We have employed low-temperature photoluminescence to estimate the total residual N concentration in semi-insulating (SI) SiC substrates where all N shallow donors are compensated in the dark. The ratio of the nitrogen-bound exciton line (Qo) to the free excitonic emission (I77) as a function of excitation power density (Pexc) was tracked for several SI 4H-SiC samples with varying residual N concentration (~ 7x1014 – 5.2x1016 cm-3). Most notably, a linear relationship was found between Qo/I77 and [N] for [N] < 1x1016 cm-3 while a sub-linear behavior was observed for samples with higher N levels. This technique should be particularly valuable to map [N] where the levels are close to or below the present SIMS detection limit of ~ 5-7 x 1014 cm-3. Results obtained for a limited number of low n-type and SI 6H-SiC substrates are also presented.