Technical Corner - Electrical Engineering Focus

This month’s article focuses on ampacity adjustment of conductors within a dwelling.

Basically, copper and aluminum wires do not like heat. When there is current flow within a wire then they begin to generate heat. Other sources of heat within a wire are from the ambient temperature of the air or the heat generated by other conductors within the same conduit to name a few. So, during design this will need to be recognized and compensated for.

Diversity of the load a cable segment feeds is also another factor that comes into play with dwellings. Diversity is looking at how much electrical load is drawing current at the same time. Turns out with dwelling units the diversity is quite significant. So, NEC 2023 section 310.12 recognizes this and allows a derating factor to be used on the service-entrance conductors, which feed the entire load of the dwelling

The concept here is that the more devices connected to the service the more chance that all of them will not be operating at the same time, so you are able to apply an 83% adjustment factor for the cabling segments shown in this illustration.

Please note the segment feeding another panelboard from Apartment 2 does not apply to this adjustment factor rule and must be sized using the typical 310.16 ampacity tables.

Example: What size aluminum wire will you need to run from Apartment 2 to a 100-amp sub-panel shown in the illustration? (Typical sub-panel installation)

Cable Type: SE Cable with 90-degree XHHW-2 conductors within (see photo)

What is interesting here is that this sub-panel does not feed the entire load of the dwelling so you would refer to table 310.16 and find that #1 AWG would be sufficient using the 75 degree column. (Refer to NEC Article 338 (illustrated version)-wire larger than #10 so use 75 degree column not the 60 degree column)

If this was the segment of wire ahead of the panel, or service, you could use table 310.12(A) which indicates one size smaller which is #2 AWG. This is allowed due to the 83% rule mentioned above. The amperage of the panel is 100A and if we take 83% of this, we would need a wire segment sized for 83 amps minimum.

When you round up you would get a #2AWG aluminum rated for 90amps.

Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me, beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director