How to draw oval line through specified coordinates?

Question

How to draw oval line through specified coordinates? I have used simple \draw [red, dashed, line width=1px] (A) to[out=180, in=180] (C); for each connection manually. And connected line dashes are not properly connected (as oval made from separate parts, at A and B points). Perhaps there is some other more effective way?

\documentclass[preview,border=12pt,12pt, varwidth=\maxdimen]{standalone}
\usepackage[americaninductors, europeanresistor]{circuitikz}
\usepackage{siunitx}
\usepackage{nccmath}
\begin{document}
    \sisetup{output-decimal-marker = {,}}
    
    \begin{circuitikz}
        
        \draw (0,0) coordinate(point0) to[R, l_=$\underline{Z}_1$, *-]  (0,4) coordinate(point1);
        
        \draw (0,4) to[L=$\underline{Z}_2$, label distance=2px, *-*] (4,4) coordinate(point2);
        
        \draw (4,4) to[L=$\underline{Z}_3$] (4,0);
        
        \draw (0,0) to[short, -*] (4,0);
        
        \draw (4,4) to[C=$\underline{Z}_5$, label distance=2px, -*] (8,4) coordinate(point3);
        
        \draw (8,4) to[R=$\underline{Z}_{67}$] (8,0);
        
        \draw (4,0) to[short] (8,0);
        
        
        \draw (0,4) -- (0,6) to[C=$\underline{Z}_4$, label distance=2px] (8,6) -- (8,4);
        
        
        \draw (point1) -- ++ (-2,0);
        \draw (point0) -- ++ (-2,0);
        
        
        \node[draw,rectangle,text centered,font=\bfseries] at ([xshift=-0.5cm,yshift=0.5cm] point1){1};
        \node[draw,rectangle,text centered,font=\bfseries] at ([yshift=0.5cm] point2){2};
        \node[draw,rectangle,text centered,font=\bfseries] at ([xshift=0.5cm,yshift=0.5cm] point3){3};
        \node[draw,rectangle,text centered,font=\bfseries] at (0,-0.5){4};


        \coordinate (A) at (1.5,5.2);
        \coordinate (B) at (6.5,5.2);
        \coordinate (C) at (4,0.8);

        \draw [red, dashed, line width=1px] (A) -- (B);
        \draw [red, dashed, line width=1px] (A) to[out=180, in=180] (C);
        \draw [red, dashed, line width=1px] (B) to[out=0, in=0] (C);


        
    \end{circuitikz}
\end{document}

Answer 1

If you're just concerned about the messy joins, the trick is to draw the shape as a single path, so that the pattern of dashes isn't stopping and starting over, but continues throughout.

You can't completely avoid inconsistency without ensuring the length of the dash pattern divides evenly into the length of the path, but a single path improves the result dramatically, even so.

\documentclass[preview,border=12pt,12pt, varwidth=\maxdimen]{standalone}
\usepackage[americaninductors, europeanresistor]{circuitikz}
\usepackage{siunitx}
\usepackage{nccmath}
\begin{document}
\sisetup{output-decimal-marker = {,}}
\begin{circuitikz}
  \draw (0,0) coordinate(point0) to[R, l_=$\underline{Z}_1$, *-]  (0,4) coordinate(point1);
  
  \draw (0,4) to[L=$\underline{Z}_2$, label distance=2px, *-*] (4,4) coordinate(point2);
  
  \draw (4,4) to[L=$\underline{Z}_3$] (4,0);
  
  \draw (0,0) to[short, -*] (4,0);
  
  \draw (4,4) to[C=$\underline{Z}_5$, label distance=2px, -*] (8,4) coordinate(point3);
  
  \draw (8,4) to[R=$\underline{Z}_{67}$] (8,0);
  
  \draw (4,0) to[short] (8,0);
  
  \draw (0,4) -- (0,6) to[C=$\underline{Z}_4$, label distance=2px] (8,6) -- (8,4);
  
  \draw (point1) -- ++ (-2,0);
  \draw (point0) -- ++ (-2,0);
  
  \node[draw,rectangle,text centered,font=\bfseries] at ([xshift=-0.5cm,yshift=0.5cm] point1){1};
  \node[draw,rectangle,text centered,font=\bfseries] at ([yshift=0.5cm] point2){2};
  \node[draw,rectangle,text centered,font=\bfseries] at ([xshift=0.5cm,yshift=0.5cm] point3){3};
  \node[draw,rectangle,text centered,font=\bfseries] at (0,-0.5){4};
  
  \coordinate (A) at (1.5,5.2);
  \coordinate (B) at (6.5,5.2);
  \coordinate (C) at (4,0.8);
  
  \draw [red, dashed, line width=1px] (A) -- (B) to[out=0, in=0] (C) to[out=180,in=180] cycle;
\end{circuitikz}
\end{document}

Answer 2

Based on my answer on your previous question and with using of the fit library:

\documentclass[12pt,, margin=12pt]{standalone}
\usepackage[americaninductors, europeanresistor]{circuitikz}
    \usetikzlibrary{fit,
                    shapes.geometric}
\usepackage{siunitx}
\usepackage{nccmath}


\begin{document}
    \begin{circuitikz}[every label/.style = {label distance=2mm,
                                             draw, minimum size=1em, inner sep=1pt,
                                             font=\bfseries}
                        ]
    \ctikzset{capacitors/width=0.1, capacitors/height=0.5,
              label distance=3pt}
% 1. loop
\draw   (0,0)   coordinate[label=below:4]   (p0)
                to[R=$\underline{Z}_1$, *-] ++ (0, 4) coordinate[label=above left:1]  (p1)  
                to[L=$\underline{Z}_2$,*-*,
                   name=A]                  ++ (3, 0) coordinate[label=above:2] (p2)
                to[L=$\underline{Z}_3$, -*, 
                   name=B]                  ++ (0,-4)
% 2. loop
        (p2)    to[C=$\underline{Z}_5$, -*,
                   name=C]                  ++ (3, 0) coordinate[label=above right:3] (p3)
                to[R=$\underline{Z}_{67}$]  ++ (0,-4) 
                -- (p0)
% 3. loop
        (p1)    -- ++ (0,2) 
                to[C=$\underline{Z}_4$]     ++ (6, 0)
                to[short,-*]                ++ (0,-2)
% input connectors
        (p1)    to[short,*-o]   ++ (-2,0)
        (p0)    to[short,*-o]   ++ (-2,0);
% group of elements forming T circuit
\node [trapezium, minimum height=40mm, trapezium stretches, yscale=-1,
       draw=red, thick, densely dashed, rounded corners=5mm,
       inner xsep=-2ex, yshift=-2ex, 
       fit=(A.center) (B) (C.center)] {};
    \end{circuitikz}
\end{document}