# DIN vs. NFPA Conductor Sizing

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#### Cory Schaeffer

A recent fire in an electrical cabinet has prompted me to look at the conductor sizes for the circuit in question. The circuit has a full load demand of 23Amps, is protected with a 25Amp circuit breaker and was built with 12AWG (4 sq. mm) wire. According to NFPA, a 12AWG conductor is good for 25Amps in a panel, and only 20Amps in a raceway or cable. My problem is that when I question the machine manufacturer, an anonymous German vendor, they tell me that I am an idiot, because 12AWG wire is good for 42Amps on their side of the pond. With just half of that load on the circuit the 12AWG wire has a temperature of 100F. Can someone explain why there would be such a vast difference between their code and ours? Thanks, Cory Schaeffer

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#### Johan Bengtsson P&L Automatik AB

First, as far as I can see the in my books, this cable should have a diameter of 2.05mm and therefore an area of 3.31mm^2, not 4mm^2 as you say below. Well, however, I have seen a recommendation to have at most some 9-10A/mm^2 for short and freely laid out wires. if there is extra insulation around you should go down to about something like 3-5A/mm^2 (depending on how hard it is for the heat to get away I suppose). 42A would give 12.7A/mm^2, a little bit on the high scale I suppose but if the wire could be cooled enough I suppose it would handle this. 23A would give 6.9A/mm^2, that is it should be ok for those short and freely laid out wires, but a little high for a cable. Pure power: 3.31mm^2 means around 5.5mohm/m this give: 42A give you 9.7W/m 23A give you 2.9W/m The temperature a wire reaches does depend on this power and some more things like: - Temperature around the wire - Insulation material and thickness - Flow of air around the cable The heat generated in the cable have to go somewhere, heat go somewhere when there is a difference in temperature, this effecively means: higher difference in temperature (between cable and air surronding it) the higher power transport. Some certain difference in temperature gives the power transport needed and that is the difference in temperature you get. Cooling the air around a wire with some number of degrees will cool the wire with the same number of degrees. Air that are not moving is an quite effective temperature insulation, this means that if the air can't circulate it gets heated around the wire, and the wire will be hotter. Note that a current twice as high will give you a temperature differece about four times as high I know this don't answer your initial question, but I hope this gives you some of the answers of "why?" I would like to know what the temperature is around the wire (like some dm or so from it is). /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------

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#### Ramer-1, Carl

Cory: I can't explain the NFPA vs DIN differences, but did come up with some other points. According to the Belden Wire & Cable catalog, a single 12 AWG conductor in a 25 degree C still air environment will experience approximately 35 degrees C temperature rise at 23 Amps. More factors come in with multiple conductors, conduit or raceway, moving air, etc. The single conductor scale puts the temperature around 140 degrees F so your 100 degree F temperature isn't a problem. Some insulations, however, are only rated to 60 degrees C (140 F), so running at 23 Amps in a 12 AWG conductor might be courting disaster, depending on the other factors mentioned above. Extrapolation from the Belden nomograph shows roughly another 25 degree C rise at 40 Amps in a 12 AWG conductor, which dictates using some of the better insulation types rated 90 degrees C or better. Your 100 degree F temperature still isn't a problem, but it does add confusion. How did the wiring get hot enough to cause a fire if the circuit was limited to 25 Amps? That's not enough to cook even the cheapest insulation. Did the circuit breaker fail? What else could have routed too much current through the cooked conductor(s), possibly bypassing protective circuitry? So it doesn't sound as much like a DIN vs NFPA disparity as a "what went haywire and found the design loophole" problem. Carl Ramer Protective Systems Design Space Gateway Support Kennedy Space Center, Florida Unsponsored personal posting

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#### Willy Smith

Cory, The electrons are better organized in Germany, so more of them can go through the wire at the same time. Just take a ride on the subways in the US and Germany, you'll see what I'm talking about and notice the difference immediately. It could also be because they're on the metric system - that 100 degree wire would only be at 38 degrees anywhere in Europe. Regards, Willy Smith Numatics, Inc. Costa Rica

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#### Anthony Kerstens

So the moral of the story is... When doing calculations, don't forget to apply the temperature derating and choosing the correct wire/cable type. Derating involves multiplying the wire/cable ampacity by a number <1 appropriately chosen from the derating table so that you end up with a larger size conductor than at room temperature. The Ontario Hydro code book table says "Based on Ambient Temperature of 30=B0C*" and "*See Table 5A ... for ambient temperatures over 30=B0C". I'm quite sure that DIN and NFPA would have equivalent instructions. Anthony Kerstens P.Eng.

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#### Willard Bridgham PE

This may sound smartnick, but it is not. Europeans in general have lousy electrical systems because systems are not so well developed as they are in US/Can and do not have a long history of use. NFPA has a lot of good data on electrical fires and Europe, you should pardon the expression, is Balkanized in this respect as each country has its own bad data, including Germans. I have never dealt with a European supplier on a project who understood electricity very well and, if they got it at all, they were unfamiliar with the NEC. I had a Brit proudly say that electricity wasn't very important to him. Electrical distribution has come lately to Europe and been retrofitted into equipment as an add-on.

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