A/C wire size.

[wwhitney]

250.122 does not care about MCA

Sure it does. The following is a valid installation:

28A MCA
40A MOCP and actual OCPD
No ampacity correction or adjustment required
#10 Cu ungrounded
#10 EGC

So now if the install uses #8 Cu instead of #10, the "ungrounded conductors are increased in size." 250.122(B) says you have to increase the EGC in size proportionately, so you need a #8 Cu EGC, unless you can invoke the exception.

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

If that holds true than it won't be long to cable manufactures have full size equipment grounds in all there cables selections.
There not going to let cables be pushed out of that % of yhe market because of this requirement.
The ony question will they still manufacture cables without full size equipment grounds to have a competitive market

 

[tortuga]

Sure it does.

Where does it say MCA in 250.122 ?
it says 'increased in size' not increased in size from calculated MCA.
The size of #8 NM is 40 amps per 334.80, thats the only size you need to be concerned with.
Thats all thats enforceable

 

[wwhitney]

Where does it say MCA in 250.122 ?
it says 'increased in size' not increased in size from calculated MCA.
The size of #8 NM is 40 amps per 334.80, thats the only size you need to be concerned with.

No, the "size" of #8 NM is not 40A. The size is #8, the ampacity is 40A.

250.122(A) says to determine the size of the EGC by looking at the OCPD rating. So with a 40A OCPD we start with #10 EGC. The size of the ungrounded conductors is immaterial for 250.122(A).

Now 250.122(B) says "hey did you use the minimum size ungrounded conductors you could have?" If so, you're done sizing the EGC, you use the value from 250.122(A). If not, how much did you upsize the ungrounded conductors? Upsize the 250.122(A) value by the same proportion, and that's the EGC size you need.

So 250.122(B) absolutely requires determining a "starting" ungrounded conductor size, and there is no language to say that should be done from the OCPD size.

Cheers, Wayne

 

[tortuga]

So 250.122(B) absolutely requires determining a "starting" ungrounded conductor size, and there is no language to say that should be done from the OCPD size.

Its right in the table:

 

[wwhitney]

Its right in the table:

Correct, and 250.122(A) tells you to do a table lookup. 250.122(B) then may modify that table lookup to get your final answer.

Cheers, Wayne

 

[tortuga]

Correct, and 250.122(A) tells you to do a table lookup. 250.122(B) then may modify that table lookup to get your final answer.

Cheers, Wayne

(B) does not apply becasue #8 NM has a 40A ocpd, you have not increased anything in size relative to the table.

 

[wwhitney]

(B) does not apply becasue #8 NM has a 40A ocpd, you have not increased anything in size relative to the table.

That's just wrong. The table doesn't say anything about conductor size. It just says 31A-60A OCPD means start with #10 EGC. You're bringing in 310.15 when there's nothing in 250.122 that references it.

In the example given, the minimum conductor size is #10, and using #8 does represent an increase in size.

Cheers, Wayne

 

[tortuga]

In the example given, the minimum conductor size is #10, and using #8 does represent an increase in size.

Thats incorrect you go by 334.80, the ampacity of #8 NM cable the chosen wiring method.

 

[kwired]

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

Yet you can run into air conditioners that essentially require 8 AWG minimum if you have 60C conductor yet also specify a MOCP of 60 amps.

Used to be common with 5 ton units. Now I think many are efficient enough they are ok on 10 AWG but are on upper end of what it can handle.

 

[suemarkp]

Sizing by cable type also bring in more irregularities with 250.122(B). Lets say you ran 8-2 MC cable with a #10 ground to a motor or HVAC load with an MCA of 32A and 75C rated terminations. This cable would be oversized because 10-2 MC would have been legal. But if NM cable, you'd have to run 8-2 because it is limited to 60C.

Ground faults don't care about wire temp rating. It is just wire size and length and the breaker rating. That determines the trip time and you want to have low enough impedance to stay in the magnetic trip portion of the breaker trip curve.

So this is just more complexity in this section (250.122(B)). Cable type shouldn't matter, just wire size and length. What is a "normal sized" conductor is not defined. You need to be able to know that in order to see if you have "oversized" the conductors. Maybe that is where "qualified person" comes in to make the judgement call. I think this code section used to specifically mention oversized because of voltage drop, but that got eliminated a few versions back. Maybe because normal voltage drop isn't defined nor does the code require a specified voltage drop to be normal or above normal.

 

[kwired]

If that holds true than it won't be long to cable manufactures have full size equipment grounds in all there cables selections.
There not going to let cables be pushed out of that % of yhe market because of this requirement.
The ony question will they still manufacture cables without full size equipment grounds to have a competitive market

They can make about anything. Suppliers generally stock what moves.

If there is a somewhat specialty item that is pretty common in some locality and needs larger EGC than is common you might find stocked cables with what is needed. This may be more common with MC or other cables than it is for NM cable though.

 

[tortuga]

Sizing by cable type also bring in more irregularities with 250.122(B). Lets say you ran 8-2 MC cable with a #10 ground to a motor or HVAC load with an MCA of 32A and 75C rated terminations. This cable would be oversized because 10-2 MC would have been legal.

As soon as you choose the wiring method (MC) then you go by the standard ampacity of your cable, so for 8AWG MC the standard size is 50A, so the 8AWG MC cable meets 250.122.
If you had the 8/2 MC on a 20A breaker now you have upsized the conductors and (B) would kick in.

What is a "normal sized" conductor is not defined. You need to be able to know that in order to see if you have "oversized" the conductors.

Its defined by the standard ampacity of the cable, nothing to do with the MCA or the equipment.

 

[wwhitney]

Thats incorrect you go by 334.80, the ampacity of #8 NM cable the chosen wiring method.

No, there is no reference to ampacity in the 2020 NEC version of 250.122. All 250.122(B) says is "increased in size". So you just look at the minimum size that could have been used for the circuit, encompassing all the rules of the NEC, and the ampacity tables do not have any special priority.

If in the same wiring method installing #10 would have been NEC compliant (with no other changes, same size OCPD), then #8 is increased in size.

[Well, I'm somewhat open to the idea that you could install a 75C rated wiring method with 75C rated terminations, but voluntarily declare that you only want to use the 60C ampacities, so you get to specify that you are using 60C ampacity for your wire sizing. But that's moot for NM cable as it does not apply.]

Cheers, Wayne

 

[wwhitney]

Its right in the table:

That's the starting (minimum) EGC size. It doesn't tell you the starting (minimum) ungrounded conductor size. Determining that requires applying the entire NEC.

There's no instruction in 250.122(B) to ignore the rest of the NEC, just take the OCPD size and do a reverse look up in Table 310.16 to determine the minimum conductor size you can use for normal circuits with that size OCPD. That's a procedure you invented with no justification.

Instead you just have to ask "if I downsized this conductor, would the install still be NEC compliant?" If the answer is yes, the conductor is "increased in size".

Cheers, Wayne

 

[jaggedben]

250.122(B) is very specific about the only reasons you can upsize conductors without upsizing an EGC, and they are the correction and adjustment factors, namely 310.15(B) and (C). So Wayne is correct that if you had a HVAC unit with an MCA of 28A, and no applicable adjustment or correction factors, then if you used 8awg you upsized them for a different reason, and per 250.122(B) you would have to upsize the EGC, too. It's true that the notion that 10awg is the baseline from which anything larger is 'upsized' is implicit here, 10awg being the minimum that all other code requirements would require for the circuit.

 

[jaggedben]

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

That last part really doesn't follow for me.

Why then does table 250.122 involve overcurrent devices at all? Do they not matter?

 

[david]

They can make about anything. Suppliers generally stock what moves.

If there is a somewhat specialty item that is pretty common in some locality and needs larger EGC than is common you might find stocked cables with what is needed. This may be more common with MC or other cables than it is for NM cable though.

Or we might start seeing a lot of 250.134 (2) by connecting to an equipment ground of the wire type.......that is otherwise run with the circuit conductors, somehow being protected from physical damage when necessary.

I'm not convinced by the wording this rule could be made applicable to NM and other types of cable.

Saying when you use NM cable your installing the wire type equipment ground in that wiring mythod, is kind of like saying when you use a metal raceway as an equipment ground your installing the metal in the raceway that makes up the equipment ground

By saying if an equipment ground of a wire type is installed leaves that up to interpretation. ( also seems to imply a raceway type wiring method)

If an equipment ground of the wire type is used might be more all inclusive

 

[electrofelon]

If that holds true than it won't be long to cable manufactures have full size equipment grounds in all there cables selections.
There not going to let cables be pushed out of that % of yhe market because of this requirement.
The ony question will they still manufacture cables without full size equipment grounds to have a competitive market

The EGC in larger cables is typically larger than minimum size. Another common issue with the EGC in cables is when they are used in parallel. But you can stick between a rock And a hard place: say you go with a larger cable size in a parallel install to get the "full size EGC in each raceway or cable". Now you have increased the size and have the 122(B) issue.

 

[tortuga]

That's the starting (minimum) EGC size. It doesn't tell you the starting (minimum) ungrounded conductor size....
There's no instruction in 250.122(B) to ..., just take the OCPD size and do a reverse look up in Table 310.16 to determine the minimum conductor size you can use for normal circuits with that size OCPD. That's a procedure you invented with no justification.

Nothing in 250.122 says how to come up with that minumum, saying I have to go by the MCA is unenforceable all I need to do is meet the OCPD for the wiring method chosen. 334.80 is in the code it limits NM to 60C.
I pass inspections like that all the time, I'll buy anyone on here a milkshake that gets red tagged on 250.122(B) for using a 8/2 NM cable on a 40A breaker.