NEC Changes For #14 Ampacity

[ActionDave]

...as for motors, if the ocd is 20A on #14 then the bc is a 20A bc, there could be a small value "short" that takes the wire to say 19A w/o ocd tripping ???

Motors have two types of protection, short circuit/ground fault, usually a breaker or fuse and overload protection at the motor. Your "small value 'short' " would be handled by the overload protection.

 

[mbrooke]

ah, my bad on the derating rant.

as for motors, if the ocd is 20A on #14 then the bc is a 20A bc, there could be a small value "short" that takes the wire to say 19A w/o ocd tripping ???

Now worry

I am going to be honest about that one. Real world has taught me otherwise. Years back my neighbor's AC, a 1985 Comfort Maker, on our duplex ended up with a stalled condenser fan. The unit would start, run 8 to 16 minutes and then trip the 20amp breaker. The compressor overload never opened once during that event. I know the overload works because 1 year prior the unit had a bad start capacitor where the overload would close, a hum could be heard and then open 1 to 2 seconds latter. A very slight dim in my unit when the overload would close. Fan ran fine the whole time. You would think this was a one time discovery, but it was not. Ive seen a good dozen other cases where a running yet strained compressor trips the breaker rather then cycling the overload. In fact if you ask a number of seasoned HVAC guys they will all tell you of encounters where a running (none shorted), AC condenser trips the OCPD. Sometimes the cause is as simple as condensing coils plugged up debris (unlike commercial units, most residential AC units do not have a high side over pressure switch resulting in elevated high side pressure building up until the compressor somehow kicks off).


At the time I never really concentrated on the MCA vs min/max breaker rating on the label (its possible the MCAs were close to the max breaker size), however nothing would surprise me if there is a discrepancy between the overload's actual trip curves built into each AC vs that of an equivalent thermal magnetic breaker. In fact (and I will have to research as I could be very wrong), I know of nothing requiring AC unit motor overloads to have a time current curve in conjunction with UL489.

I am not saying the NEC is wrong or does not know something I do not, but rather what I have encountered and heard from others working in the trades.

 

[FionaZuppa]

Motors have two types of protection, short circuit/ground fault, usually a breaker or fuse and overload protection at the motor. Your "small value 'short' " would be handled by the overload protection.

who says the short is in the motor?
i struggle with this because NEC says the OCD defines the bc, not the wire. so in essence you can put 20A ocd on #14, which goes against other NEC verbiage.

 

[iwire]

who says the short is in the motor?
i struggle with this because NEC says the OCD defines the bc, not the wire. so in essence you can put 20A ocd on #14, which goes against other NEC verbiage.

A short circuit is a much different issue than an overload

Consider the fact it is commonplace for the conductor that provides short circuit protection to be smaller than circuit conductors.

For example we can use a 10 AWG copper conductor as the means to open an overcurrent device up to 60 amps while the circuit conductors might have to be 6 AWG copper.

 

[ActionDave]

who says the short is in the motor?

Who said the short was in the motor? It could be in conductors going from the starter to the motor, every thing is still protected if done to code.

i struggle with this because NEC says the OCD defines the bc, not the wire. so in essence you can put 20A ocd on #14, which goes against other NEC verbiage.

Correct. The NEC does say OCD defines the branch circuit, and they also provide some exceptions. Motors and tap conductors are a couple of examples, and they are areas where the NEC makes good sense.

 

[FionaZuppa]

Who said the short was in the motor? It could be in conductors going from the starter to the motor, every thing is still protected if done to code.
Correct. The NEC does say OCD defines the branch circuit, and they also provide some exceptions. Motors and tap conductors are a couple of examples, and they are areas where the NEC makes good sense.

where do the motor exceptions re-define the bc?
the CCC's could potentially short before the motor, but the short could be just 19A, too much for #14, and a 20A ocd will just watch the wire overheat.

 

[ActionDave]

where do the motor exceptions re-define the bc?

210.2

the CCC's could potentially short before the motor, but the short could be just 19A, too much for #14, and a 20A ocd will just watch the wire overheat.

If the current were limited to 19A I would not call that a short.

 

[mbrooke]

where do the motor exceptions re-define the bc?

430.51 overrides the normal OCPD requirements in article 240. A special table given for short circuit and ground fault protection in 430 which governs the requirements for motor installations.

the CCC's could potentially short before the motor, but the short could be just 19A, too much for #14, and a 20A ocd will just watch the wire overheat.

Of course, thats exactly what they guard against, short circuits between the motor overloads and breaker. Of course, in typical scenarios when bare copper or aluminum conductor touches the grounded chassis of an enclosure or AC unit, the current that will flow will be well over 19amps, more likely to be 100amps plus.

 

[iwire]

the CCC's could potentially short before the motor, but the short could be just 19A, too much for #14, and a 20A ocd will just watch the wire overheat.

That is not a short and is really not going to happen.

The fact is many decades of history have proven this set up works safely.

If you personally want to use larger wire that is fine but no need for the NEC to change here

 

[romex jockey]

Now worry

I am going to be honest about that one. Real world has taught me otherwise. Years back my neighbor's AC, a 1985 Comfort Maker, on our duplex ended up with a stalled condenser fan. The unit would start, run 8 to 16 minutes and then trip the 20amp breaker. The compressor overload never opened once during that event. I know the overload works because 1 year prior the unit had a bad start capacitor where the overload would close, a hum could be heard and then open 1 to 2 seconds latter. A very slight dim in my unit when the overload would close. Fan ran fine the whole time. You would think this was a one time discovery, but it was not. Ive seen a good dozen other cases where a running yet strained compressor trips the breaker rather then cycling the overload. In fact if you ask a number of seasoned HVAC guys they will all tell you of encounters where a running (none shorted), AC condenser trips the OCPD. Sometimes the cause is as simple as condensing coils plugged up debris (unlike commercial units, most residential AC units do not have a high side over pressure switch resulting in elevated high side pressure building up until the compressor somehow kicks off).


At the time I never really concentrated on the MCA vs min/max breaker rating on the label (its possible the MCAs were close to the max breaker size), however nothing would surprise me if there is a discrepancy between the overload's actual trip curves built into each AC vs that of an equivalent thermal magnetic breaker. In fact (and I will have to research as I could be very wrong), I know of nothing requiring AC unit motor overloads to have a time current curve in conjunction with UL489.

I am not saying the NEC is wrong or does not know something I do not, but rather what I have encountered and heard from others working in the trades.

So if every circuit i ran incorporated some end point of use thermal dynamics , i wouldn't need to worry about mag trip ratings

But not everything does, ergo the TIME and MAGNITUDE any fault can assume is the focus

Evident by OCPD manufactures history of obsession with it....

~RJ~

 

[mbrooke]

So if every circuit i ran incorporated some end point of use thermal dynamics , i wouldn't need to worry about mag trip ratings

But not everything does, ergo the TIME and MAGNITUDE any fault can assume is the focus

Evident by OCPD manufactures history of obsession with it....

~RJ~

Well, can you explain further? What do you mean by end point thermodynamics? Also the mag trip? Sorry >> :dunce::ashamed1:

 

[FionaZuppa]

That is not a short and is really not going to happen.

The fact is many decades of history have proven this set up works safely.

If you personally want to use larger wire that is fine but no need for the NEC to change here

whats your definition of a BC short? if a hot in a jbox shorts, isnt that a short? i have not seen where "short" is defined by amps. to me, a short is any amps that the bc carries but does not traverse the expected or designed load device(s).

as a side note, my testing gear is morphing to the "ease of use" model, looking at a Omega devices that is easy to use and is a datalogger.
http://www.omega.com/pptst/ISD-TC.html

 

[user 100]

whats your definition of a BC short? if a hot in a jbox shorts, isnt that a short? i have not seen where "short" is defined by amps. to me, a short is any amps that the bc carries but does not traverse the expected or designed load device(s).

Actually, if a hot wire was to hit a bonded metal box that would be a ground fault.

A short circuit is where you have unintentional contact between 2 circuit conductors.

 

[FionaZuppa]

Actually, if a hot wire was to hit a bonded metal box that would be a ground fault.

A short circuit is where you have unintentional contact between 2 circuit conductors.

a "ground fault" is perhaps a special type of short. just like there is no such thing as a "square", its just a name for a special type of rectangle.

if the black wire from a single pole non-gfi BC somehow decides to bridge itself to the white wire (or metal box) and the bridge is a 960ohm bridge and it can survive 20watts without burning up, thats a short.

you re not thinking like NEC. have to think like "what if scenario", right? what if the BC shorts in some odd way that takes the amps beyond ampacity yet the breaker doesnt trip? i am not saying like 18A on 15A ocd on #14 wire. like using a 40A ocd with #14 for a single motor bc. by NEC verbiage its a 40A bc, which contradicts the ampacity table, etc.

 

[user 100]

a "ground fault" is perhaps a special type of short. just like there is no such thing as a "square", its just a name for a special type of rectangle.

if the black wire from a single pole non-gfi BC somehow decides to bridge itself to the white wire (or metal box) and the bridge is a 960ohm bridge and it can survive 20watts without burning up, thats a short.

you re not thinking like NEC. have to think like "what if scenario", right? what if the BC shorts in some odd way that takes the amps beyond ampacity yet the breaker doesnt trip? i am not saying like 18A on 15A ocd on #14 wire. like using a 40A ocd with #14 for a single motor bc. by NEC verbiage its a 40A bc, which contradicts the ampacity table, etc.

I'm not picking here but a hot making contact with an egc or bonded metal cannot be a short circuit because an egc isn't a ccc/part of the circuit and current wouldn't be taking a shortened path. I know that nobody cares about this terminology except us, but we use these terms for specific reasons and we have to differentiate between the two.

And as for what if, so what? The NEC does a pretty good job at mitigating risk, and when you follow it, your doing just that.

 

[mbrooke]

a "ground fault" is perhaps a special type of short. just like there is no such thing as a "square", its just a name for a special type of rectangle.

if the black wire from a single pole non-gfi BC somehow decides to bridge itself to the white wire (or metal box) and the bridge is a 960ohm bridge and it can survive 20watts without burning up, thats a short.

you re not thinking like NEC. have to think like "what if scenario", right? what if the BC shorts in some odd way that takes the amps beyond ampacity yet the breaker doesnt trip? i am not saying like 18A on 15A ocd on #14 wire. like using a 40A ocd with #14 for a single motor bc. by NEC verbiage its a 40A bc, which contradicts the ampacity table, etc.

Do you mean a short circuit (ground fault) through a load such as a heating element grounding out at the center?

 

[FionaZuppa]

Do you mean a short circuit (ground fault) through a load such as a heating element grounding out at the center?

answering last two posts. the gnd conductor is normally not a ccc, right, but in a bc that gnd wire ties back to the N in the panel. it can become a ccc.

i mean any short, doesnt have to be in/on/at the load. two ccc's of the bc can for some reason leak current. if you have a 40A ocd on #14 for a single motor bc and the wire (for whatever wild reason that you can think of) decides to "short" 25ft out on the 50ft run, and that "short" is a 25A short, well, the ocd laughs at it, the #14 melts some time later. maybe its buried metal conduit, the wire is compromised and water allows the "short", but get this, the amps on the hot is = to the amps on the N+gnd because the water is bridging the amps to both the N and gnd (the metal conduit).

 

[ActionDave]

i mean any short, doesnt have to be in/on/at the load. two ccc's of the bc can for some reason leak current. if you have a 40A ocd on #14 for a single motor bc and the wire (for whatever wild reason that you can think of) decides to "short" 25ft out on the 50ft run, and that "short" is a 25A short, well, the ocd laughs at it, the #14 melts some time later. maybe its buried metal conduit, the wire is compromised and water allows the "short",

And there are a few thousand engineers, UL testers, and code making authorities who have dedicated their lives to such concerns while under the watchful eye of sue happy lawyers and nit picky insurance companies. I'm confident this has been hashed out.

but get this, the amps on the hot is = to the amps on the N+gnd because the water is bridging the amps to both the N and gnd (the metal conduit).

Pitty that poor N out there with no overcurrent protection at any level.

 

[iwire]

but the short could be just 19A, too much for #14, and a 20A ocd will just watch the wire overheat.

Please explain to me how a 'short circuit' is only gong to allow just 19 amps of current to pass in a correctly wired circuit.

 

[mbrooke]

answering last two posts. the gnd conductor is normally not a ccc, right, but in a bc that gnd wire ties back to the N in the panel. it can become a ccc.

i mean any short, doesnt have to be in/on/at the load. two ccc's of the bc can for some reason leak current. if you have a 40A ocd on #14 for a single motor bc and the wire (for whatever wild reason that you can think of) decides to "short" 25ft out on the 50ft run, and that "short" is a 25A short, well, the ocd laughs at it, the #14 melts some time later. maybe its buried metal conduit, the wire is compromised and water allows the "short", but get this, the amps on the hot is = to the amps on the N+gnd because the water is bridging the amps to both the N and gnd (the metal conduit).

If something like this did happen, #14 would be the least concern. 25x120=3,000 watts at a single point. That amount of energy will start a fire even if #6 was used. In such a case, even with the correct size breaker is used you will still need a GFCI or GFP to prevent 3,000 watts from being dissipated in the time 25 amp trips a 15 amp breaker.