Instead of 'name' field, i had to use 'attribute' field.
<?php $this->widget('yiiwheels.widgets.formhelpers.WhSelectBox',array('attribute' => 'branch_name', 'size' => 'input-large', 'model' => $model, 'data' => CHtml::listData(Branch::model()->findAll(array('order' => 'branch_name')),'id','branch_name')));?>