  We present extensive radio observations of SN 2003L, the most luminous and
energetic Type Ibc radio supernova with the exception of SN 1998bw (associated
with GRB 980425). Observations from the Very Large Array are well described by
a fitting a synchrotron self-absorption model to the emission spectrum. This
model implies a sub-relativistic ejecta velocity, v ~ 0.2c, and a size of r ~
4.3 x 10^{15} cm at t ~ 10 days. The circumstellar density is suitably fit with
a stellar wind profile, n_e ~ r^{-2} and a constant mass loss rate of M_dot ~
7.5 x 10^{-6} M_{solar}/year. Moreover, the magnetic field follows B ~ r^{-1}
and the kinetic energy of the radio bright ejecta is roughly E ~ 10^{48} erg
assuming equipartition of energy between relativistic electrons and magnetic
fields. Furthermore, we show that free-free absorption does not contribute
significantly to the radio spectrum, since it implies ejecta velocities which
are inconsistent with size constraints derived from Very Long Baseline Array
observations. In conclusion, we find that although SN 2003L has a radio
luminosity comparable to that seen in SN 1998bw, it shows no evidence for a
significant amount of energy coupled to relativistic ejecta. Using SN 2003L as
an example, we comment briefly on the coupling of ejecta velocity and energy in
Type Ibc supernovae.
