I was looking to clarify some conflicting information I’ve read or heard about conductor ampacities.
I use the NEC ampacities chart as a guideline for matching breakers with wire gauges and determining the maximum amperes for which a wire is rated. They have it all laid out for many different gauges of wire, but some are not in the chart.
The InterNACHI course on this subject refers to the NEC chart, but they also show another chart (2nd page of attached file) which seems to conflict with this chart, which is one source of my confusion. If I understand correctly, is the first chart only for SE wires and the second chart for branch wiring and feeders to sub panels?
Ultimately, I am trying to fill in the maximum breaker size for the wires below: #8 CU: #6 CU: #8 AL: #6 AL: #4 AL: #3 AL:
#6 CU I see a lot and usually paired with a 60 amp breaker. I’ve seen it a couple of times with 70amp breaker and have been told this is fine, but according to InterNACHI chart it is not so i called it out in an abundance of caution. The other sizes are less common and the few times I’ve seen them it was clear the breaker was either OK or undersized, but I’d like to know for certain once and for all.
The problem with these charts is that they do not address the fact that different wiring methods can have different ampacities for the same size conductors. NM cable is always sized for 60° C from NEC Table 310.15(B)(16) but SE cable, MC cable or conduit and wire can be sized at 75° C from the same table so the same conductor sizes and materials will have different ampacities.
If you’re only dealing with NM then it’s simple use the 60° C column based on the conductor material (copper or aluminum).
For service entrance conductors and feeders that can use the 83% rule this complicates it even more.
Thanks Robert. At least that clarified WHY i am a little unclear. The difference between the NM, MC, & SE wire was helpful, and I can keep track of the different degree columns for each when evaluating wire.
Did the 2014 NEC code make all SE cable ampacity minimums a calculation, and set the minimum at 83% of the service or feeder rating (whichever is more). Do you think this standard should be my guiding rule when evaluating feeders to service panels or feeders between remote main disconnects and sub panels inside condos/townhomes?
Yes, the 2014 began using the 83% calculation for these conductors. Prior to that there was a table that was used {310.15(B)(7)-2011 NEC}. With either method the conductor sizes are the same. The 83% rule is more flexible because it allows for things like parallel conductors and ampacity adjustment (derating) factors.
The key factor in the 83% rule and the earlier table is that the feeder or service entrance conductors have to be carrying the entire load. So if you had a 200 amp disconnect adjacent to the meter the feeder conductors to the condo or town home panel can still use conductors with only 83% of the service ampacity.