I am looking into building a patio with a fireplace. I'd like the fireplace to run off of my home's natural gas. When I built the house they ran a natural gas line from the furnace to a place that we could put it outside for a patio (about a 30 foot run). Now I'll need have someone run an additional 25 feet to the fireplace location on the patio. For a total of 55ish feet with a 1/2" pipe, my very basic understanding is that the pressure is going to be very very low and that I'll never be able to get a fireplace to output anywhere close to 150,000ish BTUs that I desire. Is it even worth me testing out the pressure of the current 30 foot run, or should I just move on and build something using propane tanks?
You want a 150,000 BTU firepit? That's like a gas bonfire! Most gas fire pits I've seen are in the 30-50k BTU range, a few are 100k.
1/2" pipe run 55ft is good for about 70 cu. ft. / hour, assuming 2 PSI or less supply. At 1100 BTU per cu. ft., that will get you 77,000 BTUs, somewhere in the upper end of what I've seen for fire pits.
For 150,000 BTUs, you are going to need about 136 cu. ft. / hr so for 55ft you would need a 3/4" pipe, all the way back to your trunk line (I'm not counting your total demand on that trunk line)..
With 1/2 lb gas it will not be possible to support the fire pit you have in mind with 1/2" pipe. You can either repipe clear back to the meter with the right size of pipe or use bottle gas.