To get the diameter, wrap a cooord around the pipe and measure the length of the wrap. Take that value and divide by 3. That should get you close enough.
Without doing the math, I would say that increasing your BTU from about 220,000 by 48000, or 20% should have little affect on you system.
But get the diameter if you can.

What is the diameter of the main piping and how long is the run from the meter until it gets to the water heater?

Many NG companies have the final regulator right at the user meter and run higher pressure in their distribution system.

Without knowing how you local company is set up there is no way to tell if your company line is large enough.

All you can do is make sure your lines after the meter are large enough.

Gas lines are not often run as branching, but as 'home run' lines back to the meter.

The first step in designing the gas supply system is to make a list of all gas fixtures & there load to determine the total load.

In this case they have:

Water Heater........ 60k/btu
Furnace............ 120k/btu
Kitchen range...... 48K/bt
BBQ(frm code table) 40K/btu
TOTAL existing load 268K/btu

Per code we are to contact the gas provider to find out the actual average btu per CF of the gas they provide. When that figure is not known code says we must use 11K/btu per Cubic Foot

268K/btu divided by 11K/btu equals 24.36cu/ft per hour

Your existing range consumes 48K/btu but you will be changing that to a range that uses 96K/btu for a total demand load of 316K/but divided by 11K/btu per Cu/ft = 28.7cu/ft per hour

We then need to know the YDL(total developed length) of the pipe from the meter to the furthest fixture.

Note-TDL equals the actual physical length of the pipe plus the fitting insertion loss lengths for all fittings on the run.

Per IRC table T2413.4(1)

A 1/2" line could handled 40cu/ft out to 150ft TDL
A 3/4" line could handle 84cu/ft out to 150'TDL

The bottom line, you have no problem.

And for the record, your gas company is in the business of selling gas. The only limit on how much you can have is how much you can pay for.

I've been off GW for a bit but not forgotten my post.

I did measure the main gas intake pipe as presents in the basement room (not from outdoor main). With calipers, it appears to be 1 1/2" OD. There is a fitting and 90 degree turn with down size to 1 inch OD soon after entry, which serves the gas water heater (3 feet away), furnace (7 feet away). From the 1" OD long pipe servicing the furnace, there is a fitting, and a 90 degree turn to service the pipe leading to the out door grill: first 1" OD then it transitions 3/4" OD. From the same long line to the furnace, a fitting and 90 degree turn brings the 1" OD pipe over to the back wall of the furnace room with kitchen above. There is a fitting a 90 degree vertical turn with downsizing to 5/8" rigid pipe for the current cooktop.

I haven't gone back down to measure/plot the TDL (I have our city form) as yet as I wish to do it one time, as accurately as I can.

Thank you all for your help. I'll be back if serious questions arise.

-SW

This post was edited by SparklingWater on Thu, Jan 31, 13 at 16:09

From your 306,000 BTUH, and using the standard heating value of NG as 1 cubic foot per 1000 BTU, you need about 306 cubic feet/hr in the main
Your main at 1.5 inch would give a pressure drop of only
0.004 psi per 100 feet of length equivalent and since you have very little main it would be a fraction of that.
Your local pressure drops for 3/4 inch piping would be at most for the furnace would be 0.2 psi per 100 feet and you have significantly less than 100 equivalent feet in that line
So, your total drop from the meter is much less than 0.2 psi (at the furnace)
I would agree that your house piping is adequate.

Thanks joeplumb. I truly appreciate your interest in my question from the beginning. Reading your post tonight is very reassuring and educating. I will work to complete the TDL to have it ready for the selected GC, as our city requires it as part of its permit process. I'm excited at the prospect of using my new range down the road. I'm very appreciative of your time and experience.