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]
    )