Conductor rating - Old USE Al.

[Jolt4526]

Greetings, I'm an Electrician in Chicago, typically working on new commercial construction. I'm currently working on a project at my own house that I may be overthinking or misunderstanding entirely. This is of course residential and the house was built in the late 1970s. The service to the dwelling is 2/0 Al. USE which is feeding an old Square D panel with an 100A main OCPD. I'm looking to replace the panel with a new Square D QO, and if I can have a little more headroom without running a new service that would be ideal.

Where I'm getting hung up is I assume USE-2 wasn't around in the 1970s, so my intention is to use the 75° column in 310.16 for 2/0 Al. USE which provides an allowable ampacity of 135A. Since I don't have a definitive load calculation it would seem logical to round down to a 125A main OCPD and ensure the breaker protects the conductor.

This seems like sound logic to me, but I'm sure others on here with more experience can weigh in on this.

 

[augie47]

The NEC will allow a 2/0AL to supply a 150 amp residential service due to load diversity.
This may or may not be applicable in Chicago.

 

[Jolt4526]

I'm in the suburbs outside of Cook county, we use the NEC. At work downtown correct, the CEC has a lot of addendums.

That 150A is applicable to non-2 USE also? Is there a specific table that lays out calculating load diversity?

 

[augie47]

Art 200 would be for your load calculation
Art & Table 310.12 (2020 Code) allow the 150 amp on 2/0 AL

 

[Jolt4526]

I'm seeing my mistake now, I was referencing 310.16 instead of 310.12 for some reason. I think I got fixated on the mention of direct burial in 310.16. With a 150A main I now have to consider that such OCPD will have the possibility of not sufficiently protecting the conductor when accounting for breaker trip curve, right? I suppose these are things I never really consider at work when they are already spec'd out.

 

[LarryFine]

With a 150A main I now have to consider that such OCPD will have the possibility of not sufficiently protecting the conductor when accounting for breaker trip curve, right?

You're okay with the 150a breaker as long as your calculated load is not greater than the conductor ampacity.

 

[Jolt4526]

So is the logic that the current 100A main I have is to keep a typical consumer from easily exceeding the service conductor's rated ampacity or because it was uncommon for a house to have a calculated load to exceed 100A (despite the conductors supporting more) when it was built? I'm assuming the latter but I can see both being objectively correct.

 

[Jolt4526]

To add to my previous post, I suppose that's a theory question really, as that would be applicable to the relationship between the trip curve of any given breaker and the appropriate conductor size.

 

[don_resqcapt19]

The NEC will allow a 2/0AL to supply a 150 amp residential service due to load diversity.
This may or may not be applicable in Chicago.

The 2018 Chicago Electrical Code is a highly amended version of the 2017 NEC. Article 310 in the Chicago code is identical to that in the NEC with the exception of changes in the ampacity adjustment rules. Their adjustment table stops at 9 current carrying conductors because they do not permit more than 9 current carrying conductors in a raceway.