NEC Changes For #14 Ampacity

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mbrooke said:
Spotted over at DIY chat room, a potential overload on #14:



http://www.diychatroom.com/f18/prior-owner-nearly-burned-my-house-down-325137/
Click to expand...


A few years ago I went on a service call at an apartment complex. In the basement of the office building they had a water leak in what was used as a fitness or exercise area, carpet got all wet.

The maintenance guys had rented some dehumidifiers. They tripped the 15 amp circuit so they put the hot wire on a 40 amp breaker and it ran for a couple of days and then it tripped.

No fire but it did ruin the insulation on that #14 home run.

People do things like this all the time. If it had not tripped the 40 amp breaker they would have put it back on the 15 amp breaker when they finished drying the carpet (so they said).

People know enough to rig things but don't know the risk.
 
so back to my #14 test.

the goal is to obtain data sets for three metrics:
1) wire temp @ varying currents
2) wire insulation condition (visual) after each amp test
3) insulation condition after each amp test

i will be using a bank of T3 1000w 240v halogen bulbs to vary the current, using my amp clamp to take amps readings. will run each amp test until temps reach equilibrium for a period of 10min

will use the pink 1.5-2" thick rigid foam for the wire sandwich. the same sandwich used for all amp tests.

any feedback?
 
GoldDigger said:
How will you be measuring the temperatures? Inside or outside the wire insulation?
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i have some type-k thermo couples, but given the setup i will get something better from Omega (better suited for the physical attachment needed). will be measuring the OD of the wire insulation, etc. i can however also use a 2nd tc, strip off a small section of wire insulation and also capture the metal temp. or just capture metal temp.

one (which one), or both needed ?? given the tight sandwich, i hypothesize that the metal temp and OD of insulation will be within the error of the temp measuring device (hence "same temps").
 
I would expect there to be a measurable temp difference, I just don't know if it will be significant. I would do one metal temp just to get the information.
 
growler said:
A few years ago I went on a service call at an apartment complex. In the basement of the office building they had a water leak in what was used as a fitness or exercise area, carpet got all wet.

The maintenance guys had rented some dehumidifiers. They tripped the 15 amp circuit so they put the hot wire on a 40 amp breaker and it ran for a couple of days and then it tripped.

No fire but it did ruin the insulation on that #14 home run.

People do things like this all the time. If it had not tripped the 40 amp breaker they would have put it back on the 15 amp breaker when they finished drying the carpet (so they said).

People know enough to rig things but don't know the risk.
Click to expand...

I agree, few know the meaning behind ther rules or why things are done as it.

If I may ask, how bad was the insulation damaged? How did the exposed (in air) segments look like vs the bundled/touching insulation parts?

With a 40 amp breaker that wire probably saw 50 amps :eek:
 
FionaZuppa said:
so back to my #14 test.

the goal is to obtain data sets for three metrics:
1) wire temp @ varying currents
2) wire insulation condition (visual) after each amp test
3) insulation condition after each amp test

i will be using a bank of T3 1000w 240v halogen bulbs to vary the current, using my amp clamp to take amps readings. will run each amp test until temps reach equilibrium for a period of 10min

will use the pink 1.5-2" thick rigid foam for the wire sandwich. the same sandwich used for all amp tests.

any feedback?
Click to expand...

Sounds good, but if I may I would tweak this a bit:

1. Run the wire for more then 10 minutes. Brief overheating may not show signs like sustained overheating.

2. Make sure the foam sandwich is over 24 inches long, heat can escape from the ends.
 
mbrooke said:
Sounds good, but if I may I would tweak this a bit:

1. Run the wire for more then 10 minutes. Brief overheating may not show signs like sustained overheating.

2. Make sure the foam sandwich is over 24 inches long, heat can escape from the ends.
Click to expand...

the sandwich will be a 24"x24" with the wire running across the center. i will carve a channel in each piece to accept 1/2 the thickness of the romex, making it as symmetric as possible. there will be little chance for air movement from the two pieces being sandwiched together, i'll add some weights around the perimeter to keep it tight.

if 10min post equilibrium is not long enough, how long is long enough? 1, 12, 24, 48, 72 hrs? surely exposure to heat is the culprit in "heat damage", just not sure this experiment though is looking for heat damage, we are really only after the temps of the wire.

if i am looking for damage, each amp test may provide damage and i will need to let it cool down to do inspections. this seems to indicate that i need to replace the wire for each amp test (not sure). doable yes but i need to build the test sandwich so that its not a pita. as example, the tc's might be located in such a way that they just stay in place so i can swap the wire out with ease. or, do i just use one wire and do the study as if the wire was entering a progressive short circuit? as example, i can wire the 1kW bulbs in via toggle switches, and then switch them in progressively simulating a load that is failing and shorting in increments of 4.1 amps ?? but, not really sure i need to run it for extended periods of time, getting the temp data is enough to build another experiment where the NM wire sits in a temp controlled oven to see what happens ??

your thoughts?
 
ok, got my TC's ready and my PID calibrated. this is quality Omega TC type-k wire. my diy CD spot welder is in a box somewhere so it had to use my TIG to weld the ends (kinda tricky to do). these TC's are ready for use. i am about to order 10kW worth of T3 bulbs with holders. a few steps closer to having a functional test.


TC_calibrate.jpg



DIY_TC.jpg
 
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FionaZuppa said:
where can you find a 240v 10kW variac for $35?
Click to expand...

Once you factor in 10kw of power running days on end it becomes cheaper.

Something like this is what Id go for:


http://www.ebay.com/itm/like/121762652945?ul_noapp=true&chn=ps&lpid=82


And one of these rewound from an old microwave oven:

https://www.youtube.com/watch?v=hTaGa93lOGU

https://www.youtube.com/watch?v=RF5gzjvuNvk

You dont have to cut open the iron core, simply cutting out the secondary and putting some #6 or #4 inside is good enough. Make sure the open circuit voltage is at least a few volts so it can overcome the NM's resistance enough to pass desired current. It isn't pretty, nor is it the most hi-tech, but it gives you a variable high current at the lowest price.

One of the reasons for a variable source is that as the wire heats up current will drop through it, so the variac is used to compensate.
 
This is what the setup should look like. You don't need a "phase control switch" for this experiment so you can omit that. "PZ" will be a clamp on amp meter, and "DUT" will be where your NM hooks up, shorted at the other end of course.
 
well, i know how the variac works. i need one that could do at least 40amps, gonna cook the wire, etc.
my test is not shorting a long length of NM, it will only be about 3ft, would still need some load ohms for the variac, otherwise its just a instant short. 10kW worth of T3 bulbs is a cheap way to get variable load.

and, variac's that are somewhat close to 20A are in the $100+ range. ten 240v/1kW halogen bulbs are only $35, i'll need to add ten $1 toggle switches. and, to get the current for testing (variac aside) i need to use two-pole power.

current will be measured using my General amp clamp, i will clamp it to the #8 that will attach to the #14 NM. the impedance is obviously a function of all the components, but the additional Z that comes from the bulbs or ancillary wiring will be minimum and has no impact on the test at hand. the current will increment is steps, the amp clamp will be the tool for measuring actual current, etc.
 
FionaZuppa said:
well, i know how the variac works. i need one that could do at least 40amps, gonna cook the wire, etc.
my test is not shorting a long length of NM, it will only be about 3ft, would still need some load ohms for the variac, otherwise its just a instant short. 10kW worth of T3 bulbs is a cheap way to get variable load.

and, variac's that are somewhat close to 20A are in the $100+ range. ten 240v/1kW halogen bulbs are only $35, i'll need to add ten $1 toggle switches. and, to get the current for testing (variac aside) i need to use two-pole power.

current will be measured using my General amp clamp, i will clamp it to the #8 that will attach to the #14 NM. the impedance is obviously a function of all the components, but the additional Z that comes from the bulbs or ancillary wiring will be minimum and has no impact on the test at hand. the current will increment is steps, the amp clamp will be the tool for measuring actual current, etc.
Click to expand...


Correct, but you wont have an instant short via high current transformer.
 
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