NEC Changes For #14 Ampacity

[wwhitney]

well, i dont think its that simple.

Here's the physics:

In general the wire will have both radial and axial heat flow. If Tc is the conductor temperature, T1 is the ambient for radial flow, and T2 is the ambient for axial flow, then the heat flow is given by:

Q = K1 (Tc - T1) + K2 (Tc - T2)

For a given set of experimental conditions, Q, K1 and K2 will be constants. Q, for example, depends on the wire resistance and current; K1 depends on the wire radius and any insulation, etc.

This is a linear relationship between Tc, T1, and T2. Therefore linear extrapolation as I suggested will work. If we extrapolate to the point Tc = T2, then the only heat flow is radial, as desired.

This won't work for comparing the clamps in an ice bath to the clamps in open air, as that change in experimental conditions will change K2. But it should work for the clamps in an ice bath versus the clamps in a room temperature water bath.

Cheers, Wayne

 

[mbrooke]

Here's the physics:

In general the wire will have both radial and axial heat flow. If Tc is the conductor temperature, T1 is the ambient for radial flow, and T2 is the ambient for axial flow, then the heat flow is given by:

Q = K1 (Tc - T1) + K2 (Tc - T2)

For a given set of experimental conditions, Q, K1 and K2 will be constants. Q, for example, depends on the wire resistance and current; K1 depends on the wire radius and any insulation, etc.

This is a linear relationship between Tc, T1, and T2. Therefore linear extrapolation as I suggested will work. If we extrapolate to the point Tc = T2, then the only heat flow is radial, as desired.

This won't work for comparing the clamps in an ice bath to the clamps in open air, as that change in experimental conditions will change K2. But it should work for the clamps in an ice bath versus the clamps in a room temperature water bath.

Cheers, Wayne

Well said, my thoughts exactly.

Can you go just a bit more into the heaters at the end idea? Its doable but will need some intense thought all on its own, but I am willing to think about.

 

[FionaZuppa]

what about "A" area in your equation?

the comparison i did simply changed the heatsink from convective open air to conductive ice water. in both cases this "heatsink" pulls Q down the wire, the magnitude of that Q depends on several factors. certainly a ice water bath is more of a heatsink that clamps in ambient air. the goal was to simply see if the "heatsink" had significnat impact on wire(center) temp. this is not easily done if everything is at ambient, so to see any impact we change the "heatsink" to something that would show us some impact, thus, submerge the ends in ice water to see what impact it has. surely ice water is way more of a "heatsink" than the ends in open ambient. the ice water didnt show significant changes in wire temp, therefore we can draw a conclusion that open air ambient is also not significant.

 

[FionaZuppa]

@10A it took about 80min to get into equilibrium
at 119min T_NM=91.2F , T_Cu=91.6F (as expected in terms of these temps being equal)
made the step from 10A to 15A at the 119min mark. net measure/step @2hr.

 

[mbrooke]

@10A it took about 80min to get into equilibrium
at 119min T_NM=91.2F , T_Cu=91.6F (as expected in terms of these temps being equal)
made the step from 10A to 15A at the 119min mark. net measure/step @2hr.

Which test is this?

 

[FionaZuppa]

Which test is this?

running the sandwich test, setup described a "few" pages back. a correction factor will be done at the end.

 

[mbrooke]

running the sandwich test, setup described a "few" pages back.

The foam board test? We have lift off! What is the ambient right now?

 

[mbrooke]

again , sorry, my edit time was reached.

what i am suggesting is a std in heat density. ~1.02W/ft @ 100ft of wire seems like a good # (just as example here). however, the current ampacity table does not provide a constant heat density.

but if it were constant across all wire sizes then the table would look something like this:

(rounded up to nearest whole)
#14 20
#12 25
#11 28
#10 32
#9 36
#8 40
#7 45
#6 50
#5 57
#4 64
#3 71
#2 80
#1 92
#0 101

these max amps per wire size based on 1.02W/ft @100ft. in NEC2011 #1 is 110A in 60C column, thats 1.45W/ft @100ft, and #4 is 1.23W/ft @ 100ft. its not constant.

heatsink of insulation area only?
ok
#1 would be 10.9sq.in/ft
#4 would be 7.698sq.in/ft

thus the heatsink area density is
#1 0.133W/sq.in.
#4 0.160W/sq.in.

not even the heatsink area density is constant.

This has peaked my curiosity btw. How did you determine these numbers? Am I correct to assume wires become conservative as they go up in size?

 

[FionaZuppa]

ambient now is ~60F

as for the heat density #'s i listed, they were normalized to 1.02W/ft @100ft, using #14 @20A as the control #

NEC table allows for heat density /ft to rise as the awg size goes up. but radius of wire also increases thus heat flux is lower.

 

[wwhitney]

what about "A" area in your equation?

A is rolled up in K1 or K2. I just put all the coefficients together into one big coefficient. It's some multiple of k, so I called it K to differentiate it.

the comparison i did simply changed the heatsink from convective open air to conductive ice water.

Right. You changed both the mode of heat transfer (and hence K2) and the temperature at the end (the T2). That's why i was proposing to compare an ice bath and an ambient temperature bath, you'd be only changing T2 and keeping K2 constant.

in both cases this "heatsink" pulls Q down the wire, the magnitude of that Q depends on several factors. certainly a ice water bath is more of a heatsink that clamps in ambient air.

I agree. All those factors get rolled into K2. The rate of heat flow to the heat sink is still proportional to the temperature difference.

the goal was to simply see if the "heatsink" had significnat impact on wire(center) temp. this is not easily done if everything is at ambient, so to see any impact we change the "heatsink" to something that would show us some impact, thus, submerge the ends in ice water to see what impact it has. surely ice water is way more of a "heatsink" than the ends in open ambient. the ice water didnt show significant changes in wire temp, therefore we can draw a conclusion that open air ambient is also not significant.

Well, you changed both T2 and K2. We can't tell how to attribute the change in conductor temperature between those two changes.

Now I would think that immersion in water is a lot better heat sink than convection in air, so I would think K2_ice is bigger than K2_air. But without knowing the actual values of either one, we can't say what temperature we'd get if we could arrange for K2 = 0 so that there is no axial heat transfer.

Also, in this test the conductor was not very well insulated thermally. With the foam test, the heat sink path will be more competitive. So if the heat sink path is responsible for 2 degrees F now (still a guess at this point), then it might be responsible for 5 or 10 degrees F with foam.

Anyway, I believe I've shown how to quantify your good idea of using an ice bath so that you can measure the heat sink effect.

Cheers, Wayne

 

[wwhitney]

@10A it took about 80min to get into equilibrium
at 119min T_NM=91.2F , T_Cu=91.6F (as expected in terms of these temps being equal)

Cool! You later said the ambient is 60F, so the temperature rise is 31F. So for 20 amps you should see a 124F temperature rise, e.g. a 184F conductor temperature. If the measured results deviate from this significantly, we'll have to figure out why.

Cheers, Wayne

 

[mbrooke]

ambient now is ~60F

as for the heat density #'s i listed, they were normalized to 1.02W/ft @100ft, using #14 @20A as the control #

NEC table allows for heat density /ft to rise as the awg size goes up. but radius of wire also increases thus heat flux is lower.

My question is why though :? (my apologizes for veering off, but I think I just stumbled upon something)

 

[FionaZuppa]

My question is why though :? (my apologizes for veering off, but I think I just stumbled upon something)

a way to normalize the ampacity table. i have to go back and step through those posts. 1.02W/ft @ 100ft of wire (two CCC's) = a 50ft run of NM

you can also use heat flux density to normalize, this way the temp for any wire size on the OD is constant at ocpd level.

 

[mbrooke]

a way to normalize the ampacity table. i have to go back and step through those posts. 1.02W/ft @ 100ft of wire (two CCC's) = a 50ft run of NM

you can also use heat flux density to normalize, this way the temp for any wire size on the OD is constant at ocpd level.

I think you are on to something here.

 

[FionaZuppa]

I think you are on to something here.

i think i took #14 in existing 75C column, so 20A. i was arguing why NM was restricted to 60C and why #14 restricted to 15A ocpd. using #14 in 75C column we can derive a heat/ft constant. this is one method. the other is to derive the heat flux density of #14 @20A, from there the ampacity #'s have wire radius accounted for, thus the watts/area remains constant but watts/ft goes up for bigger wire, means bigger ampacity #'s. the latter seems to be a better method.

but as i noted, a high temp item that offers little energy to swap is not as dangerous as a lower temp item that has lots of energy to swap. so in other words, choose your poison wisely.

 

[FionaZuppa]

i will have a table of all of this,
2hr @~15A yields equilibrium at 14.50A T_NM=128.7F, T_Cu=132.4F

 

[mbrooke]

i will have a table of all of this,
2hr @~15A yields equilibrium at 14.50A T_NM=128.7F, T_Cu=132.4F

What is the current ambient?

 

[FionaZuppa]

What is the current ambient?

it was 63F at that last reading

 

[FionaZuppa]

i will have a table of all of this,
2hr @~15A yields equilibrium at 14.50A T_NM=128.7F, T_Cu=132.4F, 63F ambient

2hr @~20A yields equilibrium at 19.82A T_NM=183.2F, T_Cu=183.6F, 66F ambient
right-on wayne, post #591 :thumbsup:
this has obviously exceeded 75C(167F), but NM is rated 90C

now cooking at 25A

 

[FionaZuppa]

arrows showing the ends of the NM, and the towel the foam is sitting on.