ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Comparing ampacities

Hello!

I was inspired to adapt the NEC table 310.16 to metric wire sizes by a thread at MH. Said and done, I've put the result on a webpage.

DISCLAIMER: DON'T USE THESE VALUES!
www.global-electron.com/Ampacities_NEC.htm

I added a 70C column too. I was I bit surprised at the results.

How did you derive the figures for the standard European size cables (1.5, 2.5, 4. 6 mm etc.) in this table?

Here are the U.K. IEE ratings for Romex-style cables: http://www.tlc-direct.co.uk/Figures/Tab4.7.htm

Oh, I just made them up...

No, seriously, what I did was to fit a curve to the existing data. As I have the AWG sizes in mm² it is easy to add the in between metric sizes along the curve. Then I rounded the ampacities to the nearest lower 5 Amperes. (The original table appears to have been rounded to the nearest lower or higher 5 Amperes.)

Ah.... I wasn't sure if you had interpolated the values from the trend of the NEC values, or whether maybe you'd pulled them from Swedish wiring rules.

I've fixed the correction factors and extended the column up to 80C. (I got a few values slightly wrong the first time.)

The correction factors turn out to be the same on both sides of the pond. I found the factors had been calculated using this simple formula:

factor = root( (Temperature rating of conductor - actual ambient temperature) / (Temperature rating of conductor - reference ambient temperature) )

If you have a table with ampacities at 30C but the actual ambient is 40C and the conductor is good for 60C you calculate it as:

root((60-40)/(60-30)) = 0.82

The above formula allowed me to make a better calculation of the 70C column. This raised the ampacity for some gauges slightly.

C-H,
Just a small question.
What type of cables are we talking here?
And what type of installation method?.
Clipped direct w/ ventilation or in Conduit?.
Usual plain cables here have a maximum temperature rating of 75C, when clipped to timber in a roof and that is without the load current that they may be carrying.

I think I've got the tables into a legible form now.

Mike,

the NEC gives the names of a whole number of cables. For the time being, I haven't included them, but it appears to include all cable commonly used in the US.

Maybe some of the Americans would like to shed some light on when you can and cannot use table 310.16?

P.S. I'll try to go the other way as well and add AWG sizes to the Swedish or British tables. (It is bigger, as there are tables for clipped directly/free air, in thermal insulation and so on. Not to mention correction factors...

Edit:

I played a little more with Excel. It turns out that the ampacities of Table 310.16 match those in the column "One two core cable enclosed in conduit on a wall or in trunking" (*) in the IEE regs.

* I got the data from a cable manufacturer. The column is probably called something else in the actual regs.

[This message has been edited by C-H (edited 07-10-2004).]