diff --git a/powersimdata/input/helpers.py b/powersimdata/input/helpers.py
index f23b21bca..e74ee4193 100644
--- a/powersimdata/input/helpers.py
+++ b/powersimdata/input/helpers.py
@@ -14,14 +14,11 @@ def csv_to_data_frame(data_loc, filename):
 
 
 def add_column_to_data_frame(data_frame, column_dict):
-    """Adds column(s) to data frame
+    """Adds column(s) to data frame. Done inplace.
 
     :param pandas.DataFrame data_frame: input data frame
     :param dict column_dict: column to be added. Keys are column name and
         values a list of of values.
-    :return: (*pandas.DataFrame*) -- data frame with extra column(s)
     """
     for key, value in column_dict.items():
         data_frame[key] = value
-
-    return data_frame
diff --git a/powersimdata/input/usa_tamu_model.py b/powersimdata/input/usa_tamu_model.py
index 229fcedc8..82502ec31 100644
--- a/powersimdata/input/usa_tamu_model.py
+++ b/powersimdata/input/usa_tamu_model.py
@@ -183,14 +183,14 @@ def get_zone_name(idx):
     extra_col_bus = {
         'lat': get_lat(model.bus.index),
         'lon': get_lon(model.bus.index)}
-    model.bus = add_column_to_data_frame(model.bus, extra_col_bus)
+    add_column_to_data_frame(model.bus, extra_col_bus)
 
     extra_col_plant = {
         'lat': get_lat(model.plant.bus_id),
         'lon': get_lon(model.plant.bus_id),
         'zone_id': get_zone_id(model.plant.bus_id),
         'zone_name': get_zone_name(model.plant.bus_id)}
-    model.plant = add_column_to_data_frame(model.plant, extra_col_plant)
+    add_column_to_data_frame(model.plant, extra_col_plant)
 
     extra_col_branch = {
         'from_zone_id': get_zone_id(model.branch.from_bus_id),
@@ -201,5 +201,5 @@ def get_zone_name(idx):
         'from_lon': get_lon(model.branch.from_bus_id),
         'to_lat': get_lat(model.branch.to_bus_id),
         'to_lon': get_lon(model.branch.to_bus_id)}
-    model.branch = add_column_to_data_frame(model.branch, extra_col_branch)
+    add_column_to_data_frame(model.branch, extra_col_branch)
     return model