What was the original pipe? Black iron pipe? Was it 1/2" or 3/4" iron pipe?

What material did he use to extend the current pipe 15 feet?

I've seen many old 80k and larger furnaces running on 3/8 copper tube but I would never go with that small. I guess you could light the oven and watch flame as all top burners are turned on. If oven flame doesn't get smaller,it will work.

There is a maximum load that can be placed on a natural gas system, which obviously depends on if your service is low pressure or high pressure, and also the distance from the meter to the appliance, what is the total BTU load on your system, etc. Also, what other appliances extend from the same "trunk line" (like your fireplace) should be taken into account.

A qualified plumber or mechanical (HVAC) contractor should be able to do a load calculation. That said, we have never installed 3/8" gas line to a range. It *sounds* too small to meet the demand. We install 3/4" frequently for the range in kitchen remodels.

Yes, all burners and oven on will equal a max BTU. It should be listed on the rating plate affixed to the appliance somewhere. Many times, in ranges, it is inside the oven on a wall, or in the bottom drawer area, etc.

The truth is that I don't know. I would never do that - I'd never use copper for gas line and I've never used less than 1/2". Even if 3/8" copper was acceptable by code -- it would be cutting things too close, why risk the callback.

Figuring out the correct gas pipe size requires that you map out the length of each segment of gas line, any elbows, etc and calculate the btu's. There's an amount of work that goes into it.

80k btu/hour over 20ft with several 90 elbows - I'd be doing it in 3/4 and probably not do all the math to be honest. 40k btu/hour or less I'd do in 1/2" for that length.

If I was looking at a long run leading up to that, esp with large gas appliances I might go to the trouble of calculating the entire branch if I was adding on to it.