NEC Changes For #14 Ampacity

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can you check my math. the calculated temps are above 90C. this scenario seems like a hazard (if my math is right).
note: it's open air calcs with no consideration for heat loss. in real world some of the heat would leak out and final temp would be slightly less, but not that much less.

Code: 
[SIZE=1]per Eaton spec 1pole 10-70A breakers 
1min-3max sec trip @ 104F (40C) to trip rated 40A @ 750% (120v/0.4=300A)

using 3sec case (75ft of #14 NM, two CCC's)
1W = 1J/s
300^2 * 0.4 = 36kW = 36kJ/s
36kJ/s * 3s = 108kJ(total)

energy per ft
108kJ/150ft = 0.72kJ/ft

copper heat capacity (noting that copper wire is an alloy)
3.45 J / (cm^3 K)

#14 0.0641" dia = 0.16025cm
1ft(volume) = 0.080125^2cm * 3.14 * 30cm = 0.6048cm^3

2.08656J/K * 1/0.72kJ = 0.002898/k = ~+345K = +161F
temp of wire = 104 + 161 = ~265F

if my math is right, the copper (150ft, 75ft #14NM two CCC's) would get up to ~265F during a 300A short for 3s.


**********************************************

per Eaton spec 1pole 10-70A breakers 
3min-11max sec trip @ 104F (40C) to trip rated 40A @ 375% (150A)

using 11sec case (150ft of #14 NM, two CCC's)
1W = 1J/s
150^2 * 0.8 = 18kW = 18kJ/s
18kJ/s * 11s = 198kJ(total)

energy per ft
198kJ/300ft = 0.66kJ/ft

copper heat capacity (noting that copper wire is an alloy)
3.45 J / (cm^3 K)

#14 0.0641" dia = 0.16025cm
1ft(volume) = 0.080125^2cm * 3.14 * 30cm = 0.6048cm^3

2.08656J/K * 1/0.66kJ = 0.00316/k = ~+316K = +109F
temp of wire = 104 + 109 = ~213F

if my math is right, the copper (300ft, 150ft #14NM two CCC's) would get up to ~213F during a 150A short for 11s.[/SIZE]
 
the % over currents are still below mag trip thresholds. the Eaton graphs though do have a table that says the 40A ocpd has an instantaneous trip in the 300-550A range, which would have to be mag trip, but its quite a range.

so, perhaps the next gen ocpd is std thermal trip + electronic mag trip. wouldnt take much to install a shunt, diodes, opamp, and a cap&resistor to dampen micro-sec spikes, this would allow the mag trip current to move closer to real world short currents. i surely over simplified it, but thats about what it takes to sense currents with accuracy.
 
FionaZuppa said:
can you check my math. the calculated temps are above 90C. this scenario seems like a hazard (if my math is right).
note: it's open air calcs with no consideration for heat loss. in real world some of the heat would leak out and final temp would be slightly less, but not that much less.

Code: 
[SIZE=1]per Eaton spec 1pole 10-70A breakers 
1min-3max sec trip @ 104F (40C) to trip rated 40A @ 750% (120v/0.4=300A)

using 3sec case (75ft of #14 NM, two CCC's)
1W = 1J/s
300^2 * 0.4 = 36kW = 36kJ/s
36kJ/s * 3s = 108kJ(total)

energy per ft
108kJ/150ft = 0.72kJ/ft

copper heat capacity (noting that copper wire is an alloy)
3.45 J / (cm^3 K)

#14 0.0641" dia = 0.16025cm
1ft(volume) = 0.080125^2cm * 3.14 * 30cm = 0.6048cm^3

2.08656J/K * 1/0.72kJ = 0.002898/k = ~+345K = +161F
temp of wire = 104 + 161 = ~265F

if my math is right, the copper (150ft, 75ft #14NM two CCC's) would get up to ~265F during a 300A short for 3s.


**********************************************

per Eaton spec 1pole 10-70A breakers 
3min-11max sec trip @ 104F (40C) to trip rated 40A @ 375% (150A)

using 11sec case (150ft of #14 NM, two CCC's)
1W = 1J/s
150^2 * 0.8 = 18kW = 18kJ/s
18kJ/s * 11s = 198kJ(total)

energy per ft
198kJ/300ft = 0.66kJ/ft

copper heat capacity (noting that copper wire is an alloy)
3.45 J / (cm^3 K)

#14 0.0641" dia = 0.16025cm
1ft(volume) = 0.080125^2cm * 3.14 * 30cm = 0.6048cm^3

2.08656J/K * 1/0.66kJ = 0.00316/k = ~+316K = +109F
temp of wire = 104 + 109 = ~213F

if my math is right, the copper (300ft, 150ft #14NM two CCC's) would get up to ~213F during a 150A short for 11s.[/SIZE]
Click to expand...




Im checking it now. Is this hand notation or via a computer program?

BTW, what ever it is, I like it! :D:cool:
 
FionaZuppa said:
the % over currents are still below mag trip thresholds. the Eaton graphs though do have a table that says the 40A ocpd has an instantaneous trip in the 300-550A range, which would have to be mag trip, but its quite a range.
Click to expand...


Such a range is normal due to manufacturing tolerances. The thermal trip also has a range and its in fact one reason (besides ambient temperature) whey trip curves start around 125% and NEMA/UL testing lets a breaker carry 134% indefinitely (if I remember correctly).


so, perhaps the next gen ocpd is std thermal trip + electronic mag trip. wouldnt take much to install a shunt, diodes, opamp, and a cap&resistor to dampen micro-sec spikes, this would allow the mag trip current to move closer to real world short currents. i surely over simplified it, but thats about what it takes to sense currents with accuracy.
Click to expand...

FWIW one can argue AFCIs are electronic mag trip breakers since high mag trips are what helped catapult the parallel arc fault theory to infamy. Although if the mag trip goes electronic Id imagine might as well make the thermal part electronic also.

But, being who I am I remain true to old technology in this case. A current spike can kill the electronics rendering what is basically a life safety device useless.
 
The afci instructions state right on them to remove them from megging Mr MBrooke

I always wonder what mother nature has provided , over the years, to my installs .....

~RJ~
 
mbrooke said:
Im checking it now. Is this hand notation or via a computer program?

BTW, what ever it is, I like it! :D:cool:
Click to expand...

done in notepad, copied into forum, wrapped with
Code: 
 tags, font downsized. its the default forum font.
a current spike can be controlled to protect the electronics, its a voltage spike or something like ring wave that can cause havoc.
 
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FionaZuppa said:
did the numbers work out correctly?
Click to expand...

They look correct for now, but anyone who has insight let us both know if we are correct :thumbsup:


Only thing I am tempted to disagree with is the 3 second tripping time... but this part has me confused more then you.
 
mbrooke said:
They look correct for now, but anyone who has insight let us both know if we are correct :thumbsup:


Only thing I am tempted to disagree with is the 3 second tripping time... but this part has me confused more then you.
Click to expand...
i took the Eaton BR 1-pole trip graph, they plot min and max #'s vs % over rated. thus in my data i show the min/max times and used worse case scenario (to match the thinking of NEC) for both end-of-run short circuit scenarios (same breaker, two different lengths of #14NM, etc). i suspect trip time is much less than 3sec, can easy plot it via Excel tables and graphs, but no need as we are looking at worse case scenario for the published breaker data, etc.

that said, the over temp of the wire does not necessarily mean fire. ever touch a red hot flat-top stove really fast? you dont get burned. however, hold your finger there for more than 1ms and youch. the Q now becomes, does the temp rise from 104 to 265F in delta-t of 3s with sudden stop of input energy cause any damage? the NM sheath and wire insulation and heatsinks around the NM sheath need to absorb all the kJ back to ambient temp. i am not sure if the sudden release of energy due to short circuit which clears by breaker in 3s is enough to cause damage. the wire doesnt sit at 265F, thats the final temp starting at 104F, etc. from a purely temp view its not a good scenario. it would need to be tested to find real results.

so whos gonna test a 75ft run of #14NM on a 1pole 120v 40A ocpd and then short the ends together??? but in a test the ocpd should be eliminated and a control device inserted to allow the amps to flow for set times, we cant count on a ocpd (breaker) to be a fixed variable, etc.
 
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FionaZuppa said:
i took the Eaton BR 1-pole trip graph, they plot min and max #'s vs % over rated. thus in my data i show the min/max times and used worse case scenario (to match the thinking of NEC) for both end-of-run short circuit scenarios (same breaker, two different lengths of #14NM, etc). i suspect trip time is much less than 3sec, can easy plot it via Excel tables and graphs, but no need as we are looking at worse case scenario for the published breaker data, etc.

that said, the over temp of the wire does not necessarily mean fire. ever touch a red hot flat-top stove really fast? you dont get burned. however, hold your finger there for more than 1ms and youch. the Q now becomes, does the temp rise from 104 to 265F in delta-t of 3s with sudden stop of input energy cause any damage? the NM sheath and wire insulation and heatsinks around the NM sheath need to absorb all the kJ back to ambient temp. i am not sure if the sudden release of energy due to short circuit which clears by breaker in 3s is enough to cause damage. the wire doesnt sit at 265F, thats the final temp starting at 104F, etc. from a purely temp view its not a good scenario. it would need to be tested to find real results.

so whos gonna test a 75ft run of #14NM on a 1pole 120v 40A ocpd and then short the ends together??? but in a test the ocpd should be eliminated and a control device inserted to allow the amps to flow for set times, we cant count on a ocpd (breaker) to be a fixed variable, etc.
Click to expand...


Should still be considered imo. Pushing wire over 90*C isnt my idea of safe.
 
mbrooke said:
Should still be considered imo. Pushing wire over 90*C isnt my idea of safe.
Click to expand...

temp alone does not mean much. as example, a 1ucm^3 of material that has low hc (say 0.00001j/cm^3k) can have it's temp rise high w/ little input energy, so just a bit of energy there, touching it would heatsink that energy out very fast into say another material with hc=20kJ/cm^3k, the new material temp changes by 0.00001k, just not a lot of energy being exchanged, etc. ultimately it is energy that causes fire, etc.
 
FionaZuppa said:
temp alone does not mean much. as example, a 1ucm^3 of material that has low hc (say 0.00001j/cm^3k) can have it's temp rise high w/ little input energy, so just a bit of energy there, touching it would heatsink that energy out very fast into say another material with hc=20kJ/cm^3k, the new material temp changes by 0.00001k, just not a lot of energy being exchanged, etc. ultimately it is energy that causes fire, etc.
Click to expand...

So while the conductor heat briefly, the insulation acts as a heat sink? I genuinely dont know going beyond the copper itself heating.
 
mbrooke said:
So while the conductor heat briefly, the insulation acts as a heat sink? I genuinely dont know going beyond the copper itself heating.
Click to expand...

the wire insulation, the air between wire and sheath, the sheath itself and anything beyond the sheath are all heatsinks. heat will flow from wire to outside the sheath. all of this heatsink stuff has been omitted from my math. what we know for sure is, everything beyond the wire will start to heat up, the copper metal itself can withstand 265F, but the unknown is what impact this release of energy has done on things past the copper. from my physics background i suspect minimal damage w/o compromising integrity of the insulation on the copper, but only a lab test can confirm that, and is warranted.

but it begs the Q, if NEC allows a 40A ocpd with #14NM via an "exception" then where's the data that provides a conclusion that this is safe?
 
FionaZuppa said:
the wire insulation, the air between wire and sheath, the sheath itself and anything beyond the sheath are all heatsinks. heat will flow from wire to outside the sheath. all of this heatsink stuff has been omitted from my math. what we know for sure is, everything beyond the wire will start to heat up, the copper metal itself can withstand 265F, but the unknown is what impact this release of energy has done on things past the copper. from my physics background i suspect minimal damage w/o compromising integrity of the insulation on the copper, but only a lab test can confirm that, and is warranted.

but it begs the Q, if NEC allows a 40A ocpd with #14NM via an "exception" then where's the data that provides a conclusion that this is safe?
Click to expand...


exactly what I am wondering. Im puzzled :?
 
yeah, i now about the N-M calcs, it summarizes the physics into a neat equation accounting for heat loss. the issue with it is this, N-M math uses heat loss variables. my math eliminated the heat loss and closely follows worse case scenario of say NM being sandwiched between insulating batts, or in the case we saw some posts back, the wire migrated into the bay that was full of insulation. as long as there exists a TA lower than conductor temp there will always be heat flow, but the flow flux will varying depending on insulation properties which has impact on if the wire itself is compromised. notice in the N-M equation that Rca is in denominator, make that # high and ampacity goes down.

all that said, N-M equation allows NEC to define ampacity using known property values of wire types, and, the result is per NEC definition of "ampacity", the current in amperes a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

the wire temp rise of #14 under load-end full short on a std 40A ocpd @ 75ft (two ccc's) seems like it could be (has possibility) in that hazard zone. if you test 1,000 ocpd's in this test scenario perhaps just one of them takes the full 3sec to trip. isnt it this 1/1000th scenario the exact thing NEC tries to account for in the verbiage? in this instance its not a unknown thing, its a real world thing and, at least by math, seems like a known hazard.

all this however stemmed from the post about ocpd's, which is tied back to NEC exceptions, which may need to be modified. my original Q is around #14 ampacity and why has CEC gone back to 20A?
 
so here, check my math, N-M for a #14 wire (used N-M example but changed some values)

close estimate, #14 stranded XHHW in 1" steel conduit with 30C ambient
Code: 
T[SIZE=2]c = 90C(max)
Ta = 30C
delta TD = 2
Rdc = 2690uOhm/ft
Yc=0
Rca = 17.1

I=sqrt((90-32)/(2690*17.1))
= sqrt(58/45999)
= 0.03551kA[/SIZE]
I = "ampacity" = 35.51A

regardless of why NM is bound to 60C column, this tells me i should be able to use a 20A ocpd for this run, but NEC says ocpd max 15A for #14 wire. doesnt make sense to me. perhaps NEC should say "15A max ocpd for #14 NM"? even if we apply the 125% over ocpd rating, the amps is only 18.75, still way below 35.51. and 125% over 20A ocdp is still only 25A, 10.51 less than what N-M says is "safe". is the NEC applying a 50% safe factor to the N-M numbers and then choosing the closest ocpd? if so why?

given the ampacity definition and the N-M stuff, it is not clear to me what safety factor is baked into the N-M equations, i say this because the ampacity definition already says "can carry continuously under the conditions of use without exceeding its temperature rating"
 
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any amps w/o heat loss means the wire temp will continuously rise to failure. real world is to test it, etc. i am still working on my test.
 
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