PHP - Error en numero de filas de una consulta MySQL

$conn = new mysqli($servername, $username, $password);

$sql = "SELECT codP, frac FROM kadena.Borra;";

$result = $conn->query($sql);

while ($row = $result->fetch_assoc()) {

	$sql2 = "SELECT id FROM catalog WHERE codCatalogo=".$row["codP"].";";

	$res2 = $conn->query($sql2);

	$conta = $res2->num_rows + 0;

	echo $sql2." -- " . $conta . "<br>";

	if ($conta > 0) {

		while ($fila = $res2->fetch_assoc()) {

			$cadSql = "UPDATE catalog SET fracciones=" . $row["frac"] . " WHERE Id = " . $fila["Id"] .";";

			$conn->query($cadSql);

			echo $cadSql . "<br>";

		}

	}

}

echo "fin**";