30*C AC Wire Resistance 4

[Mbrooke]

Does anyone know the 30*C AC resistance (or impedance) of 14, 12 and 10 gauge wire?

 

[Mbrooke]

You know that the 30C temperature referenced is the ambient temperature, not the conductor temperature right?

Sure. However, in reality, the conductor will never reach 75*C, even at 15 amps. Also given the fact resi circuits tend to have a few amps loading at most, makes it likely the conductor will be actually a few degrees above ambient.

 

[Mbrooke]

@Waross: Please see these attached documents on why the magnetic pickup was lowered on 15 and 20 amp single pole breakers:

https://files.engineering.com/getfi...446a-a591-911e3177a168&file=KeepingPace-7.pdf

https://files.engineering.com/getfi...66a-a2b9-6d6538ab5d46&file=KeepingPace-15.pdf

https://files.engineering.com/getfi...f71-9769-ff6220475e7b&file=KeepingPace-37.pdf

And of course, what started it all:

https://files.engineering.com/getfi...&file=Circuit_Breakers_The_Myth_of_Safety.pdf

There was a major industry effort to lower the magnetic trip threshold on all 15 and 20 amp breakers when it was theorized that thermal tripping resulted in fires.

 

[Mbrooke]

And of course, we have this video:

https://youtu.be/emxfsOUTkUg?t=1547

Current 125% over the instantaneous trip function is what provides arc fault protection as validated by several major UL studies spanning two decades.

However- it was soon realized that because the NEC, specifically 250.4 A 5 doesn't define effective ground fault current path and nothing limits the run length of final circuits (unlike most foreign codes and IEC 60364) it was theoretically possible to have short circuit levels below 150 amperes at the furthest point of some residential circuit.

75 amps was settled upon being the lowest short circuit value that may be found in a dwelling based upon the longest reasonable run for a circuit (large home, panel in the basement).

Thus the original AFCI was drawn up as being nothing more than coil with a pickup of 75 amps. However it was soon realized 75 amps would nuisance trip on motors starting, light bulbs burning out, ect.

For this reason the electronic AFCI was created that ironically would distinguish between a vacuum cleaner starting and an arcing short circuit.

What makes me, electricians and so many others irate to no end is that instead of NFPA-70 adding a table restricting final circuit lengths for a given wire size and breaker rating they mandated an expensive failure prone device that is really nothing more than an empty fire extinguisher. If it can't tell between dangerous arcing and a switch mode power supply, who says it will actually trip on dangerous arcing? How says the electronics will still be functioning in 40 years, especially when UL claims multi kv power surges are causing insulation break down and high current arcing in from over driven staples?

 

[sparkyusa]

Bravo MBrooke for the info posted.

I'm starting to realize why an ecg went from insignificant, smaller 50's version to today's full size version

simply stated ~R~ matters

~S~

 

[Mbrooke]

Well, consider the fact those behind article 250 once thought the earth (terra firma) opened circuit breakers... no one can refute grounding and bonding theory has always been a struggle for them.

R does matter, it is the basis of all life and property protection.

 

[7anoter4]

Lately I have no contact with the practical use of conductors and I think it has been reviewed here almost all possible cases. But as an old cable manufacturer I can mention the following:
there are two types of standards: one for use and another for manufacturing. The manufacturing standard has tolerances because it cannot be achieving a single value. The copper wire is pulled through diamond dies that are consumed with the time and diameter of the conductor pulled increases.
Another problem is the twisting of the thread wires into the compound conductors that stretch the wire according to the stranding force. Then, the diameter and thickness of the insulation differ and depending on the type of the extrusion die ressure type or vacuum type depends on it the centering. There are still many elements that can change the resistance-R and inductivity-L Depending on the frequency is resistance skin effect and proximity effect if there are close each to other several wires of different phases.
The temperature of the conductor and the environmental is another problem. That's why there are standards of use that are based on the experience of using conductors.
In reality, if the calculation goes by a standard, one should also take into account tolerances and not be too limited to the standard.

 

[waross]

8-102 Voltage drop (see Appendices B and D)
After reading your post concerning the legal action that the CSA (Authors of the CEC) have instituted in regards to Copyright infringement, I leave it to you to locate a copy of the CEC.
Suffice to say that rule 8-102, Appendix B and Appendix D explain the derivation, purpose and use of Table 68.
The number of codes and reference books piled on your desk is immaterial if the stack does not include a copy of the CEC, when you chose to post false information as to the purpose and use of a table.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

@Waross: Posting small excerpts from the code is not copy right infringement.

False information? Nice try. Do you seriously believe the CEC doesn't have other inferences in creating table 68 besides those claimed at face value?

 

[Mbrooke]

@7anoter4: Right, this is one reason why Table 310.16, IEC 60364-5-52, ect are conservative as well as anything to do with EFLI. In reality the size of the wore will vary slightly, and if you ever get yellow, orange or brown copper you'll be guessing about the chemical and molecular structure. No doubt is this taken into account by standards which typically list current carrying capacity and Z assuming the conductor is on the end to the far end of the tolerance spectrum.

 

[Mbrooke]

And of course my second goal/objective/motive in asking for 30*C resistances:

https://forums.mikeholt.com/threads...-what-is-correct-method.2560032/#post-2657132

This question gets asked literally every other day by electricians and engineers fulfilling NEC requirements in particular 110.24. More often than not a unified consensuses is not reached.

I'd like to make a table that can be placed into NFPA-70 where the serving transformer, service drop and service conductors are converted into simple ohm values which can be added together by anyone then used to divide +6% of the nominal voltage to obtain the maximum short circuit current at the service disconnecting means. This must be based on a 30*C conductor impedance at most.

For determining maximum disconnection times I already have chapter 9 table 9 values at 75*C. These when added to a trafo's equivalent ohm value are used to divide -6% of the nominal voltage to obtain ground fault loop current.

No matter what I'd like to hit the ground running on actual values and then add simplfiers or modifiers when making the tables if the math says I can do so without exceeding worse case scenarios.

 

[waross]

What about power factor?
You may add pure resistance arithmetically but you cannot add impedances unless the PF or X/R ratios are equal.
You probably think that the current on the neutral must always be the difference between the L1 current and the L2 current in a single phase circuit.
Not so, despite what they teach in high school and trade school, and a lot of text books.
(And this is not a CSA conspiracy.)

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

While I recognize there’s a world of difference between #10 at 120V and 795 at 115kV, it’s still weird. Yes, I try to get the R component “right” when calculating line parameters, but the industry did just fine for a very long time by introducing a very small error that made the math significantly simpler, setting Z = X, letting R = 0. The difference between a line angle of 90 and a line angle of 80 is really small. Read Blackburn, the math is all based on Z = X. Close enough.

Sure, that wimpy wire stuff is different, but X still matters, even if no longer overwhelmingly dominate. How exactly is the given spacing between phase and neutral maintained? If you know R to a tenth of a percent but X is still +/- 2.5% have you gained much?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[FreddyNurk]

I'm starting to think this is less about fault loop impedance or the resistive component of copper at 30C and more about attempting to tell the world that we're wrong and don't follow standards.

 

[sparkyusa]

If the focus of 110.24 holds us to calculating AIC for proximal validity, why can't it do the same for distal validity? ~S~

 

[Mbrooke]

@FreddyNurk: If you mean the industry was wrong in mandating AFCIs and AFDDs, along with ever expanding GFCI requirements each code cycle (wait for the day when a commercial electrical room with 5 84 space panels is full of dual function breakers) you can bet your top dollar I'm going to challenge that in real time.

Not just because it is a civic duty for man to question authority, but also I know for a fact AFCIs were originally created in the hopes of mimicking the British (and IEC) wiring system of low magnetic pickup breakers, fused plugs and RCDs. That was the goal as it was theorized the advantages of this particular system mitigated fires even if the particular way of doing things was originally implemented for other reasons. Yet despite this, AFCI are now being aggressively mandated on a global scale as something new claiming to provide the same parallel arc fault protection that original research confirmed was already provided by IEC wiring practices.

Whats left is series arc fault protection where I and a growing number of people have come to realize 90% of all electrical fires are not caused by arcing but rather joule heating.

In other words, AFCIs and AFDDs provide little if any fire protection. What stands is pure fraud, which only escalates breadth and scope as people remain silent.

 

[Mbrooke]

What about power factor?
You may add pure resistance arithmetically but you cannot add impedances unless the PF or X/R ratios are equal.

PF or circuit reactance is negligible in smaller wire sizes (under 6WAG) when calculating fault current. This is why the equation Zs=Ze+(R1+R2) uses R1 and R2 instead of Z1 and Z2.

I mean you could add reactance into the equation, however the final difference is so negligible that using 75*C or -6% voltage will always give lower loop currents.

You probably think that the current on the neutral must always be the difference between the L1 current and the L2 current in a single phase circuit.

Not so, despite what they teach in high school and trade school, and a lot of text books.
(And this is not a CSA conspiracy.)

Which is why I'm trying to create a few simple tables for calculating 110.24 and EFLI. Electricians should be able to easily get these answers on their own for the ease of it.

 

[Mbrooke]

While I recognize there’s a world of difference between #10 at 120V and 795 at 115kV, it’s still weird. Yes, I try to get the R component “right” when calculating line parameters, but the industry did just fine for a very long time by introducing a very small error that made the math significantly simpler, setting Z = X, letting R = 0. The difference between a line angle of 90 and a line angle of 80 is really small. Read Blackburn, the math is all based on Z = X. Close enough.

Sure, that wimpy wire stuff is different, but X still matters, even if no longer overwhelmingly dominate. How exactly is the given spacing between phase and neutral maintained? If you know R to a tenth of a percent but X is still +/- 2.5% have you gained much?

David, as always your replies are thought provoking

NM, MC and even conduit is literally the polar opposite of a transmission line, at least with smaller wire sizes. R dominates so much, that ignoring X produces no ill effect. The wire will be guaranteed close, certainly within a cable assembly.

Assuming 75*C R for fault loop impedance (lower current than actual) and 30*C R for short circuit currents (higher current than actual) will always produce conservative values that will fulfill their intended goals of tripping the breaker quickly and not exploding the device respectively.

With larger sizes X does begin to matter, and at that point it may be necessary for a sparky to calculate Z= the square root of (Rsqr + Xsqr)

 

[waross]

I'd like to make a table that can be placed into NFPA-70 where the serving transformer, service drop and service conductors are converted into simple ohm values which can be added together by anyone then used to divide +6% of the nominal voltage to obtain the maximum short circuit current at the service disconnecting means. This must be based on a 30*C conductor impedance at most.

So you are going to add the X of the transformer to the R of the circuit.

That's sounds simple.
Not correct, but simple.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

I'm thinking the X can be turned to an equivalent R value. I haven't tried it, but its one of the things I'm working on.

Of course nothing will compare like vector math.

 

[stevenal]

Faulted circuit conductors don't remain at ambient temperature, they heat - quickly. If you want precision, you should model the resistance dynamically. If using a constant resistance for simplicity, the 75C value will probably be more accurate.