# 1039.740 What restrictions apply for using emission credits?
The following restrictions apply for using emission credits:
(a) *Averaging sets.* Emission credits may be exchanged only within an averaging set. For emission credits generated by Tier 4 engines, there are two averaging sets—one for engines at or below 560 kW and another for engines above 560 kW.
(b) *Emission credits from earlier tiers of standards.* (1) For purposes of ABT under this subpart, you may not use emission credits generated from engines subject to emission standards identified in appendix I of this part, except as specified in § 1039.102(d)(1) or as follows:
| If the maximum power of the credit-generating engine is * * * | And it was certified to the following standards identified in appendix I of this part * * * | Then you may use those banked credits for the following Tier 4 engines * * * |
| --- | --- | --- |
| (i) kW < 19 | Tier 2 | kW < 19. |
| (ii) 19 ≤ kW < 37 | Tier 2 | kW ≥ 19. |
| (iii) 37 ≤ kW ≤ 560 | Tier 3 | kW ≥ 19. |
| (iv) kW > 560 | Tier 2 | kW ≥ 19. |
(2) Emission credits generated from marine engines certified to the standards identified in appendix I of this part for land-based engines may not be used under this part.
(c) *NO*<sub>X</sub> *and NO*<sub>X</sub>* + NMHC emission credits.* You may use NO<sub>X</sub> emission credits without adjustment to show compliance with NO<sub>X</sub> + NMHC standards. You may use NO<sub>X</sub> + NMHC emission credits to show compliance with NO<sub>X</sub> standards, but you must adjust the NO<sub>X</sub> + NMHC emission credits downward by twenty percent when you use them, as shown in the following equation:
NO<sub>X</sub> emission credits = (0.8) × (NO<sub>X</sub> + NMHC emission credits).
(d) *Other restrictions.* Other sections of this part specify additional restrictions for using emission credits under certain special provisions.
[69 FR 39213, June 29, 2004, as amended at 70 FR 40464, July 13, 2005; 81 FR 74140, Oct. 25, 2016; 86 FR 34506, June 29, 2021]