def symmetric_transmission_constraint_rule(backend_model, loc_tech):
    """
    Constrain e_cap symmetrically for transmission nodes. Transmission techs only.

    .. container:: scrolling-wrapper

        .. math::

            energy_{cap}(loc1::tech:loc2) = energy_{cap}(loc2::tech:loc1)

    """
    lookup = backend_model.__calliope_model_data['data']['lookup_remotes']
    loc_tech_remote = lookup[loc_tech]
    return backend_model.energy_cap[loc_tech] == backend_model.energy_cap[
        loc_tech_remote]
