Oversized main disco

Home built in 2001, panel and meter appear to be original. So I just wanted to make sure I am correct on this. Hard to believe it was originally wired this way, but who knows, right?

Service Panel has a 200 amp breaker, but the feeders are only TWHN-2, which according to my handy chart is only good for 100 amps. Just wanted to make sure I wasnt missing something here.
Looks like the main breaker could be newer, but still, this is a 2000 sf home, shouldnt have been wired with only 100 amps 20 years ago.


THWN-2 is an insulation type not the conductor size. It looks #2/0 copper (200 amp) from the photo.


Yeap, what Robert said, it will usually say AWG after the size.

1 Like

Gotcha, thanks.
I thought it looked like 2/0, but then when I saw that, it looked smaller, LOL.

Thanks for saving me from being thrown under the bus. haha

here is the chart I was looking at, now it makes sense. :grinning: :man_facepalming:


Don’t let your mind play trix with you :wink: There is a name for this… when you read or hear something and your mind makes a different association with the object.

1 Like

That table is for standard conductor ampacities. For a dwelling service the service conductors are required to only be 83% of the OCPD size so 200 amps * 83%=166 amps minimum conductor size for 200 amps. From that table #2/0 Cu at 75° C= 175 amps so you’re good.


Isn’t that only if they are carrying 100% of the load?

Do you mean the 83% rule? Isn’t the panel in the photo carrying 100% of the load?

I assume that it is carrying 100%. I just wanted to point out the 100% because many people undersize feeders (like to sub panels) using the 83% rule for ‘other’ conductors.

Why would you assume? The photo’s clear show that the meter is feeding only the service. If you wanted to reference sub-panels you could have just said that. :slightly_smiling_face:


I could have if my input wasn’t always attacked. I also see it on services with 2 -200 AMP panels. 4/0 AL to each panel. Violation.


I asked because the way you worded the question I thought that maybe I posted the wrong information.

Regarding your example, #4/0 Al has an ampacity of 180 amps, next size up rule permits a 200 amp OCPD so there might not be any violation.

1 Like


What I was talking about is this.


the 83% only applies if the entire load is on the conductors (single set). If you have 2 200 AMP panels the 4/0 AL would be limited to 180 AMPS since the entire load is split and not to one panel. 2017 NEC pp-70-149

Getting old. I have passed this very example. My point was supposed to be that the 83% rule only applies to carrying the entire load.

See I let my mistakes stand and make no excuses.

What is the calculated demand load? The service size isn’t based on the year the house was built. It is based on the calculated demand load.

1 Like

Yeah, I was Just implying I have never seen a 100 amp service on any single family home built in the past 60 years or so.
I know it could still be acceptable

really ???

1 Like

Yeah. Typically if it’s only 100 amp, it’s either built pre 60’s, or it’s a small <1k sf apartment/ condo.
That’s been my experience in my area

amazing…I worked a lot of new construction in the 70’s (never as an electrician) but the standard panel was usually 100 amp for Your basic 1500 to 2000 sq ft home…we must have been way behind…


That’s all I got. 100 amp - 40x25 - 2 levels. They are all like that around here.