100% width nested table layout within a parent table with an extra TD

Question

I'm trying to make a nested table layout so when you click on a certain row element the inner table will show up as a dropdown, my main problem here is that the inner table layout need to occupy 100% of the space available and because the parent layout have one more TD than the inner table i can't find the way to get this done. Here you have an image of table design:

Here's the HTML:

              <table class="table table-striped">
            <thead>
              <th><input type="checkbox"></th>
              <th>UBICACION</th>
              <th>EQUIPO</th>
              </th>
            </thead> 
            <tbody>                 
              <tr>
                <td><input type="checkbox"></td>                  
                <td><a href="#">Edificio 4</a></td>
                <td></td>
              </tr>
              <tr>
                <td><input type="checkbox"></td>                  
                <td><a href="#">Azotea</a>
                </td>
                <td></td>
              </tr>
              <tr class="current">
                <td><input type="checkbox"></td>
                <td><a href="#">Mástil 1</a>
                  <table class="table-equipment">
                    <thead></thead>
                    <tbody>
                      <tr>
                        <td></td>
                        <td>
                          <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                          <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35</p>
                        </td>
                        <td><i class="fa fa-exclamation-triangle"></i></td>
                      </tr>
                      <tr>
                        <td></td>
                        <td>
                          <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                          <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35</p>
                        </td>
                        <td><i class="fa fa-exclamation-triangle"></td>
                      </tr>
                      <tr>
                        <td></td>
                        <td>
                          <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                          <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35</p>
                        </td>
                        <td><i class="fa fa-exclamation-triangle"></td>
                      </tr>                                                    
                    </tbody>
                  </table>
                </td>
                <td></td>
              </tr>
              <tr>
                <td><input type="checkbox"></td>
                <td><a href="#">Tripode Autosoportado ORG1</a></td>
                <td></td>
              </tr>
              <tr>
                <td><input type="checkbox"></td>                  
                <td><a href="#">Mástil 3</a></td>
                <td></td>
              </tr>
              <tr>
                <td><input type="checkbox"></td>                  
                <td><a href="#">Mástil 2</a></td>
                <td></td>
              </tr>                                                                                
            </tbody>
          </table>

And here's the SCSS:

    * {
  box-sizing: border-box;
  font-smoothing: antialiased;
  text-rendering: optimizeLegibility;
}
*:focus {
    outline: none !important;
}
@media all and (-ms-high-contrast:none), (-ms-high-contrast:active) {
  .box {
      box-shadow: 0 -1px 2px 0 rgba(0, 0, 0, .12), -1px 0 2px 0 rgba(0, 0, 0, .12), 0 2px 2px 0 rgba(0, 0, 0, .16), 0 0 2px 0 rgba(0, 0, 0, .12)
  }
}
[type=reset],
[type=submit],
button,
html [type=button] {
  -webkit-appearance: button
}
[type=button]::-moz-focus-inner,
[type=reset]::-moz-focus-inner,
[type=submit]::-moz-focus-inner,
button::-moz-focus-inner {
    border-style: none;
    padding: 0
}
body {
  background-color:#f5f5f5;
  font: 300 16px/1.6 'Ubuntu', sans-serif;
  color: #40576a;
}
body,html{
  height:100%;
}
button:focus{
  outline: none;   
}
button,
input {
  overflow: visible
}
button,
select {
  text-transform: none
}
button,
input,
optgroup,
select,
textarea {
  font-family: sans-serif;
  font-size: 100%;
  line-height: 1.15;
  margin: 0
}
fieldset {
  border: 1px solid silver;
  margin: 0 2px;
  padding: .35em .625em .75em;
}
h2 {
  background-color:#2b8e00;
  color:#fff;
  font-size:14px;
  font-weight: bold;
  margin:0 0 1em 0;
  padding:1em;
  text-align: left;
  text-transform: uppercase;
}
h3 {
  border-bottom:1px solid #2b8e00;
  color: #40576a;
  font-size: 14px;
  font-weight: 700;
  margin:2.5em 0 1.5em 0;
  padding: 0 0 .5em 0;
  text-transform:uppercase;
  &:first-child {
    margin-top: 0;
  }
}
html {
  font-size: 62.5%;
}
input[type="text"], textarea, select {
  background-color: transparent;
  background-image: none;
  border-radius: 0;
  border: none;
  border-bottom: 1px solid rgba(0,0,0,.26);
  box-shadow: none;
  color: rgba(0,0,0,.87);
  display: block;
  font-size: 16px;
  outline: 0;
  padding: 0 0 3px 0;
  text-shadow: 1px 1px 1px rgba(0,0,0,.004);
  width: 100%;
}
legend {
  box-sizing: border-box;
  color: inherit;
  display: table;
  max-width: 100%;
  padding: 0;
  white-space: normal;
}
ul {
  list-style:none;
  padding-left:0;
}  
.audit-box {
  height:auto;
  padding:0 0 2em 0;
  h2 {
    background-color:#2b8e00;
    color:#fff;
    font-size:14px;
    font-weight: bold;
    margin:0 0 0 0;
    padding:1em;
    text-align: left;
    text-transform: uppercase;
  }
}
.audit-error-report {
  article {
    border-top: 1px solid #2b8e00;
    padding:15px 0;
    overflow:hidden;
    &:first-child {
      border-top:0;
      padding-top:0;
    }
  }
}
.box {
  background-color: #fff;  
  box-shadow: 0 2px 2px 0 rgba(0,0,0,.16),0 0 2px 0 rgba(0,0,0,.12);
  margin-bottom:1em;
  transition: all 0.3s cubic-bezier(.25,.8,.25,1);
  @media only screen and (min-width:992px) {
    margin-bottom:0em;
  }  
}
.btn {
  -moz-user-select: none;
  -ms-touch-action: manipulation;
  -ms-user-select: none;
  -webkit-user-select: none;
  animation-duration: 0.0001s;
  animation-name: mui-btn-inserted;  
  background-color: #FFF;
  background-image: none;
  border-radius: 2px;
  border: none;
  color: #2b8e00;
  cursor: pointer;
  display: inline-block;
  font-size: 14px;
  font-size: 14px;
  font-weight: 500;
  height: 36px;
  letter-spacing: .03em;
  line-height: 18px;
  line-height: 36px;
  margin-bottom: 6px;
  margin-top: 6px;
  overflow: hidden;
  padding: 0 26px;
  position: relative;
  text-align: center;
  text-transform: uppercase;
  touch-action: manipulation;
  transition: all .2s ease-in-out;
  transition: background-color .3s;
  user-select: none;
  vertical-align: middle;
  white-space: nowrap;
  i {
    margin-left:1em;
  }
  &:focus, &:hover {
    box-shadow: 0 0 2px rgba(0,0,0,.12),0 2px 2px rgba(0,0,0,.2);
    background-color:rgba(0,0,0,.050);
    border:none;
    outline:none;
  }
  &:active, &:focus, &:hover {
    outline: none;
    text-decoration: none;
    color: #2b8e00;
    font-weight: 500;
  }  
}
.btn-blue {
  font-weight: bold;
  background-color: #40576a;
  color:white;
  line-height: normal;
  padding:10px;
  &:hover {
    color:white;
    text-decoration: none;
  }
  &.simple {
    color:#40576a;
    background-color: transparent;
  }
}

.btn-green {
  color: #2b8e00;
  font-weight: bold;
  &:hover {
    text-decoration: none;
  }  
  i {
    display:inline-block;
    margin-right:3px;
    vertical-align: text-bottom;
  }
}
.btn--raised {
  box-shadow: 0 0 2px rgba(0,0,0,.12),0 2px 2px rgba(0,0,0,.2);
}
.btn-right {
  @media only screen and (min-width:768px) {
    float:right;
  }
}
.btn-row {
  margin:2em 0 0 0;
  height:50px;
  line-height: 50px;
  a {
    display:inline-block;
  }
}
.center {
  text-align:center;
}
.chart {
  min-height: 250px;
  width:100%;
}
.chart-box {
  height:auto;
  overflow: hidden;
  padding:0 0 1em 0;
  @extend .box;
  &.no-title {
    padding:1em;
  }
  &.left-align {
    .data, .labels {
      p {
        text-align:left;
      }
    }
  }
  h2 {
    nargin:0 0 2em 0;
  }  
  .data {
    p {
      text-align:left;
    }
  }
  .labels {
    p {
      font-weight:500;
      @media only screen and (min-width:480px) {
        text-align:right;
      }
    }
  }
  .td-select {
    width:150px;    
  }  
}
.c-mask {
  -webkit-transition: opacity .3s,width 0s .3s,height 0s .3s;
  background-color: #000;
  height: 0;
  left: 0;
  opacity: 0;
  overflow:hidden;
  position:fixed;
  top: 0;
  transition: opacity .3s,width 0s .3s,height 0s .3s;  
  width: 0;
  z-index: 100;  
}
.divider {
  background-color: #2b8e00;
  display: block;
  height: 1px;
  margin:2.5em 0;  
}
.dropdown {
  position:relative;
}
.dropdown-menu {
  border:none;
  border-radius:0;
  box-shadow: 0 2px 2px 0 rgba(0,0,0,.16),0 0 2px 0 rgba(0,0,0,.12);
}
.form-audits {
  margin: 2em 2em;
  position: relative;  
  input {
    height: 32px;
  }  
  input:focus, textarea:focus {
    border-color: #40576a;
    border-width: 2px;
  }
  input:focus~label, textarea:focus~label {
    color:#40576a;
    font-weight: 400;
  }
  textarea {
    min-height: 64px;
  }
  .form-group {
    display: block;
    padding-top: 15px;
    margin-bottom: 20px;
    position: relative;
    label {
      position: absolute;
      top: 0;
      display: block;
      width: 100%;
      color: rgba(0,0,0,.54);
      font-size: 12px;
      font-weight: 400;
      line-height: 15px;
      overflow-x: hidden;
      text-overflow: ellipsis;
      white-space: nowrap;
    }
  }
}
.form-group {
  overflow:hidden;
  &.textarea {
    margin-bottom:20px;
    overflow:hidden;
  }
}
.form-wrapper {
  h2 {
    background-color: transparent;
  }
}
.header-buttons {
  border-bottom: 1px solid #2b8e00;
  height: 40px;
  margin-bottom:20px;
  padding:0;
  h3 {
    border-bottom:none;
    color: #40576a;
    float:left;
    font-size: 14px;
    font-weight: 700;
    line-height: 40px;
    margin:0;
    padding: 0;
    text-transform: uppercase;  
  }
  button {
    float:right;
    margin-right:0;
  }  
}
.is-active {
  -webkit-transition: opacity .3s;
  height: 100%;
  opacity: .7;
  transition: opacity .3s;  
  width: 100%;
}
.location-details {
  input[type="checkbox"] {
    margin-left:0;
  }
  h4 {
   color:#2b8e00;
   font-size:14px;
   font-weight: 700; 
  }
  .fa-exclamation {
    color:red;
    margin-left:5px;
  }
}
.main {
  padding:0 0 4em 0;
}  
.menu-box {
  @extend .box;
  padding:1em;
  h2 {
    color:#40576a;
    font-size: 1em;
    font-weight: bold;
    text-align:left;
  }
  i {
    color: #2b8e00;
    display:block;
    font-size: 4em;
    text-align:center;
  }
  p {
    color: #40576a;
    font-size: 14px;
    font-weight: 400;
    text-align:left;    
  }   
}
.pictures {
  img {
    margin-bottom:20px;
    @media only screen and (min-width:768px) {
      margin-bottom:0;
    }
  }
}
.progressbar {
  counter-reset: step;
  overflow: hidden;
  text-align:center;
  margin:0 0 20px 0;
  li {
    color: #2b8e00;;
    float: left;
    font-size: .8em;
    list-style-type: none;
    position: relative;    
    text-transform: uppercase;
    width: 33.33%;
    &.active:before, &.active:after {
      background: #27AE60;
      color: white;
    }
    &:first-child:after {
      content:none;
    }
    &:after {
      background: #40576a;
      content: '';
      height: 2px;
      left: -50%;
      position: absolute;
      top: 9px;
      width: 100%;
      z-index: 0;       
    }
    &:before {
      background: #40576a;
      border-radius: 3px;
      color: #fff;
      content: counter(step);
      counter-increment: step;
      display: block;
      font-size: 1em;
      line-height: 20px;
      margin: 0 auto 0 auto;
      position:relative;
      text-align: center;      
      width: 20px;
      z-index: 1;
    }
  }
  &.single {
    margin-bottom:0;
  }
}
.right {
  float: right;
}
.row {
  margin-bottom:20px;
}  
.search-box {
  input[type="text"] {
    background-color: #fff;
    padding: 0 1.7em 0 .5em;
    position:relative;
    height:2em;
    width:100%;
  }  
  label {
    position:absolute;
    right:2em;
    top:4.5em;
  }
}
.site-container {
  background-color:#f5f5f5; 
}

table {
    border-collapse: collapse!important;
    font-size:14px;
    width: 100%;
    thead {
        border-bottom:2px solid #ddd;
        tr {
            th {
                padding-left:8px;               
            }
        }   
    }
    tbody {
        td {
            border-bottom:1px solid #c7c7c7;
            padding-left:8px;
        }
    }
    .current {
        background-color: #eee !important;
    }
}
.table-equipment {
    background-color: #eee;
    border-top:1px solid #c7c7c7;
    margin-top:10px;
    width:100%;
    p {
        line-height: 10px;
    }
    td {
        border-left:1px solid #c7c7c7;
        padding:20px 0 20px 10px;
        &:first-child {
            border-bottom:none;
            border-left:none;
            width:30%;
        }
    }
    .client {
        color: #40576a;
        font-weight: 600;
    }
    .equipment {
        color: #7f8080;
        font-size: 14px;
    }
    .fa-exclamation-triangle {
        color:red;
        font-size:18px;
    }
}
.table-results-audits {
    .box {
        padding:20px 10px;
    }
    label {
        font-weight: normal;
    }
    select {
        display:inline-block;
        width:70px;
    }
    input {
        margin-left: 0.5em;
        display: inline-block;
        width: auto;
    }
    ul {
        margin:0;
    }
    .dataTables_filter, .dataTables_paginate {
        text-align:right;
        white-space: nowrap;
    }
    .input-sm {
        height: 30px;
        padding: 5px 10px;
        font-size: 12px;
        line-height: 1.5;
        border-radius: 3px;
        padding-top:0;
        padding-bottom:0;
    }
    .table-controls {
        overflow: hidden;
        margin-bottom:10px;
    }   
}

Answer 1

I guess you can achieve this with a combination of colspan, table-layout:fixed and providing widths to the columns .

Made a small change by moving your child table as a sibling of the .current tr , ie moved it outside. and removed a empty column from your child table.

<tr class="current">
        <td>
            <input type="checkbox">
        </td>
        <td><a href="#">Mástil 1</a></td>
        <td></td>
    </tr>
   <tr><td><!--- your subtable ---></td></tr>

       html,body{
            margin:0;
            padding:0;
        }

        table{
            table-layout:fixed;
            border-collapse: collapse;
        }
        table td{
            border:1px solid #ccc;
            padding:2px;
        }

        table.table-equipment td{
                border:1px solid red;

        }

        tr.sub-current td{
            padding:0;
        }

<table class="table table-striped">
    <thead>
    <tr>
        <th width="20%">
            <input type="checkbox">
        </th>
        <th width="60%">UBICACION</th>
        <th width="20%">EQUIPO</th>
    </tr>
    </thead>
    <tbody>
    <tr>
        <td width="20%">
            <input type="checkbox"/>
        </td>
        <td width="60%"><a href="#">Edificio 4</a></td>
        <td width="20%"></td>
    </tr>
    <tr>
        <td>
            <input type="checkbox">
        </td>
        <td><a href="#">Azotea</a>
        </td>
        <td></td>
    </tr>

    <tr>
        <td><input type="checkbox"></td>
        <td><a href="#">Mástil 3</a></td>
        <td></td>
    </tr>
    <tr>
        <td><input type="checkbox"></td>
        <td><a href="#">Mástil 2</a></td>
        <td></td>
    </tr>
    <tr class="current">
        <td>
            <input type="checkbox">
        </td>
        <td><a href="#">Mástil 1</a></td>
        <td></td>
    </tr>
    <tr class="sub-current">
        <td colspan="3">
            <table class="table-equipment" >
                <tbody>
                <tr>
                    <td width="80%">
                        <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                        <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena
                            Panel 2.5x0.35</p>
                    </td>
                    <td width="20%"><i class="fa fa-exclamation-triangle"></i></td>
                </tr>
                <tr>
                    <td>
                        <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                        <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena
                            Panel 2.5x0.35</p>
                    </td>
                    <td><i class="fa fa-exclamation-triangle" ></i></td>
                </tr>
                <tr>
                    <td>
                        <p class="client">PANEL_VDF_GSM_UMTS_S3 VODAFONE</p>
                        <p class="equipment">Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena Panel 2.5x0.35, Antena
                            Panel 2.5x0.35</p>
                    </td>
                    <td><i class="fa fa-exclamation-triangle" ></i></td>
                </tr>
                </tbody>
            </table>
        </td>
    </tr>
    <tr>
        <td><input type="checkbox"></td>
        <td><a href="#">Tripode Autosoportado ORG1</a></td>
        <td></td>
    </tr>


    </tbody>
</table>

Check if this solves your problem.

P.S : Assuming you can move the child table out of the tr

Answer 2

The easy way to handle this is to make use of the colspan attribute on your td that needs to cover two tds. There is a nice example here: w3schools

A simple example would be:

<table>
    <thead>
        <Tr><th>id</th><th>details</th></Tr>
    </thead>
    <tbody>
        <tr><td>1</td><td colspan="2">stuff</td></tr>
        <tr><td></td><Td>details</Td><td>more details</td></tr>
    </tbody>
</table>

https://jsfiddle.net/9yrxuwkm/