Inventories

Inventories are valued at the lower of cost or market. Cost is determined by the first-in, first-out (“FIFO”) method. We establish an allowance for excess and obsolete inventories based on product line changes, the feasibility of substituting parts and the need for supply and replacement parts.



	September 30, 2017			December 31, 2016
	(in thousands)
Raw materials	$	57,652		$	43,438
Work in process	3,588			2,279
Finished goods	6,070			3,017
	67,310			48,734
Less: Allowance for excess and obsolete inventories	(1,193		)	(1,382		)
Total, net	$	66,117		$	47,352

The related changes in the allowance for excess and obsolete inventories account are as follows:



	Three months ended					Nine months ended
	September 30, 2017			September 30, 2016		September 30, 2017			September 30, 2016
Allowance for excess and obsolete inventories:	(in thousands)
Balance, beginning of period	$	1,642		$	1,065	$	1,382		$	757
Provisions for excess and obsolete inventories	(206		)	112		54			420
Inventories written off	(243		)	—		(243		)	—
Balance, end of period	$	1,193		$	1,177	$	1,193		$	1,177