size of lintel over drains

[sm_jamieson]

I need to lintel over the drains - the span is about 500mm (2 drain pipes and a water pipe), although I may build a brick support in the middle of the span. The building over is a single-storey brick-built extension. But assuming a span of 500mm, is a concrete lintel 100mm wide x 70mm high OK (like the two I just bought from Wickes !), or do I need to use a lintel 150mm (two courses) high ? Thanks, Simon.