A/C wire size.

[wwhitney]

trip curve

Two questions about trip curves:

1) Is the width of the region shown purely attributable to variation or calibration differences between different units? I.e. if we could repeatedly test a single given circuit breaker at the specified cold start ambient 40C temperature, would we give a single curve on the graph within that region?

2) Can anyone point me towards an annotated trip curve where an algebraic formula is given for the shape of the curve in each region? For a thermal magnetic breaker, my first thought is that there are only two regions, the thermal region where trip time t is proportional to 1/I², and a magnetic region, where trip time is constant. I imagine that's a bit too simplistic, though.

Cheers, Wayne

 

[Eddie702]

It also shows just how complicated answering a seemingly simple question can become.

It shows that the code is not written properly if all the smart minds on this forum can't come to a consensus, then how is the average contractor or inspector supposed to come to the correct answer?

The way I look at it (which is probably wrong) is that wires are selected for HVAC equipment in 440 and standard motors in 430 allow for 125% of the load on a standard motor or in the case of HVAC equipment based on 125% of the largest motor (usually the compressor) in multi motor equipment. The equipment ground is sized based on the overcurrent protection which in most cases exceeds the "normal" current rating of the wire so motors and HVAC equipment are the only equipment that I am aware of that in a normal installation the over current protection does not match the conductor size. Although I am sure there are other cases

In any case the equipment ground must be sized to a minimum as in table 250:122 That table has not changed in decades and #10 copper has been allowed for a 60 amp breaker forever. If a 60 amp breaker has been determined to be able to carry any fault that a 60 amp breaker will let through then oversizing the circuit conductors both hot and equipment ground should be allowed as ling as 250:122 is complied with;

How about Tap conductors? In the case if tap conductors the equipment ground may be undersized in relation to the over current protection ahead of the tap and in that case the equipment ground does not have to be sized larger that the ungrounded conductors of the tap.

 

[Elect117]

Reading and Configuring Time-Current Curves « Knowledge Base – Design Master Software

1) Is the width of the region shown purely attributable to variation or calibration differences between different units? I.e. if we could repeatedly test a single given circuit breaker at the specified cold start ambient 40C temperature, would we give a single curve on the graph within that region?

That is how I understand it.

2) Can anyone point me towards an annotated trip curve where an algebraic formula is given for the shape of the curve in each region? For a thermal magnetic breaker, my first thought is that there are only two regions, the thermal region where trip time t is proportional to 1/I², and a magnetic region, where trip time is constant. I imagine that's a bit too simplistic, though.

I can only find where they write it like piecewise functions or define each section of protection with their limit.

https://library.e.abb.com/public/114371fcc8e0456096db42d614bead67/2CDC400002D0201_view.pdf

 

[wwhitney]

Or am I mistaken, and we can construct a reasonable example where a normal sized EGC depends on a minimum impedance of the ungrounded conductors in order to be adequate during a fault?

If I had the time I would.

Thinking about this some more, it seems to me that if are concerned about conductors/EGC surviving a bolted fault, what we end up with is a criterion of a minimum conductor size based on the available fault current.

That is, if for sufficiently large fault currents, the OCPD opens (say magnetically) in a fixed time period (say 1 cycle), then the worst case I²t would occur when I is the full available fault current. E.g. for a fault immediately after the OCPD where the impedance contribution of the conductor and EGC between the OCPD and the fault is small enough to be negligible.

Then the temperature rise of any conductor carrying the fault current (including the EGC, which is likely the smallest conductor in the fault current path) will depend on I²t/A², where A is the cross sectional area. The square factor arises as one factor due to decreasing resistance with area (and thus decreasing power dissipation per unit length I²R and thus decreasing total energy I²Rt), and another factor due to increasing conductor mass per unit length, and thus increasing energy required to raise the conductor temperature a given amount.

The result is that if there is a maximum allowable conductor temperature rise (say, that the EGC does not get hot enough to damage the insulation of other conductors it may be in contact with), we get a maximum allowable I²t/A², which coupled with a worst case maximum fault current I and a fixed t in the instantaneous range gives us a minimum allowable A.

So then why doesn't the NEC have a rule of the form "given an AFC of X, the minimum conductor size is Y"? I mean, I haven't worked out an example of the above (partially because I'm unclear why the formula in post 100 has logs in it and is missing a square exponent on A; seems like for short time spans the heat generated has no time to be conducted away and the right hand side should just be proportional to the temperature rise), but surely it will say that with a 50,000A fault a #14 conductor would experience excessive temperature rise?

Regardless, getting back to the point of 250.122(B), it seems to me that the result is that if for a given circuit with a given size EGC, the EGC will avoid excessive temperature rise during a fault located anywhere on the circuit for any fault current in the instantaneous OCPD range up to the AFC, that same statement will be true even when the ungrounded conductor is upsized. As in either case, the worst case is that the fault is just after the OCPD and the fault current is the full AFC, and so it's the same worst case.

Cheers, Wayne

 

[mtnelect]

This might help to explain.

 

[jerryalan]

Another case where that technically applies is, when converting a 240v electric range circuit using #6NM w/a #10 EGC to a 120v gas range circuit, the #10 EGC is now not large enough for a 15a breaker.

this confuses me, "#10 EGC not large enough for a 15a breaker", what am i missing?

 

[LarryFine]

this confuses me, "#10 EGC not large enough for a 15a breaker", what am i missing?

In a 15a circuit, the EGC is the same 14ga as the circuit conductors. When the 14ga wires are "increased" in size to 6ga, the EGC is supposed to be proportionately enlarged.

So, technically, the EGC should "continue" to be the same gauge as the circuit conductors, which in this case means the #10 EGC is undersized for the new 15a circuit rating.

 

[infinity]

In a 15a circuit, the EGC is the same 14ga as the circuit conductors. When the 14ga wires are "increased" in size to 6ga, the EGC is supposed to be proportionately enlarged.

So, technically, the EGC should "continue" to be the same gauge as the circuit conductors, which in this case means the #10 EGC is undersized for the new 15a circuit rating.

IMO the exception added to the 2020 NEC for 250.122(B) has pretty much ended this argument. It's obvious that this EGC sized for a 60 amp circuit would still be large enough for a 15 amp circuit.

Exception: Equipment grounding conductors shall be permitted to be sized by a qualified person to provide an effective ground fault current path in accordance with 250.4(A)(5) or (B)(4).

 

[Eddie702]

#10 EG is good for 60A. Don't understand how a # 10 could not be large enough for a 15 amp circuit.

 

[infinity]

#10 EG is good for 60A. Don't understand how a # 10 could not be large enough for a 15 amp circuit.

I agree which is exactly what the exception is saying.

 

[wwhitney]

#10 EG is good for 60A. Don't understand how a # 10 could not be large enough for a 15 amp circuit.

If the circuit is long enough, #10 would be too small.

E.g. suppose we decide that we want the bolted fault current to be at least 120A, for sufficiently fast breaker operation (not sure how fast that would be at 8 times rated current). And say our load is a true 12A but can tolerate a 10% voltage drop or 12V. That means our maximum impedance for faults, or 120V/120A, is the same as our maximum impedance for normal operation, or 12V/12A. So our EGC should be the same size as our circuit conductors.

Southwire's voltage drop calculator says that at 120V, 12A over a 2-wire #10 copper AWG circuit would have a 10.07% VD at 480 ft. So if this circuit is longer than 480ft, #10 circuit conductors and #10 EGC would not meet our criteria and we would need #8 circuit conductors and #8 EGC. Which would work for up to 725 ft.

Cheers, Wayne

 

[ISES117]

customer has an a/c unit being installed and label says min. circ ampacity is 26 max ocp is 40. can you run #8 for unit as customer wants?

Acknowledge
If there is no voltage drop, I would run a #10 with a #12 grounding conductor. Overkill cost money.

 

[curt swartz]

Acknowledge
If there is no voltage drop, I would run a #10 with a #12 grounding conductor. Overkill cost money.

Hope you are not being serious.

 

[infinity]

If the circuit is long enough, #10 would be too small.

True but it's not really germane to the question you quoted from eddie702 which is from Larry mentioned converting a 60 amp range circuit to a 15 amp circuit, Larry stated "Another case where that technically applies is, when converting a 240v electric range circuit using #6NM w/a #10 EGC to a 120v gas range circuit, the #10 EGC is now not large enough for a 15a breaker." jerryalan asked a similar question.

this confuses me, "#10 EGC not large enough for a 15a breaker", what am i missing?

#10 EG is good for 60A. Don't understand how a # 10 could not be large enough for a 15 amp circuit.

 

[wwhitney]

True but it's not really germane to the question you quoted from eddie702 which is from Larry mentioned converting a 60 amp range circuit to a 15 amp circuit

Ah, missed that bit of context from this older thread, thanks.

Cheers, Wayne

 

[infinity]

Ah, missed that bit of context from this older thread, thanks.

Cheers, Wayne

I figured as much. At 135 posts it's easy to get lost in the weeds.

 

[Jaybone812]

this confuses me, "#10 EGC not large enough for a 15a breaker", what am i missing?

250.122(b) says when the ungrounded circuit conductors are increased in size for any other reason than ampacity adjustments required by 310.15(b) or (c) the cir mild of the egc must be increased in proportion to the increase of the ungrounded conductors . A # 6 and #6 cable assembly comes with a #10 egc, the #6 ungrounded conductors are quite a bit larger than what’s required for a 15 amp circuit so since they were increased for no reason the egc must increase and you can’t increase a egc that’s part of a cable assembly so unless you are deemed qualified to use 250.122(b) ‘s exception , it’s technically a violation


Sent from my iPhone using Tapatalk

 

[infinity]

since they were increased for no reason the egc must increase and you can’t increase a egc that’s part of a cable assembly so unless you are deemed qualified to use 250.122(b) ‘s exception , it’s technically a violation

In your opinion do you think that the exception applies? The bar for the word qualified in this scenario is pretty low.

 

[Jaybone812]

In your opinion do you think that the exception applies? The bar for the word qualified in this scenario is pretty low.

lol I 100% agree with you in terms of the bar that has been set call someone qualified and it’s very low. In all reality a #10 egc in that cable assembly is more than sufficient in size to provide an effective ground fault current path for any 15 amp circuit, so I’d say yea it applies here , but if I was a wiring inspector allowing the use of the exception would depend on their understanding and explanation as to why the feed is increased in size to begin with
If there’s a ac condenser that has a Mca of 30 and a MOP of 50 and they run a # 6 cable nm cable assembly to the disconnect location , and when asked why they told me they ran 6-2 nm cable to the disconnect because it’s a 50 amp circuit so wire has to be good for 50 amps
I would not call someone like that a qualified individual and I would not allow them to use the exception. If someone didn’t understand how to size a branch circuit for a condenser why would I allow them to determine if the conductor is sized appropriately to provide a the circuit with an effective path for ground fault current ?
So for me personally allowing the use of the exception I guess it would be dependent on if they actually understand why the 250.122(b) violation was cited and understand why they have to apply the exception to satisfy the code. If they don’t know or can’t understand or explain why they did it any why you should allow it I don’t consider them qualified and wouldn’t let them use the exception

 

[suemarkp]

I would set a lower bar, especially for HVAC techs who are allowed to install wiring and may not do any voltage drop calculations. If they said "I'm leaving expansion in case they go to a 5 ton with a 41A MCA and this cable is sufficient for that", it should be fine for a much smaller unit. It also happens when you remove a system that had a larger wire than you need now with a unit that is more efficient and has a lower MCA. No need to rip out a cable when the old system with a 30A cable that was sized fine now has a 15A unit on that same wiring.