Electrical – Understanding Aluminum Wire Restrictions: What You Need to Know

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I am (hopefully) getting some major electrical work done soon by a professional electrician. While it probably won't matter much (meter is only a few feet from existing panels, and new panels will be in the same location) as copper vs. aluminum cost will be minimal, I looked at local amendments (Montgomery County, MD, currently NEC 2017 but likely 2020 soon) and they currently include:

Section 310.15(B) Tables. Delete the first sentence and substitute with the following: The minimum size of conductors must be as shown in Table 310.15(B)(16) through Table 310.15(B)(19), as modified by Table 310.15(B)(1) through (B)(7), except that the minimum size of alumimum and copper-clad aluminum conductors must be No. 2. The use of aluminum conductors is limited to service entrance and feeder applications only.

(emphasis added)

Clearly, wire from meter to outside disconnect (electrician told me he has to check with the County to find out whether they are requiring that yet or not, as the County is in the process of updating code) could be aluminum. And I am pretty sure the next leg to the main panel could be aluminum. I am also pretty sure that if, for example, I had EVSE (someday…) or on-demand electric water heating (hopefully never) that I could not use aluminum wire even if all relevant components are rated for aluminum – i.e., meaningless fear.

But I am curious about feeder. Does that mean, as I think it does, that wires between the main panel and any subpanels could be aluminum, with a minimum size of 2 AWG. In other words, if I had a 60A feed to a subpanel, that could be 6 AWG copper THWN or 2 AWG aluminum but not the aluminum 3 AWG UF or aluminum 4 AWG THWN that would be allowed in other jurisdictions?

Best Answer

Yes, that's what it means. However you get a reprieve on large appliances.

You have 3 types of wiring generally.

  • Service wires are from the power company to the main disconnect/breaker.
  • Feeder wires are from the main disconnect to the last breaker panel.
  • Branch circuit wiring is from a panel to a point-of-use ("utilization equipment").

My general thinking is that #2 aluminum is about the same price as #10 copper.

They are saying feeder can't be aluminum unless it's #2 or larger, which is fine by me given the above price point.

Also that aluminum is banned for branch circuits of any size; for instance wiring to an 80A EVSE must be #4Cu not #2Al. But here's the trick. EVSEs and most other very large appliances need disconnect switches. A 4-space subpanel qualifies as both a disconnect and a subpanel, turning the long run into feeder. Thus the #4Cu to an 80A EVSE only needs to be 2 feet long. The rest can be AL.

Likewise, a 60A hot tub can be run with #2AL (overkill) to a "hot tub subpanel" there and then copper to the tub proper.

There is nothing wrong with aluminum heavy feeder; it has always been reliable - but then, heavy feeder is typically torqued properly and used on terminals actually rated for aluminum. There was a problem in the 1970s when aluminum wire was used for small branch circuit wiring, however those terminals were not properly rated for aluminum, and nobody was using torque screwdrivers for the small stuff. (we only learned of the importance of that about 10 years ago).

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