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# Wire Amps, for give size.

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## Wire Amps, for give size.

(OP)
If I had rectangular strips of copper that were not industry standard size so no ampacity table existed for them. How would I calculate Ampacity? It seems that trying to match a specific circular mill area in the standard wire is not accurate.
I guess another way to ask is what is the ampacity of a given circular mill. When arranged in a rectangle (line a Bus) vs round ( like a wire)

This is your life and its ending one moment at a time.

### RE: Wire Amps, for give size.

AppleJaxJap (Electrical)(OP)2 Feb 23 01:45
"....I guess another way to ask is what is the ampacity of a given circular mill. When arranged in a rectangle (line a Bus) vs round ( like a wire)..."
I look at it in two different main factors:
1. The ampacity is dependent on the temperature:
(a) for bare copper busbar the temperature is limited to say > 70 but < 100 o C.
(b) for insulated conductor is dependent on the type of insulation , say PVC limited to 70 o C; VPE limited to 90 o C etc.....
2. The temperature rise is dependent on the natural cooling; mainly by conduction , radiation and convection by air etc....
Say a 100 mm 2 material be it copper or aluminum; it can be in round (5.6 dia), square (10x10), (20W x 5T) or (20H x 5T ) etc.... of the same cross-sectional area . When say 1A flowing through them, they will be heated up equally. But, their surface temperature/temperature-rise differ due to different cooling surface area and their deposition... etc..
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

Rule of thumb is 1000A per in^2. 1000A for a 4 x 0.25 bus bar.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

### RE: Wire Amps, for give size.

I think the OP's problem is that they want to use a circulation cross section.

I guess the answer would be to compare similar cross section area to perimeter ratios from standard tables to find the best comparison that relates the conducting area and cooling ability.

### RE: Wire Amps, for give size.

(OP)
@che12345 yes this is what i mean

A square conductor that is 1 sq inch has a perimeter of 4 inches
that same 1 sq inch round wire has a perimeter of 3.55 inches
so yes %100 temperature/temperature-rise differs due to different cooling surface area
is there a way mathematically to get from one point to another?

@davidbeach is there any documentation on this? the rectangular wire I'm looking at has insulation. so 1000A per in^2. 1000A for a 4 x 0.25 bus bar aT what temp?

This is your life and its ending one moment at a time.

### RE: Wire Amps, for give size.

Ampacity is ostensibly defined as the amount of current that results in a 30 degC temperature rise. The physical form of the wire determines its heat transfer, which determines the temperature rise. A cheap conservative answer would be to use the largest inscribable circle in the square cross-section of the wire.

Note, however, that ampacity only applies to an isolated wire in air, Anything that affects the heat transfer will technically alter the ampacity

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Wire Amps, for give size.

Dear Mr AppleJaxJap (Electrical)(OP)2 Feb 23 07:1

"....so yes %100 temperature/temperature-rise differs due to different cooling surface area......is there a way mathematically to get from one point to another?..."
Please refer to the "CDA publication 22" This is a very old and out of print book. Google it, you still can get some information out of it. In chapter 3, Calculation of current-carrying capacity ....heat loss by convection, radiation and conduction are illustrated in detail.
Attention: There are some typo errors. Caveat emptor.
Please inform me if you face difficulties from Google, or from public libraries/ technical universities etc...
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

You didn't define the question well enough. Under what conditions do you want the ampacity capacity? Free air outside or insulated in a 3-phase bus bar configuration in a box are very different ampacities.

Here's the book being referenced.

https://electrical-engineering-portal.com/res/Copp...

#### Quote:

This should be a good start, find your closest match.

https://www.copper.org/applications/electrical/bus...

In the real world those numbers will mean little unless these "wires" are uninsulated, mounted in free air outside and without the sun shining on them. This info is useless for bars in an enclosure.

### RE: Wire Amps, for give size.

(OP)
I am only saying bus bar to convey shape. It is not a bus bar it is a rectangular insulated wire.
I am trying to determine the ampacity difference in 2 wires of the same KCMILL, with the same insulation installed in the same location at the same temp.
All else being equal how does the shape of a wire affect ampacity and is their math that gets me from one shape to another?

This is your life and its ending one moment at a time.

### RE: Wire Amps, for give size.

Ampacity of a conductor?
It depends!
Here are some of the factors that determine rated ampacity;
Maximum temperature.
Conditions of use.
Duty cycle.
Wind speed.
Installers in Canada are governed by the following rule tables.
These tables are for field installations and do not cover panel building.
For that you must use a code to which I do not have access.

Canadian Electrical Code, Part I
302 © 2015 CSA Group
Table 1
Allowable ampacities for single unshielded copper
conductors, rated not more than 5000 V, in free air
(based on an ambient temperature of 30 °C*)
(See Rules 4-004, 4-006, 8-104, 12-2210, 12-2260, 26-142, 42-008, and 42-016 and
Tables 5A, 5B, and 19.)
Size,
AWG or kcmil
Allowable ampacity†
60 °C‡ 75 °C‡ 90 °C‡§ 110 °C‡ See Note (3) 125 °C‡ See Note (3) 200 °C‡ See Note (3)
Table 2
Allowable ampacities for not more than three copper conductors,
rated not more than 5000 V and unshielded, in raceway or cable
(based on an ambient temperature of 30 °C*)
(SIMILAR TEMPERATURE RANGES)

Table 5A
Correction factors applying to Tables 1, 2, 3, and 4 (ampacity
correction factors for ambient temperatures above 30 °C)

Table 5B
Correction factors for Tables 1 and 3 (where from two to
four single conductors are present and spaced less than
25% of the largest cable diameter)

Table 5D
Current rating correction factors where spacings are maintained
(in ventilated and ladder-type cable trays)

Table 12
Allowable ampacity of flexible copper conductor cord and equipment wire
(based on an ambient temperature of 30 °C)

Table 12A
Allowable ampacities for portable copper conductor power cables (amperes per conductor)

Table 12B
Temperature correction factor
(See Tables 12A and 12E.)

Table 12C
Conductor rating correction factor
(See Tables 12A and 12E.)

Table 12D
Layering correction factor

Table 12E
Allowable ampacities for Type DLO cables in
a permanent installation in cable tray

Table 28

Determining conductor sizes in the secondary circuits of motors
(See Rule 28-112.)
Classification of service
Percentage of nameplate current rating of motor
5-minute
rating
15-minute
rating
30- and 60-
minute rating
Continuous
rating
Short-time duty
Operating valves, raising or
lowering rolls, etc.
110 120 150 —
Intermittent duty
Freight and passenger elevators,
tool heads, pumps, drawbridges,
turntables, etc.
85 85 90 140
Periodic duty
Rolls, ore- and coal-handling
machines, etc.
85 90 95 140
Varying duty 110 120 150 200
Note: For motor-generator arc welders see Section 42.
Resistor duty classification Duty cycles
Carrying capacity of
conductors in per cent of full
Light starting duty 5 s on 75 s off 35%
Heavy starting duty 10 s on 70 s off 45%
Extra-heavy starting duty 15 s on 75 s off 55%
Light intermittent duty 15 s on 45 s off 65%
Medium intermittent duty 15 s on 30 s off 75%
Heavy intermittent duty 15 s on 15 s off 90%
Continuous duty Continuous duty 110%

Table 36A
Maximum allowable ampacity for aluminum
conductor neutral supported cables

Table 36B
Maximum allowable ampacity for copper
conductor neutral supported cables

Table 57
Allowable ampacities for Class 2 copper conductors
(based on an ambient temperature of 30 °C†)

Table 58
Ampacities of up to four insulated copper conductors in raceway
or cable for short-time-rated crane and hoist motors
(based on an ambient temperature of 30 °C)

Table 66
Ampacities of bare or covered conductors in free air, based
on 40 °C ambient, 80 °C total conductor temperature,
and 610 mm/s wind velocity

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Wire Amps, for give size.

AppleJaxJap: In addition to the lengthy list supplied by waross, you also might need to consider the frequency of your current waveform. High frequencies require very large cross-sections to convey current since the usable conductor is limited to what is called "skin effect" - in essence, it isn't the cross-section that's important, but the perimeter. For more normal line frequencies, the whole conductor cross-section can be used.

In one of your previous posts, you indicated that you have insulated wire - just two different shapes (rectangular and/or circular). Given the identical environment conditions (proximity to other conductors, insulation type and thickness, ambient temperature, same cooling method, etc.) and line frequency conditions, the perimeter does not matter. just the cross-section. So if the two conductors being investigated have the same cross-sectional area (same circular mils or square inches), then they have the same ampacity. Their observed surface temperatures might be slightly different, but not enough to require an infinite tuning of the geometry.

For the record: most larger electric machines (motors, generators, condensers, transformers, etc.) use rectangular conductors, often in multi-turn construction where individual conductors are in close (i.e. "touching") proximity to one another.

Last thing: if you look at typical ampacity tables, you'll see that ampacity doubles when the cross-sectional area doubles. So the incremental difference will be (roughly) linear. This means something with 32 CM will carry 32/28 more current than something with 28 CM, all else being equal.

Converting energy to motion for more than half a century

### RE: Wire Amps, for give size.

Dear Mr AppleJaxJap (Electrical)(OP)2 Feb 23 16:05
".... It is not a bus bar it is a rectangular insulated wire......All else being equal how does the shape of a wire affect ampacity and is their math that gets me from one shape to another? "

With the same cross-sectional area, the main factors that affect the ampacity/temperature rise are:
(a) it can be of different shapes which determine the cooling surface area. Cooling surface area of a rectangular > square > round etc... This is not applicable for round or square.
(b) cooling by convection and radiation is dependent on the deposition. Convection by a rectangular bar in horizontal flat > rectangular bar in vertical .....etc. This is again not applicable for round or square.
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

(OP)
@che12345 thank you! You seem to be the only one who understands what I'm asking. and I think we are close.

All else being equal. All else being equal, how does the shape of a wire affect ampacity, and is their math that gets me from one shape to another?

I will ask it another way.
Let’s say you have a standard 500KCMil wire that had a rating of 380A.
Now let's say you have the same 500KCMil area wire, but it was in the shape of a square, so it had more surface area.
All else being equal.
I would assume that the square one would be able to carry more Amps because it has 1.128x more surface area to dissipate heat.
Is that math to get me the ampacity of the square wire?

This is your life and its ending one moment at a time.

### RE: Wire Amps, for give size.

he shorted answer is that the ampacity is determined by the conductor's ability to radiate the heat generated by the passage of the current and by the temperature limit.
All things being equal, starting with the heat generated by the current.
Just shape. The ability to reject the internally generated heat.
The greater the perimeter, the more area per unit length to reject heat by radiation and convection. In some installations, conduction may also be a factor.
Shape: The more surface area per unit length, the higher ampacity.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Wire Amps, for give size.

Also orientation: Mounted on edge may be better for convection cooling than mounted flat.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Wire Amps, for give size.

I doubt surface area matters much, if at all. I doubt you can just use a 1.13 multiplier. Especially when if this wire is put against another wire so one whole flat side is against another wire.

### RE: Wire Amps, for give size.

Lionel hit on the issue that I used to deal with.
We would wind coils using square or rect wire.
The result was a much higher density of conductor.
And this changed the heat calculations.
In some cases it helps dissipate heat better with no dead air space between windings.
But in other cases the increased density simply resulted in more heat than the system could handle.
You have to evaluate case by case.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Wire Amps, for give size.

It's the shape that determines the heat transfer, so the comparison would be 4 flat plates vs a cylinder.

see https://doc.comsol.com/5.5/doc/com.comsol.help.hea...

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Wire Amps, for give size.

Deer Mr. AppleJaxJap (Electrical)(OP)3 Feb 23 03:40
"#1......All else being equal, how does the shape of a wire affect ampacity, and is their math that gets me from one shape to another?.....Now let's say you have the same 500KCMil area wire, but it was in the shape of a square, so it had more surface area....All else being equal, I would assume that the square one would be able to carry more Amps because it has 1.128x more surface area to dissipate heat...."
Yes. that the square one would be able to carry more Amps ..... This is applicable for a.c. and d.c.; based on the cooling surface.

" #2. Is that math to get me the ampacity of the square wire?.. "
Referring to my earlier post, the approximate dc current rating for flat and round bars are given in detail.
Attention: (a) for ac, the skin effect, due to frequency, size, shape and thickness etc... shall be taken into consideration. Where Rf/R0 is always > 1.0. But R60HZ/ R 50Hz is close to 1.
(b) the skin effect is independent of the magnitude of the current flow. There are other factors etc....
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

Dear Mr Deer Mr. AppleJaxJap (Electrical)(OP)3 Feb 23 03:40
1. For most cases the bar is mounted in free air, the natural cooling by Convection > radiation > conduction. For convection, the heat dissipation from the vertical surface (Wv) is about 1.29 time more efficient/effective than the heat dissipation by the horizontal surface (Wh).
2. Take a case say :
(a) [5H x 2W] i.e. vertical mount, the cooling surface by convection is Wv=2x5=10, + Wh =2 ; ignoring the less effective convection by the bottom horizontal surface.
(b) [5W x 2H] i.e. horizontal mount, the cooling surface by convection is Wv=2x2=4, + Wh =5 ; ignoring the less effective convection by the bottom horizontal surface.
From above (a) and (b), it is clear that vertically mount is preferred. Note: (Wv) is about 1.29 time more efficient/effective than Wh.
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

(OP)
@che12345 thank you I appreciate your time and effort on this.

This is your life and its ending one moment at a time.

### RE: Wire Amps, for give size.

Dear Mr AppleJaxJap (Electrical)(OP)5 Feb 23 22:55
I would like to submit the emissivity factor which had not been touched on, for your consideration.
1. Radiation (Wr ) is considered to travel in straight lines from all surfaces. It is proportional to the temperature difference to the fourth power. It is proportional to the relative emissivity (e) between the body and the surrounding.
2. Typical absolute emissivity of copper busbar: bright metal e=0.1, heavily oxidized e=0.7, dull non-metallic paint e=0.9 ....
3.For a single busbar with emissivity from e=0.1 to e=0.9 is reported to have an increase of >20% in current rating.
4. Dull non-metallic paint which are inexpensive in cost, can be manually brush/spray on at site. The thickness is immaterial. Note: dull powder coating and heat shrink on tubing are NOT cost effective and difficult to implement at side or on an existing installation.
Che Kuan Yau (Singapore)

### RE: Wire Amps, for give size.

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