diff --git a/bacnet-stack/include/config.h b/bacnet-stack/include/config.h
index 7f709f1f..a5a509c0 100644
--- a/bacnet-stack/include/config.h
+++ b/bacnet-stack/include/config.h
@@ -137,7 +137,16 @@
 /*
 ** Set the maximum vector type sizes
 */
-#define MAX_BITSTRING_BYTES        (15)
-#define MAX_CHARACTER_STRING_BYTES (MAX_APDU-6)
-#define MAX_OCTET_STRING_BYTES     (MAX_APDU-6)
+#ifndef MAX_BITSTRING_BYTES
+#define MAX_BITSTRING_BYTES (15)
+#endif
+
+#ifndef MAX_CHARACTER_STRING_BYTES
+#define MAX_CHARACTER_STRING_BYTES (MAX_APDU-6)
+#endif
+
+#ifndef MAX_OCTET_STRING_BYTES
+#define MAX_OCTET_STRING_BYTES (MAX_APDU-6)
+#endif
+
 #endif