Covers NG-fired spark ignition internal combustion engines; also NG-fired simple cycle turbines. Does not cover compression ignition engines, despite a link for 40 CFR Subpart IIII being listed on the General Permits page under the GP-5 heading.

"Exemption Category 38"; powerpoint presentation (see p. 4), pertaining to well pad installations, provides a list of requirements, which if fulfilled, will allow Part 89 non-road engines and stationary IC engines to be exempted from permitting requirements. Questions 1 and 2 in the FAQ addresses the circumstances under which an engine on a well pad would qualify as a compressor station to be permitted under GP-5. Note the upper threshold on NOx required to qualify for EC38 is based on the ozone season.

The Technical Support Document (TSD) for the January 31, 2013 version of GP-5 provides the BAT rationale for each of the emission limits in that document. In many cases, they are considerably more stringent than were provided in the immediately-preceding version. Here's my tabulation of what I found in the TSD vs. the spreadsheet of JJJJ requirements on the Pennsylvania website:

	.LE.100 bhp (either LB or RB)		.GT.100 bhp and .LE.500 bhp				.GT.500 bhp
			LB		RB		LB		RB
	GP-5	JJJJ	GP-5	JJJJ	GP-5	JJJJ	GP-5	JJJJ	GP-5	JJJJ
NOx	2.0 g/bhp-hr (3-way catalyst); given the lack of a limit on HC, I'd say it's ambiguous whether this is more stringent or even acceptable relative to JJJJ	NOx + HC = 6 g/bhp-hr for .LE. 25 bhp (40CFR part 1054); NOx + HC = 2.83 g/bhp-hr for .GT. 25 bhp and .LE. 100 bhp (40CFR 1048 101(c) (sic)); note I'm ignoring emergency engines throughout	1.0 g/bhp-hr	1.0 g/bhp-hr	0.25 g/bhp-hr (NSCR)	1.0 g/bhp-hr	0.5 g/bhp-hr	1.0 g/bhp-hr	0.2 g/bhp-hr (NSCR)	1 g/bhp-hr
CO	2 g/bhp-hr (3-way catalyst)		2.0 g/bhp-hr	2.0 g/bhp-hr	0.30 g/bhp-hr (NSCR)	2.0 g/bhp-hr	93% CO reduction or 47 ppmvd @ 15% O2	2.0 g/bhp-hr (JJJJ); 93% CO reduction or 47 ppmvd @ 15% O2 (ZZZZ)	0.30 g/bhp-hr (NSCR)	2.0 g/bhp-hr
HC	no limit	see limit for NOx	0.70 g/bhp-hr (NMNEHC)
HCHO	no limit; believed not to exceed 1.45 TPY for a 500 hp engine	no limit; CO viewed as a surrogate parameter	no limit	no limit	no limit	no limit	0.05 g/bhp-hr	no limit	76% reduction or 2.7 ppmvd @ 15% O2	no limit (JJJJ); 76% reduction or 2.7 ppmvd @ 15% O2 (ZZZZ)
VE	shall not exceed either of the following limitations: .GE. 10% for a period or periods aggregating more than 3 minutes in any one hour or .GE. 30% at any time		same for all categories		same for all categories		same for all categories		same for all categories