diff --git a/src/calliope/preprocess/data_sources.py b/src/calliope/preprocess/data_sources.py
index f11bb668..8b47ed84 100644
--- a/src/calliope/preprocess/data_sources.py
+++ b/src/calliope/preprocess/data_sources.py
@@ -5,6 +5,7 @@
 import logging
 from collections.abc import Hashable
 from pathlib import Path
+import warnings
 
 import numpy as np
 import pandas as pd
@@ -154,7 +155,7 @@ def node_dict(self, techs_incl_inheritance: AttrDict) -> AttrDict:
                     techs_incl_inheritance[tech].get("base_tech", None)
                     == "transmission"
                 ):
-                    self._raise_error(
+                    self._raise_warning(
                         "Cannot define transmission technology data over the `nodes` dimension"
                     )
                 else:
@@ -361,6 +362,10 @@ def _check_processed_tdf(self, tdf: pd.Series):
         if len(unique_index_names) != len(tdf.index.names):
             self._raise_error(f"Duplicate dimension names found: {tdf.index.names}")
 
+    def _raise_warning(self, message):
+        """Format warning message and then raise Calliope ModelWarning."""
+        warnings.warn(self.MESSAGE_TEMPLATE.format(name=self.name, message=message), exceptions.ModelWarning)
+
     def _raise_error(self, message):
         """Format error message and then raise Calliope ModelError."""
         raise exceptions.ModelError(