Amp Ratings

[mkgrady]

I get confused by the tables for allowable amperage for the smaller wires (14, 12 and #10). I understand the ratings are before any derating factors for ambient temperature and more than 3 CCC in a raceway. And that the breaker size can't be larger than 15, 20 and 30 amps.

My confusion comes in when it is a motor load. which is an exception to the 15, 20, 30 amp limit. Say for example an HVAC unit has an MCA of 22 amp and a MOCP of 25. Are there any circumstances that would allow the wire size of any type to be #12?

 

[kwired]

240.3 is where it tells you there may be other rules in other articles that may override the general rules of art 240. Both 430 for motors and 440 for air conditioning equipment are in that list.

For the air conditioning situation since you mentioned it - 440.4(B) is what is used on most equipment - it tells us what is supposed to be on the nameplate.

Minimum circuit ampacity - already has 125% of largest compressor plus all other loads factored into it so no calculating is necessary, your conductor ampacity must be the MCA or higher, before any adjustments.

So if the MCA is 22 like you mentioned then 310.15(16) says a 60 deg copper conductor must be 10 AWG minimum. Only time you need to use 60 C conductor most cases is if using NM cable, otherwise 75 C conductor is normally acceptable - in which case it can be 12 AWG.

Overcurrent protection is different with motors and air conditioning then most general applications as you usually need to be able to hold a surge of starting current, and the small conductor rule of 15,20,30 amp protection for 14, 12, and 10 AWG doesn't apply to motors or A/C's.
Motor overload protector still prevents overloading and the branch circuit device that can be 1.75 to 2.50 times full load rating or even more in some cases is primarily there for protection during short circuits and ground faults.

That 22 MCA unit may call for 40 or 50 amp breaker pretty easily.

 

[ramsy]

..Only time you need to use 60 C conductor most cases is if using NM cable, otherwise 75 C conductor is normally acceptable - in which case it can be 12 AWG..

Mkgrady is right to suspect #12 on this circuit - since the operating temperature exceeds 60°C.

110.14(C)(1)(A) prohibits 22A on #12 in equipment under 100A, not listed above 60°C.

 

[infinity]

My confusion comes in when it is a motor load. which is an exception to the 15, 20, 30 amp limit. Say for example an HVAC unit has an MCA of 22 amp and a MOCP of 25. Are there any circumstances that would allow the wire size of any type to be #12?

If you used MC cable or conduit and wire you could use the 75° C ampacity to size the conductors as long as you had 75° C terminals on the circuit breaker and disconnect (if used). So #12 MC cable instead of NM could be code compliant on a unit with a 22 amp MCA.

 

[ramsy]

..as long as you had 75° C terminals on the circuit breaker..

There are many existing fuse boxes 100A or less with 75°C breakers, and others with missing or illegible labels.

I believe, 110.14(C)(1)(A)(2)&(3) prohibits forcing the bus to 75°C, without confirming its listed or labeled for the purpose.

The UL White Book, ELECTRICAL EQUIPMENT FOR USE IN ORDINARY LOCATIONS (AALZ), under "Distribution and Control Equipment Terminations" states:
“A 75°C conductor temperature marking on a circuit breaker or switch normally intended for wire sizes 14-1 AWG does not in itself indicate that 75°C insulated wire can be used unless 1) the circuit breaker or switch is used by itself, such as in a separate enclosure, or 2) the equipment in which the circuit breaker or switch is installed is also so marked.”

 

[mkgrady]

It seems there may be agreement that a 12-2 MC cable can be used for a Hvac mca of 22 amps with a breaker size to be determined by the morning. So that I understand the rules better, let's say I have an oven that has a full load of 22 amps. Do we get the same answer? Is 12-2 MC cable able to serve this non motor/hvac load?

 

[ActionDave]

It seems there may be agreement that a 12-2 MC cable can be used for a Hvac mca of 22 amps with a breaker size to be determined by the morning. So that I understand the rules better, let's say I have an oven that has a full load of 22 amps. Do we get the same answer? Is 12-2 MC cable able to serve this non motor/hvac load?

Cooking equipment is not in Table 240.4 (G).

 

[ramsy]

Yes, if appliance terminations, breakers and fuse-box/panels are all listed or labeled for 75°C, most likely with 100A or more service equipment.

No, if not listed or labeled for 75°C. Circuit ampacity must follow the weakest link in the chain, or Table 310.15(B)(16), 60°C column. #12 = 20A max.

 

[kwired]

It seems there may be agreement that a 12-2 MC cable can be used for a Hvac mca of 22 amps with a breaker size to be determined by the morning. So that I understand the rules better, let's say I have an oven that has a full load of 22 amps. Do we get the same answer? Is 12-2 MC cable able to serve this non motor/hvac load?

In addition to replies to this:

240.3 does refer you to art 422 for appliances.

If that 22 amps is a continuous load, your base minimum ampacity turns into 27.5 amps.

You also can have situations where you can supply such cooking appliances from branch circuit tap conductors with branch circuit overcurrent protection up to 50 amps.