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Ampacity of Copper Conductors
9

Ampacity of Copper Conductors

Ampacity of Copper Conductors

(OP)
Anyone know why Table 310.15(B)17) lists 4/0 copper at 360*C but wire spec sheets list it at 480 amps? Same for other wire sizes... what is the actual ampacity? Am I correct to think NFPA70 assumes no wind while wire manufacturers assumes wind?




RE: Ampacity of Copper Conductors

You are comparing bare copper wire with insulated copper wire.
The Wire insulation provides thermal insulation as well as electrical insulation.
Note also that your 4/0, insulated wire has an ampacity of 405 Amps at 90 degrees C.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

(OP)
I guess thats behind it. Any idea what bare copper is rated at 90 or 100*C?

RE: Ampacity of Copper Conductors

3
The NEC current ratings in free air is based on a wind speed of 2 ft/sec (610 mm/sec).
It should be noted the following:
  • Insulation provides extra thermal resistance that makes this cable operate warmer than the bare conductor of the same size exposed to the same ambient conditions. Therefore, it should be expected that the rated Apmt of the insulated conductor < bare cable amp.
  • For overhead applications, Insulated conductors are not limited by the insulation and sagging but for bare conductor only sagging is the limiting factor.
  • The term standstill is used in some ampacity tables for ambient without wind. Also, the ampacity rating includes 961 watts/sq. foot sun, 0.5 coefficients of emissivity and absorption primarily for bare cables.

RE: Ampacity of Copper Conductors

In the Canadian code, the equivalent table lists bare copper or cable with special insulation at:
110 degrees C, 455 Amps
125 degrees C,485 Amps
200 degrees C, 590 Amps
Note (3) These ampacities apply to bare wire or under special circumstances where the use of insulated conductors
having this temperature rating is acceptable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

(OP)
@Cuky:

Quote:

Insulation provides extra thermal resistance that makes this cable operate warmer than the bare conductor of the same size exposed to the same ambient conditions. Therefore, it should be expected that the rated Apmt of the insulated conductor < bare cable amp.


Now this is getting interesting! smarty I originally assumed the opposite- that a covering would increase dissipation due to increase surface area not carrying electricity. Then I doubted my train of thought when I posted here and I would have agreed with this. However, a code expert pointed me to 310.15 B 21 which seems to actually show bare copper as having a lower current rating:



None the less it is still lower than most manufacturer data.

I looked at publicly available PJM documents which seem to use 2ft wind only for Emergency conditions and no wind for steady state operating though in theory this is a separate issue outside the scope of this thread but none the less makes me wonder where the true ampacity of 4/0 stands.

RE: Ampacity of Copper Conductors

Thank you for your information Cuky.
I don't see our information as being contradictory, rather we are sharing information from different sources based on slightly different parameters.

Quote (Cuky)

but none the less makes me wonder where the true ampacity of 4/0 stands.
That's easy.
The ampacity is whatever the AHJ says it is. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

(OP)
I think we can all agree on the AHJ having the last word.

Any idea at what temperature annealing takes place?

RE: Ampacity of Copper Conductors

In NEC Annex B it is noted:
"For additional information concerning the application of these ampacities, see IEEE
STD 835-1994, Standard Power Cable Ampacity Tables."
Let’s take IEEE 835/1994 page 33:
0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable
90ºC - Copper Conductor - Concentric Stranded in Free Air - Single Isolated Cable
40ºC Air Ambient
In free air there are 4 possibility:
no sun no wind 434 A ; 2 Ft/s 542 A
full sun no wind 344 A ; 2 Ft/s 477 A
NEC Table 310.15(B)(17) 4/0 90oC 405 A 30oC air.
Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F)
36-40oC Conductor temp. 90oC 0.91
That means according to NEC 4/0 copper conductor insulated for 90oC in 40oC air =
405*0.91= 368.55 A . No any condition meats IEEE 835/1994.
For ampacity calculation there are 2 possibilities :
according to Neher and McGrath[corrected according to IEEE 835/1994] or IEC 60287.

RE: Ampacity of Copper Conductors

For ampacity calculation there are 2 possibilities :
according to Neher and McGrath[corrected according to IEEE 835/1994] or IEC 60287.
From NEC table 5
Type XHHW, XHHW-2, XHHN insulated core dia. 16.21 mm [0.638 inches]
From NEC table 8 dcopper= 13.41 mm[ 0.528”] 40oC AIR
IEEE Method 435 A NO SUN NO WIND
IEC Method 448 A NO SUN NO WIND

RE: Ampacity of Copper Conductors

(OP)
Alright, so I now want to ask: do I assume wind or no wind?

RE: Ampacity of Copper Conductors

Calculated in "no sun no wind" condition for bare conductor the ampacity is lower than for covered case due to decreased heat dissipation by radiation: the radiation factor for covered is close to black [92%] but for bare decrease up to 23%.
Ventilation can amplify convection and thus compensate for the reduction in radiation.
How much? I will have to use the calculation method from IEEE 738 and that as soon as I can.

RE: Ampacity of Copper Conductors

2
Seasonal ratings are bad enough. Adding time of day, day of year, and wind speed plus cloud cover is just asking for trouble unless you have a fully automated dynamic rating scheme and a way to enforce those ratings. No long term profit in squeezing out the last first cost dollar.

RE: Ampacity of Copper Conductors

(OP)
@ David, where would you put 4/0 under your seasonal ratings?

@7anoter4: Can you explain the theory behind radiation factor?

RE: Ampacity of Copper Conductors

Calculated according IEEE 738/2007 IEEE Standard for Calculating the Current-Temperature of Bare Overhead Conductors for no sun, wind 2ft/s and ε[Emissivity (0.23 to 0.91)]=0.23 the ampacity of 4/0 copper [75oC conductor 25oC air]=487 A

RE: Ampacity of Copper Conductors

I agree with davibeach and I did not introduce the sun exposure in my calculation.

RE: Ampacity of Copper Conductors

(OP)
487- close to that table. Any idea of the current at 100*C?

RE: Ampacity of Copper Conductors

The heat evacuation from cable surface to air has two parts-convection and radiation.
The radiation depends on surface emissivity-ε it is the ratio between the black body emissivity
4.96 kcal/m^3.hr.(100oK)^4 and the this body emissivity q=vol.4.96*ε*(T/100)^4 where T it is the body temperature in Kelvin degrees and vol it is the body volume [m^3] q[= kcal/hr].

RE: Ampacity of Copper Conductors

In the same conditions[Ta=25oC,no sun,2ft/s] I=580 A.

RE: Ampacity of Copper Conductors

(OP)
I owe you now.

Can you explain how surface emissivity works?

RE: Ampacity of Copper Conductors

4/0 what? Could potentially deal with CU, AAC, ACSR, ACSS (generally don't have ACSS that small). The facility ratings come out of the Planning group. For protection I just look at the highest rating, which is for the coldest ambient and make sure nothing trips at that point. Not sagging it to the ground on high load on hot days is the responsibility of the system operators.

RE: Ampacity of Copper Conductors

Emissivity is just a measure of how well something emits thermal energy. If something is a good absorber, it is also a good reflector. Poor absorber, poor reflector.

Mike

RE: Ampacity of Copper Conductors

(OP)
4/0 copper. Which yes I know is typically not found on any new lines- but go back 100 years it was common.

RE: Ampacity of Copper Conductors

(OP)
mparenteau: Is this why condenser coils are black and evporator coils are white?

RE: Ampacity of Copper Conductors

"If something is a good absorber, it is also a good reflector. Poor absorber, poor reflector."

That is not correct. A good absorber is a good emitter but a poor reflector. A reflector is a poor absorber and a poor emitter.

Shiny copper is a good reflector in the infrared, as is gold.

Condenser and evaporator coils are generally designed for convective heat transfer and not radiative heat transfer. Car radiators not really radiators, either. Heating radiators in homes are rarely black, they usually are white or silver. Go figure.

RE: Ampacity of Copper Conductors

Did you get your reflections mixed, CompositePro?
The condenser coils are black for best radiation, opposite of absorbtion.
The evaporator coils may be unpainted because many paints and coverings don't do well in a wet, freeze-thaw cycle.
In refrigerators, the light colours are probably to facilitate identifying and cleaning dirt.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

Sometimes answering a specific question gives the impression that I do not agree with what has been said by other specialists in the field. I am sorry if I left this impression.
I do not have experience in as many fields as waross or vast and deep knowledge in the field of cables, grounding and many others like cuky2000.
Because in the 60 years since I finished my studies I only dealt with design, I got in the habit of looking for explanations by calculation and less from my own experience.
Usually the questions do not contain enough data to be able to calculate.
This is not the case with MBrooke where the accuracy of the question forces me to a precise answer.
By the way, I agree with the views expressed by all participants and consider that I have still
more to learn from my participation in this very interesting and instructive forum.


RE: Ampacity of Copper Conductors

Emmissivity shows up in the formula for heat transfer by radiation.

Reference - https://www.engineeringtoolbox.com/radiation-heat-...

For objects other than ideal black bodies ('gray bodies') the Stefan-Boltzmann Law can be expressed as

q = ε σ T4 A (2)

where

ε = emissivity coefficient of the object (one - 1 - for a black body)

RE: Ampacity of Copper Conductors

Thank you for the mention, 7anoter4.
I have learned quite a bit from your posts.
Keep up the good work.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

@CompositePro:

Yes...thanks for keeping me honest.

To restate: Good absorber/good EMITTER is what I should have said. Poor absorber/good reflector.

Mike

PS This can be shown by the equation: a + r = 1, where:

a = absorption (or emissivity)
r = reflectivity

this assumes no transmission, else the equation is a + r + t = 1.

RE: Ampacity of Copper Conductors

(OP)
Can I ignore conduction and radiation for short circuit calcs?

RE: Ampacity of Copper Conductors

Short circuits are just that, short! Better not worry about heat being transferred during such conditions, assume adiabatic conditions!

RE: Ampacity of Copper Conductors

(OP)

Quote:

adiabatic

Schweet!!!!!!!!!!!!

What final short circuit temp do I not want to exceed.

RE: Ampacity of Copper Conductors

What is the melting point of copper?
(Not as off the wall as you may think.
I remember years ago, an instruction sheet for drying the ends of mineral insulated cable prior to termination.
"If an open flame is used to dry the cable, care must be taken to not melt the copper".)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ampacity of Copper Conductors

(OP)
Good question- I don't know for hard drawn copper.

My fear is annealing.

RE: Ampacity of Copper Conductors

See IEEE 80/2013 section 11.3.1.1 Calculation of TCAP.11.3.1.2 Resistivity of clad steel rod Table 1
Copper, annealed soft-drawn Fusing temperature Tm (°C)=1083
As far as I know above 300oC copper softens and losses its mechanical strength.
From Materials for electrical engineering by Jaromír DRÁPALA:

RE: Ampacity of Copper Conductors

Please see Onderdonk or Preece formulas for fusing of conductors!

RE: Ampacity of Copper Conductors

(OP)
Need some more info Parchie- I like where you're going with this.


Any idea how much strength hard drawn loses at 300*C?

RE: Ampacity of Copper Conductors

Sorry. In my post of 21 july 8:01 it is noted:
"In the same conditions[Ta=25oC,no sun,2ft/s] I=580 A.
I meant this:
In the same conditions[Ta=25oC,full sun,2ft/s] I=580 A.[ε=0.23,α=0.23]].Sorry.

RE: Ampacity of Copper Conductors

(OP)
580 amps is not bad at all. Thanks for crunching these numbers! Debt noted, I owe you now.!

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