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Wire Parameter Calculator September 20, 2007

This Javascript web calculator will calculate the resistance and ampacity for copper wire based on the gauge. Both metric (mm) and American Wire Gauge (AWG) are supported. Note: Ampacity is based on a curve fit to MIL-STD-975. To see the wire table that this calculator is based on as well as important information about wire insulation temperature ratings, click here.



Wire Size

Optional Inputs:

Wire Temperature Deg. 
Wire Length
Number of Wires in Bundle

Results (per each wire):

Resistance Ohms
Single Wire Ampacity Amps
Wire Bundle Ampacity (per wire) Amps
Copper Diameter
Copper Area
Copper Weight


1. Gene - October 11, 2007

Having an option to select aluminum wires instead of copper would also be handy.

2. Brad - October 11, 2007

Hi Gene,

That is a good idea. The spec this calculator is based on does not cover aluminum wires so I will have to think about how to scale and adapt it. For sure the resistance is easy to calculate, but the ampacity is the tricky one.


3. Paul - October 23, 2007

Who uses Aluminum wire these days? I thought they banned that stuff in the 70’s? Aluminum melts at a relatively low temperature and doesn’t conduct as well. I wouldn’t waste time calculating that.

4. Paul - October 23, 2007

Thanks for the Calculator BTW. It’s the best one I’ve used so far.

[Paul, Glad you like it! Thanks, Brad]

5. al - October 25, 2007

Can you determine the different speeds of a motor by checking the windings or wires of a 3 speed single phase motor using an ohm meter?

[Al, I don’t think so - or at least I don’t know how. Brad]

6. Steve - October 26, 2007

Paul, aluminum wire is not “banned” per se. Nor was its use discontinued due to the melting temperature, but the galvanic effect when bonded with other metals causing corrosion. It is still used in controlled environments.

7. A. James Flynn - November 9, 2007

All of those drop feeders to your homes and bussiness are “ALUMINUM” so Paul you are one of those who uses “ALUMINUM” wire these days!

8. Peter Manins - November 17, 2007

If I put in a multistrand copper cable: 0.19 mm wire diameter, 252 strands, I get nonsense.
For a start, the calc is internally limited to 15 strands in a bundle, and the bundle amperage is lower than the single wire amperage at 0.777 Amp. Yet this is an 8 AWG power cable for ~60 Amp!

Also, I would like to see the voltage drop for the length in volts/Amp. This is important for 12 volt systems.

9. Brad - November 17, 2007

Hi Peter,

I should clarify that the “Number of wires in Bundle” is not for multi-stranded wire, but for multiple insulated wires (e.g. in a harness). When there is a bundle of multiple wires, each wire is derated to carry less current. Above 15 wires in a bundle, there is no additional derating. This is just the MIL-STD-975 way of doing things.

Calculating the voltage drop is a good idea. I will plan to add that feature.


10. Peter Manins - November 18, 2007

Right, sorry. I have a totally different use in mind with my comments. And not being familiar with the MIL standard does not help!

Nevertheless, a calculator for multistrand wire would be very useful - to me.

Voltage drop in V/A is just resistance! Took me a bit to realise that!


11. Gio - November 27, 2007

Thanks for the calculator! It is a really handy and by far the best one I have found to-date. I hate to ask but is there any chance of a downloadable version that can run on windows as out here in the boonies there is no way to log on and use it when it is most needed…

12. Brad - November 27, 2007

Hi Gio,

I’m glad you like it. To use the calculator without an internet connection, save the web page to your hard drive. For example, with IE, make sure the menu bar is displayed, and then choose file - save – save as web page complete.


13. Kelly - December 12, 2007

What about insulation temperature ratings? For example 200C Kapton wire can handle more current than 60C PVC. Some places say 12 AWG 200C wire can handle 55 Amps.

14. Brad - December 12, 2007

Hi Kelly,

You are right. Insulation temperature ratings are an important factor. They are addressed in the attachment linked in the main write-up above. I also updated the write-up to make this clearer. As for 55 Amps for a 12 AWG 200C wire, you are going to get different results depending on which spec you go by. The reason is the temperature the wire will reach due to self heating is greatly affected by the environment in which it is used and the specs are linked to the intended environment.


15. PeterJan - December 22, 2007

Hi Guys
Is it possible to make the standard in this calculator selectable so you can calculate against NEC or NFPA79 or (I’m Dutch) IEC60204?

16. Brad - December 24, 2007


If you can get me a copy of the applicable part of those specs, I will look into it.


17. SHAFIQUE - December 24, 2007

Please send me the information about Formulas for using circular wire in place of flat wire. I shall be so much thankful to you.


18. Brad - December 24, 2007


We have the above calculator for round wire, and for flat wire please see:
If these do not calculate what you need, please let me know.


19. Randy - December 28, 2007

I have a rectangular conductor that I would like to know the ampacity of. It is not a PCB trace, it is a turn in an electric machine.

I have the following information for an operating point (all values are per turn).

1. Operating current 245A,rms (line to neutral)
2. Operating temperature 165 C
3. Copper losses per turn 36.79W
4. Cross sectional area 4.3mm width (X) 2.84mm height =12.212mm^2

What I dont know
5. Length of the turn (I have an idea what is fesible, but I dont know exactly)

Lets call it right now a single conductor, in reality there are 4 turns per slot of this conductor. However that answers your “wires per bundle question” which I believe is more properly termed strands per turn at least in machine design lingo.

I know that the machine can do this, what I am curious about is how much current I can pump before this coil melts essentially. I know that its actually more complicated than that and that the stacking of coils, skin effect and induced fields will actually make the heating different, but it would give me a good start just where the copper itself melts.

20. Brad - December 28, 2007

Hi Randy,

That is a good question and indeed it is not a simple question. Even if we knew the power loss in the wire, we do not know the thermal resistance from the wire to the ambient temperature and I suspect this thermal resistance is very different in a machine than in a cable bundle since the heat path is very different. Of course copper/insulation melt/burn at specific temperatures not specific currents. Finding the ampacity amounts to solving a thermal problem where the heat source is the joule heating of the wire. I am afraid the general ampacity curves for wire would not be applicable to your machine problem, and I unfortunately don’t have curves or equations for machines. I think you would have to do a detailed finite element thermal analysis or determine the thermal impedances experimentally.


21. S. P. Bhardwaj - January 4, 2008

I have a generator of 250KVA and 230V 3-Phase. The length of wire is 50 meters. Of what cross-section and of how many strands cable should use?

22. Brad - January 6, 2008

S. P. Bhardwaj,

You will first need to calculate the current. Then, based on the allowable voltage drop and ohms law, choose a wire that has low enough resistance to work. In addition, the wire you choose will have to be rated to handle the current and meet all applicable local codes. The number of strands inside the cable varies by design and just the rated current is important. I added the following calculator to my site [1]. You may also find [2] helpful.


[1] http://circuitcalculator.com/wordpress/2008/01/05/3-phase-calculations/
[2] http://baen.tamu.edu/users/stark/AGSM325_files/Lecture/4%20WiringVoltDrop%20Handout.325.pdf

23. Larry Steenstry - January 17, 2008

Could you please settle a question we have? Please define ‘Wire Bundle Ampacity’. Is the number shown, amps per wire in the bundle, or is it the total amps in the bundle?


24. Denny - January 18, 2008

Hi Brad,

Does your calculator apply to single wires coiled around a generator core? I’m trying to calculate the number of turns required and wire diameter required for some driver coils to saturate the cores with 12VDC @ 2A.



25. Brad - January 29, 2008


See comment 20 above. I don’t think this calculator would apply for your situation either.


26. Brad - January 29, 2008

Hi Larry,

Good question. Wire Bundle Ampacity is per wire. I added a note to the calculator where results the results are displayed.


27. paul - February 7, 2008

Hi friends,
Does any one know of a free software that could help me to do load calculations of motors, required power cables, selection of breakers, short circuit calculations etc. for a medium type project? At least to save the time I thought using software is better rather than manual computing. I have gone through browsing, found all those are very expensive.

28. Rob - February 25, 2008

Hey Brad, thank you so much for this calculator. I use it probably once/week on average!

One thing I’m looking for right now and missing is the ability to calculate the mass of copper given the inputs. It should be difficult since you’ve got diameter and length. I could look up density (which is what I will do). Of course, it would just be an estimate (issues such as insulation mass and stranded vs. un-stranded) but would be useful.

Thanks again!

29. Brad - February 28, 2008

Hi Rob,

Adding the mass calculation is a good idea. I’ll put it on my “to do” list. For reference, the density I found for copper is 8.96 g/cm^3.

[Update 2/29/08: I added the mass calculation to the calculator. Brad]