologist
This is an interesting question. What is your application or is it just curiosity?

curiosity.

Just as comparison on 310.16 a 1/0 THHW has an ampacity of 170A and on 310.17 in free air installation 1/0 THHW has an ampacity of 260A.

Now what is the ampacity of the same size copper material (wire or just a solid piece of copper rod) without the insulation? or same size steel rod?
 

gar

Senior Member
Location
Ann Arbor, Michigan
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EE
121227-1721 EST

edward:

"Reference Data for Radio engineers", by ITT. Fusing currents of wires p55 in my book. Copper, aluminum, german silver, iron, and tin. An equation is provided. Data from Automatic Electric Co.

These are approximations and you have to make judgements for your purpose relative to these ultimate failure points.

For example #6 copper is 668 A. This is the highest of the metals referenced above.

.
 

Jraef

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"Ampacity" as defined in the NEC and other codes, is not so much how many amps a conductor CAN carry, i.e. conductivity, but rather how many amps it can SAFELY carry. Then from there you have to define "safely". For example when you insulate a conductor and the insulation catches on fire, it is no longer safe. If it is non-insulated but it warps or distorts to where something else touches it, then it is not safe.

Steel is not a safe conductor under any definition I am aware of, and that has to do with the conductivity issue. It is conductive, but it presents a lot more resistance than something like copper or aluminum, so for the same physical dimensions it will carry a LOT less current before it distorts. Think arc welding...

Copper conductivity is slightly worse than aluminum, but aluminum comes with other issues that can make it less safe in many applications. Gold or silver is even better, but if you think we have a problem with wire thieves now, just try using them! Copper is not the best conductor, it is the optimum conductor.
 

mike_kilroy

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United States
keeping Jraef's good comment about how the rating came about for safety, isn't the answer simply equiv. circular mills area? If a wire X with Y mm^2 area is rated Z amps, would not a similar metal with same Y area be rated the same Z amps?
 

mivey

Senior Member
keeping Jraef's good comment about how the rating came about for safety, isn't the answer simply equiv. circular mills area? If a wire X with Y mm^2 area is rated Z amps, would not a similar metal with same Y area be rated the same Z amps?
There are other factors: shape and emissivity come to mind.
 

gar

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EE
121227-2004 EST

don:

My guess it is more like a longer term fusing point, maybe call it steady state, possibly minutes. I did not try to track down the original source. The comments indicated that the values could vary considerably.

Obviously the value is dependent upon material, its melting point, resistivity, ambient temperature, and radiation characteristics (emissivity).

.
 

mivey

Senior Member
121227-2004 EST

don:

My guess it is more like a longer term fusing point, maybe call it steady state, possibly minutes. I did not try to track down the original source. The comments indicated that the values could vary considerably.

Obviously the value is dependent upon material, its melting point, resistivity, ambient temperature, and radiation characteristics (emissivity).

.
Most likely steady-state with an assumed ambient, emissivity, absorptivity, conductivity, etc as well as a thermal limit (usually below the point where the material is warped, materially damaged, mechanically weakened, etc).
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
121228-0803 EST

edward:

Since you may not have easy access to the reference I provided here is some of the information.

The equation used is I = K*d(3/2) where I is fusing current in amperes, K a constant based on the material, and d the wire diameters in inches.

For # 6 wire (0.1620" dia) the values provided are:

668 A, K=10244, copper
495 A, K= 7585, aluminum
341 A, K= 5230, german silver
208 A, K= 3148, iron
107 A, K= 1642, tin

Not stated, but we can assume that these are uninsulated wires in still air, vertically mounted and of sufficient length that the end terminations don't greaterly alter the heat transfer at the center of the wire. Probably very little tensile load.

.
 

Besoeker

Senior Member
Location
UK
keeping Jraef's good comment about how the rating came about for safety, isn't the answer simply equiv. circular mills area? If a wire X with Y mm^2 area is rated Z amps, would not a similar metal with same Y area be rated the same Z amps?
Not as a rule.
The temperature at which insulated conductors can run is likely to be determined by the temperature limitations of the insulation. For the most common insulated cables here that is either 70C or 90C.
Bare conductors can normally carry more current firstly because they are not constrained by the insulation and secondly there isn't the covering which is both electrically and thermally insulation to the heat transfer into the surrounding air is more effective.
 

Julius Right

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Electrical Engineer Power Station Physical Design Retired
The current flowing through conducting medium produces losses[heat losses].Part of the heat losses will raise the conductor temperature and another part it is evacuated through insulation and other layers of insulating material producing temperature drop up to outside medium[ air, ground or other]. The total sum of this temperature drops [insulation, jacket and outside medium] will be the conductor temperature.
Other source of heat -as other cable- in vicinity, may raise more the temperature of the considered cable.

I think we have to take into consideration the regime of the current flowing through the conductor. There is a short-time, intermittent and a steady state.
The insulation-the close layer sensible to temperature- can overcome a more elevated temperature for short-time. For instance, PVC usually undergoes 70 dgr.C in steady state and 130 dgr. short-time, EPR and XLPE 90 dgr.C steady and 250 short-time.
The heat is not evacuated fast. In a case of short-time heating one may consider no heat will be evacuated and all the losses will raise the conductor temperature.
Second, the heat passes from one layer to another in 3 ways: conduction, convection and radiation.
In most of materials as insulation, jacket, (serving) and Earth, the conduction is the way.
The air conduction way is negligible but the convection and radiation are significant.
In our case -1/0 copper conductor THHW-NEC 310.15.16- the cable is 3 core cable-or 3 single-core cable-in earth [or in cable raceway] where the cable are reciprocally heating and the heat is evacuated to the outside medium, slightly. In this case the cable is more heated and less cooled so reach the maximum permitted insulation temperature at a lower level current.
In the second case-NEC 310.15.17 -the cable is a single-core in free air so the heat is evacuated easily and no other source of heat is in its vicinity. So a more elevated current can flow to reach the same temperature.
 

Besoeker

Senior Member
Location
UK
The current flowing through conducting medium produces losses[heat losses].Part of the heat losses will raise the conductor temperature and another part it is evacuated through insulation and other layers of insulating material
Under steady state conditions it all dissipated through the outer surface of the conductor whether it is insulated or not.
 

Besoeker

Senior Member
Location
UK
121228-0803 EST

edward:

Since you may not have easy access to the reference I provided here is some of the information.

The equation used is I = K*d(3/2) where I is fusing current in amperes, K a constant based on the material, and d the wire diameters in inches.

For # 6 wire (0.1620" dia) the values provided are:

668 A, K=10244, copper
495 A, K= 7585, aluminum
341 A, K= 5230, german silver
208 A, K= 3148, iron
107 A, K= 1642, tin

Not stated, but we can assume that these are uninsulated wires in still air, vertically mounted and of sufficient length that the end terminations don't greaterly alter the heat transfer at the center of the wire. Probably very little tensile load.

.
Good stuff.
It might also be worth mentioning that the shape of the conductor also matters. Bus bars are generally rectangular in section to get a higher current carrying capacity for the same amount of copper than a square or round bar for example.
 

hurk27

Senior Member
Good stuff.
It might also be worth mentioning that the shape of the conductor also matters. Bus bars are generally rectangular in section to get a higher current carrying capacity for the same amount of copper than a square or round bar for example.

I agree with this as more surface is exposed you have more thermal transfer, but this also applies to higher frequencies as the skin effect takes into consideration the impedance of the conductor is lowered.
 

Besoeker

Senior Member
Location
UK
I agree with this as more surface is exposed you have more thermal transfer, but this also applies to higher frequencies as the skin effect takes into consideration the impedance of the conductor is lowered.
Actually, it applies to all frequencies from DC upwards. Increasing the frequency pushes the current towards the outside of the conductor (hence skin effect) so less of the cross-sectional area is used which increases the impedance.
 
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