CSS Alignmnet границы зависит от ширины контейнера

#wrap {
  display: flex;
  flex-direction: column;
  width: 800px;
  #header {
    width: 100%;
    box-sizing: border-box;
    border: 1px solid black;
  }
  #grid {
    width: 100%;
    display: grid;
    grid-template-columns: 1fr 1fr 1fr;
    border-left: 1px solid black;
    box-sizing: border-box;
  }
  span {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 30px;
    border-right: 1px solid black;
    border-bottom: 1px solid black;
  }
}

<div id = "wrap">
  <div id = "header">Header</div>
  <div id = "grid">
    <span>Item</span>
    <span>Item</span>
    <span>Item</span>
    <span>Item</span>
  </div>
</div>