Romex Ampacity on A/C unit

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Geraldselectric

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I am studying a question as follows:

What size romex is required to supply a 6000va A/C unit?

6000/240v=25a
25a x 125% = 31.25a

The answer states at 75 deg., romex required would be #10 (35a). I thought you were limited to 60 deg. when dealing with romex because of 334.80? Am I missing something? Thanks.
 
celtic

celtic

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Location
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Geraldselectric said:
The answer states at 75 deg., romex required would be #10 (35a). I thought you were limited to 60 deg. when dealing with romex because of 334.80? Am I missing something? Thanks.
Click to expand...
Yes...

See the astrisk next to #10 in T310.15 ?
At the bottom of the table, the astrisk says - SEE 240.4(D)

240.4(D) states:
(D) Small Conductors.
Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.
Click to expand...

Ya with me?
 
Dennis Alwon

Dennis Alwon

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Geraldselectric said:
If using romex, doesnt 334.80 require using the 60 deg. column and 31.25a would require a #8 romex and 35a OCPD?
Click to expand...


I tend to agree with you-- The asteriks are there but it does not say for romex so I believe you are limited to 60C column. Those asteriks allow you to use the higher ampacities but for the temp rating of the wire or cable.

The OCPD can be greater than 35 amps-- where did you get that info. #8 is good for 40 amps and you can protect an a/c unit at 175% of the motor rated load current. art 440.22
 
H

haskindm

Senior Member
Location
Maryland
I can't believe that a test question would use the term "ROMEX" since that is a brand name for type NM Cable. If the test did use this term it is a good indication that the test writer lacks some basic code knowledge, so I would question their ability to provide correct answers. If the question does refer to NM cable, then the requirement in 334.80 would apply, limiting the ampacity to that of 60 degree conductors and would override the asterisk in table 310.16.
 
G

Geraldselectric

Member
The test question does state Nonmetallic-sheathed cable. But, I believe the writer forgot about the 60 deg. limitation and it is confusing me. Another question is:

What size Nonmetallic-sheathed cable is required to supply a branch circuit for a 16,000va cooktop and 12,000va oven using note 2 per table 220.55?

Using note 2, you have unequal ratings and add them up.

28k - 12k = 16k x 5% = 80%, 8k x 180% = 14.4kw / 240v = 60 amps

The answer is #6 with 70a OCPD. But again, I disagree because I am wiring with romex. I believe the answer should be #4 with a 70a OCPD. Also, shouldn't the demand factor from col. C be 11k, not 8k as above because there are 2 units? 11k x 180% = 19.8kw/240v = 82.5 amps?

Does everyone agree with the 60 deg. limitation? I would think there would be an easier method by now with the NEC on conductor sizing. Even the question writers get confused.
 
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Dennis Alwon

Dennis Alwon

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haskindm said:
If the question does refer to NM cable, then the requirement in 334.80 would apply, limiting the ampacity to that of 60 degree conductors and would override the asterisk in table 310.16.
Click to expand...

Just to be clear I want to clarify this statement. You can still use 12/2 NM cable for load of 25 amps at 60C for certain load as a/c, etc. The asterik is still effective in the sense that it will continue to override 240.4(D) demand of 20 amp for #12/2 NM. You just have to use the 60C column.
 
H

haskindm

Senior Member
Location
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If it was easy, everyone would be doing electrical work. Nobody ever said it would be easy or simple. Question writers, instructors, and even code panel members are human too, we all get confused sometimes. That is why they put erasers on pencils... and spell check on computers.
 
celtic

celtic

Senior Member
Location
NJ
I thought about this at work today...

The only things I can think....
- the authors wanted you to STOP at 6000/240=25A
- there is a typo...75/60 col.
 
G

Geraldselectric

Member
Yes, 240v is given in the question as above. The problem is that the answer is using the 75 deg. column to size the conductor, as if I was using THHN, not NM Cable. Thanks to all of you that replied!
 
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Davis9

Davis9

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MA,NH
Look at the first line in 240.4(D) Small Conductors.

It refers you to (E) and (G) of the same section. This over rides (D) and allows the ampacity adjusted.

tom:smile:
 
480sparky

480sparky

Senior Member
Location
Iowegia
Geraldselectric said:
The problem is that the answer is using the 75 deg. column to size the conductor, as if I was using THHN, not NM Cable.
Click to expand...

Interesting. Dare we ask where you are getting these questions?
 
G

Geraldselectric

Member
Davis9 said:
Look at the first line in 240.4(D) Small Conductors.

It refers you to (E) and (G) of the same section. This over rides (D) and allows the ampacity adjusted.

tom:smile:
Click to expand...

How does it over ride 334.80? I am not sizing a tap (E) and question is not looking for OCPD (G). Can you explain how your answer allows for the ampacity adjustment to the 75 deg. column when 334.80 states you must use the 60 deg. column? Thanks.
 
celtic

celtic

Senior Member
Location
NJ
Davis9 said:
Look at the first line in 240.4(D) Small Conductors.

It refers you to (E) and (G) of the same section. This over rides (D) and allows the ampacity adjusted.

tom:smile:
Click to expand...
240.4(G) might apply:

240.4(G) Overcurrent Protection for Specific Conductor
Applications. Overcurrent protection for the specific conductors
shall be permitted to be provided as referenced in
Table 240.4(G).

Table 240.4(G) Specific Conductor Applications
Air-conditioning and refrigeration equipment circuit conductors ~ 440, Parts III, VI
Click to expand...
...but that STILL doesn't explain the 75? notation in regards to NM (334.80).
 
M

Michael15956

Senior Member
Location
NE Ohio
Geraldselectric said:
How does it over ride 334.80? I am not sizing a tap (E) and question is not looking for OCPD (G). Can you explain how your answer allows for the ampacity adjustment to the 75 deg. column when 334.80 states you must use the 60 deg. column? Thanks.
Click to expand...
Maybe the writter of your question was referring to NM-B, it is rated for 90C.
 
infinity

infinity

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New Jersey
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Journeyman Electrician
Michael15956 said:
Maybe the writter of your question was referring to NM-B, it is rated for 90C.
Click to expand...


But it still must be used at the 60 degree ampacity due to 334.80, which has no exceptions.
 
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