A/C wire size.

[electrofelon]

It's no different than plugging in a lamp with a 16 gauge cord into a receptacle on a 20A circuit.

And of course the ampacity figures in the NEC are somewhat arbitrary and conservative. It's very unlikely the nunnery or kitten store is going to burn down because there is some 12 gauge on a 30 or 40 amp breaker.

 

[ActionDave]

And of course the ampacity figures in the NEC are somewhat arbitrary and conservative. It's very unlikely the nunnery or kitten store is going to burn down because there is some 12 gauge on a 30 or 40 amp breaker.

And A/C units and motors are one of the places where the NEC rules actually make some sense because they are rooted in some science rather than emotion, tradition, and voodoo.

 

[jonsplace]

#10 ground is good for up to 60 a breaker or fuses. it has been that way for decades.

Many electricians don't understand that the wire does not have to match the breaker or fuse size because it is a motor circuit. The motors in the unit are thermally protected to prevent conductor overload.

The MCA is the "minimum" circuit amps of the wire, you can always use larger wire if you want but are not obligated to do that.

The MOCP is the 'maximum" size breaker or fuses that the "minimum MCA wire size" is allowed to be used with.

You can always use a smaller MOCP but are not obligated to do that either. But the MOCP must be large enough for the equipment to start.

Yeah, I believe in a motor control system you can put a #16 AWG on a 40a breaker don't have the book in front of me

 

[Fred B]

Voltage drop is one reason to upsize the wiring but not only reason. so to play devils advocate (Inspector), he likely will not know the rational of the increase in size. 250.122 is not very ambiguous though. But not sure of the exception and WHO is the "qualified person" here.

NEC 2023: 250.122(B) Increased in Size.
If ungrounded conductors are increased in size for any reason other than as required in 310.15(B) or 310.15(C), wire-type equipment grounding conductors, if installed, shall be increased in size proportionately to the increase in circular mil area of the ungrounded conductors.
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)

Artical 100:
Qualified Person.
One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved. (CMP-1)

So lets get rid of 122(B) in its entirety when you add the exception.

 

[infinity]

So lets get rid of 122(B) in its entirety when you add the exception.

The 2023 is yet another attempt at the NEC trying to get this right and again they've failed with this "qualified" nonsense.

I would start with this list:
1) Start with metal raceway that qualifies as an EGC, then no 250.122(B).
2) Had larger wire on the truck so I upsized the ungrounded conductors, then no 250.122(B).
3) Converted at 50 amp receptacle to a 20 amp, then no 250.122(B).
4) Upsized for voltage drop then 250.122(B) applies.

 

[kwired]

So were the EGCs upsized on these installations per 250.122(B)?

Some yes others no.

A common application is to run 4 AWG aluminum URD quad to the center pivot, that sometimes can be up to about 2800 feet away. These machines usually have a 30 amp disconnect though actual recommended fuse to install may only be a 15 or 20, will vary with system length and number of drive motors on it. BTW is very rare for every motor to run at same time and would be for pretty short time if they did. If you would clamp an ammeter on at any given time they seldom would draw much more than maybe 6 or 8 amps at any given point in time. First few seconds after switching direction of travel probably the time they would draw the most current as nearly every motor could run for a few seconds as it gets re-aligned so to speak for going the other direction.

Anyway with the #4 quadplex you are mostly running it for voltage drop reasons, though some the irrigation only guys maybe don't realize that is why. But #4 quad has all four conductors same size so it does comply with 250.122(B) even if you didn't think about that requirement. Should you happen to be running larger sizes they typically have one reduced size conductor though there are some available with all conductors same size. But at some those long distances it still introduces enough impedance and raises the trip time for fuses during a fault.

 

[kwired]

Yeah, I believe in a motor control system you can put a #16 AWG on a 40a breaker don't have the book in front of me

I didn't look either but thinking that one is possibly is if the control conductor doesn't leave the control cabinet it originates in. Run to a remote located switch and now you have lesser limits on overcurrent protection.

 

[jap]

It's no different than plugging in a lamp with a 16 gauge cord into a receptacle on a 20A circuit.

Sure it's different.
The Receptacle outlet should have 20 amp wiring to it.

JAP>

 

[david]

Technically true, but I would hope an inspector wouldn't pull that. The next AC unit may have an MCA of 31 and now you'd be ripping out that #10 and installing #8. And you'd still have a #10 EGC in the latter case.

I must be in real trouble because the AC tech who installed mine put in #6 NM cable because he only looked at the 50A breaker and not the MCA. Now I have a smaller AC unit on that same wire with an MCA in the high 20's and a 40A breaker.

Imo 250.122 (B) dosn't apply to NM cable, a wire type equipment ground if installed, could only be referring to a raceway type installation.
You do not install a wire type equipment ground in a cable the manufacture does, so the if installed has to be something other than a cable type wiring mythod

 

[wwhitney]

Imo 250.122 (B) dosn't apply to NM cable, a wire type equipment ground if installed, could only be referring to a raceway type installation.

No, it applies to NM cable. When you install NM cable, you are as a side effect installing the EGC therein. If the NM cable has the wrong size EGC because of 250.122(B) requirements, you just need to use a different wiring method.

Cheers, Wayne

 

[Elect117]

wire size goes up, resistance goes down, fault current at the load goes up, required ampacity for the EGC goes up.

 

[wwhitney]

wire size goes up, resistance goes down, fault current at the load goes up, required ampacity for the EGC goes up.

It's not that simple. The combined ungrounded plus EGC impedance determines the fault current, which determines the fault duration based on the OCPD trip curve and source characteristics. Then the EGC has to withstand the fault current for the fault duration. The shorter the duration, the smaller the EGC can be for a given fault current. EGC ampacity shouldn't enter into it, as ampacity is a continuous rating, so would only apply if the fault goes on forever and never clears. Which would obviously be a problem.: - )

The real issue is "wire length goes up; ungrounded conductor size is increased to provide adequate voltage at the load; if the EGC conductor size doesn't also go up, then the fault current may go so low that the fault duration becomes excessive."

Cheers, Wayne

 

[david]

No, it applies to NM cable. When you install NM cable, you are as a side effect installing the EGC therein. If the NM cable has the wrong size EGC because of 250.122(B) requirements, you just need to use a different wiring method.

Cheers, Wayne

In that case you could never use uf cable for a swimming pool pump that you supplied with 240 volts.
I installed the # 12 uf cable because I needed the min #12 awg copper equipment ground. No the pump only called for 14 awg supply conductors so you need to increase the eguipment ground proportionally

At least not for the exception you mentioned

We are going to have to disagree on what the phrase if installed means.

Imo if installed means in addition to the equipment ground that the wiring instalation provides.

You would be required to install a wire type in a nonmetallic raceway, you might chose to install one in a metal raceway. You do not install one or have control over the size of one in a cable such as NM or uf

 

[wwhitney]

In that case you could never use uf cable for a swimming pool pump that you supplied with 240 volts.
I installed the # 12 uf cable because I needed the min #12 awg copper equipment ground. No the pump only called for 14 awg supply conductors so you need to increase the eguipment ground proportionally

There's no problem in your example, the EGC never is required to be larger than the ungrounded conductors, so #12 EGC for #12 ungrounded is always compliant.

Where you could get into trouble with NM cable is if you're supplying HVAC equipment with an MCA of 28A and an MOCP 40A, and you choose to run #8-2 NM (for the interior portion of the circuit). Did you do that because the voltage drop on #10 would be too much? Oops, that's a 250.122(B) violation, you need a #8 EGC as well, so you better use a different wiring method (sadly, per 250.119(B) you can't use #8-3 NM and relabel one of the #8s green, unless "the conditions of maintenance and supervision ensure that only qualified persons service the installation.")

Or did you do that because you always match conductor size to the OCPD size? Then you can invoke 250.122(B) Exception, no problem.

Cheers, Wayne

 

[LarryFine]

What if you used the #8 NM because you had no #10 or your helper just goofed?

 

[wwhitney]

What if you used the #8 NM because you had no #10 or your helper just goofed?

Apply 250.122(B) exception, you're qualified to say that 10-2 NM would have been fine, so the #10 EGC in either 10-2 or 8-2 will provide "an effective ground fault current path."

Cheers, Wayne

 

[david]

There's no problem in your example, the EGC never is required to be larger than the ungrounded conductors, so #12 EGC for #12 ungrounded is always compliant.

Where you could get into trouble with NM cable is if you're supplying HVAC equipment with an MCA of 28A and an MOCP 40A, and you choose to run #8-2 NM (for the interior portion of the circuit). Did you do that because the voltage drop on #10 would be too much? Oops, that's a 250.122(B) violation, you need a #8 EGC as well, so you better use a different wiring method (sadly, per 250.119(B) you can't use #8-3 NM and relabel one of the #8s green, unless "the conditions of maintenance and supervision ensure that only qualified persons service the installation.")

Or did you do that because you always match conductor size to the OCPD size? Then you can invoke 250.122(B) Exception, no problem.

Cheers, Wayne

True residential pools was a poor example, you get the idea the main idea that you have no control over the size equipment ground in cables, # 6awg uf. Or in conduit when you pull two different size ungrounded conductors, or conductors larger because you needed to to get the job completed.

Non of that maters because the exception can be used by the installer for a multiple of reasons

 

[tortuga]

Given 250.122(B), that's not enough info to decide if #10 is sufficient.

The wire size 250.122(B) is referring to is the OCPD sizes in T250.122, so in any case if your using a #8 on a 40 you never need a equipment ground larger than 10, unless a different article requires it.

 

[wwhitney]

The wire size 250.122(B) is referring to is the OCPD sizes in T250.122, so in any case if your using a #8 on a 40 you never need a equipment ground larger than 10.

That's just not true. An example I made up in the another thread: you could have an HVAC unit with an MCA of 28A and an MOCP of 40A. You run #8 NM for the interior portion of the run. If you made that choice because you upsized for voltage drop because it's a long run, you need a #8 EGC under 250.122(B).

Cheers, Wayne

 

[tortuga]

That's just not true. An example I made up in the another thread: you could have an HVAC unit with an MCA of 28A and an MOCP of 40A. You run #8 NM for the interior portion of the run. If you made that choice because you upsized for voltage drop because it's a long run, you need a #8 EGC under 250.122(B).

Cheers, Wayne

250.122 does not care about MCA, all that matters is the 40A breaker is a standard size for #8 NM
Cheers