Derating Question

[Jimmy7]

I ran two 12-2 romex cables to a laundry area and I had to change over to emt to a 4" square to get into the room.
These are 120 volt circuits. I believe I have to derate, correct? This would be four current carrying conductors, so I would use 310.15(C)(1) which would be 80%. I then would use 310.16 90 degree column ( I have always been taught to use the 90 degree column. Where is that written?) Anyway, a #12 would be 30 amps at 90 degrees - 30 amps x 80% = 24 amps, so I can use 20 amp breakers, correct?

 

[augie47]

If you are using 90 deg conductors you are correct.'
It is discussed in 110.14

 

[Jimmy7]

I stripped back the 12-2 romex and sleeved it to the 4” square. Do the conductors inside romex have 90 degree insulation?

 

[Elect117]

See 334.18 for more detail based on your installs configuration.

You can use the 90° Column to derate cables that are rated for 90°C (romex is one of them).

 

[augie47]

I stripped back the 12-2 romex and sleeved it to the 4” square. Do the conductors inside romex have 90 degree insulation?

Commonly done but per Code those conductors are not ones listed in 310,104 so they are not rated as a conduit wiring method such as THWN.

 

[Jimmy7]

It’s already installed, is there any problem with leaving it, or should it be ripped out?

 

[augie47]

It’s already installed, is there any problem with leaving it, or should it be ripped out?

It is highly unlikely it will ever be a problem but any installation not to Code can be a potential liability.

 

[Jimmy7]

I’m not trying to being a wise guy, but asking a serious question. If that’s the case, then you could never use a non-metallic cable to emt transition connector, correct? or is it that you just can’t strip the cable and put it in the EMT?

 

[augie47]

Technically you can't use the stripped NM conductors in a conduit system.

 

[infinity]

I’m not trying to being a wise guy, but asking a serious question. If that’s the case, then you could never use a non-metallic cable to emt transition connector, correct? or is it that you just can’t strip the cable and put it in the EMT?

The cable gets sleeved into the EMT and then enters into the box. I believe that a minimum of 1/4" of jacket is required in the box. Not being allowed to remove the jacket is stupid but unless the conductors are marked you're not supposed to remove it.

 

[Jimmy7]

I went onto a wire manufacture’s website and was looking at both #12 solid and stranded thwn-2 . It appears that the #12 solid had a 90 deg ampacity of 20 amps, and #12 stranded had a 90 deg ampacity of 30 amps. Is this typical of solid wire to have a lower ampacity at 90 degrees? I don’t think I would have known this in the field. I would see the -2 and think okay 90 degree rating and look at the #12 90 degree column and derate.

 

[augie47]

Can you post that website..... I've not seen any manufacturer that had a different rating between solid and stranded.
If you have not, you might look at Table 310.104

 

[Jimmy7]

https://encorewire-prod.imgix.net/media/products/product-sheets/copper/commercial/EncoreWire-Copper-THHN_2023.pdf

Is 310.104 the correct article?

 

[augie47]

That is interesting. I'm not sure why they rated them differently but you will see the footnotes take into account a number of Articles other than 310.
As far as the NEC is concerned the base ampacities are shown in 310.15(B)(16) with a couple of conductors having dual rating depending on wet or dry locations (again, 310.104)
The "usable" ampacities shown in 310.B15 can be effected by 110.14, 240.4 and other articles.
For your application a #12 solid, stranded, and of any standard insulation would have an ampacity of 20 amps based on 240.4(D)


(references are based on 2017 NEC)

 

[infinity]

Is 310.104 the correct article?

That is for over 1000 volts.

The table notes say that the ampacities listed take into account 240.4(D) for small conductors. Look at the fine print:

2 Ampacity of conductors are based on NFPA 70 (NEC) Table 310.15(B)(16). See 110.14(C), 240.4(D) and 310.15(B) for other limitations where applicable.

 

[Dennis Alwon]

I ran two 12-2 romex cables to a laundry area and I had to change over to emt to a 4" square to get into the room.
These are 120 volt circuits. I believe I have to derate, correct? This would be four current carrying conductors, so I would use 310.15(C)(1) which would be 80%. I then would use 310.16 90 degree column ( I have always been taught to use the 90 degree column. Where is that written?) Anyway, a #12 would be 30 amps at 90 degrees - 30 amps x 80% = 24 amps, so I can use 20 amp breakers, correct?

You are correct. If you look at 334.112 you will see that the NEC dictates that NM cable must use 90C conductors. The fact that they aren't marked doesn't change the fact that they are 90C

334.112 Insulation.

The insulated power conductors shall be one of the types listed in Table 310.4(1) that are suitable for branch-circuit wiring or one that is identified for use in these cables. Conductor insulation shall be rated at 90°C (194°F).

 

[Jimmy7]

You are correct. If you look at 334.112 you will see that the NEC dictates that NM cable must use 90C conductors. The fact that they aren't marked doesn't change the fact that they are 90C

Would you leave the installation I mentioned earlier, or rip it out?

 

[Dennis Alwon]

Would you leave the installation I mentioned earlier, or rip it out?

Since it is already done I would leave it but remember you can get called on it if the inspector sees it. Personally I think it is ridiculous to have that rule but it is what it is, as they say.

 

[Jimmy7]

One last question regarding the ampacity chart earlier in this thread.

In this case with the #12 solid, do you derate of 20 amp listed in 90 deg manufacturer chart, or can you derate off 30 amps listed in the code book?
I guess you could’t use the Code book, because of 240.4 ( D), it can’t be over 20 amps., Correct?

 

[infinity]

One last question regarding the ampacity chart earlier in this thread.

In this case with the #12 solid, do you derate of 20 amp listed in 90 deg manufacturer chart, or can you derate off 30 amps listed in the code book?
I guess you could’t use the Code book, because of 240.4 ( D), it can’t be over 20 amps., Correct?

Throw away that chart and use the proper values from the NEC.