20 or 25mm?

Hi. Can anyone tell me the correct diameter mdpe pipe that should be used to replace old lead water mains supply pipe, is it 20mm or 25mm?

Thanks.

pixey

No disadvantage in going bigger, the cost is quite low.

As it happens, I had my lead main replaced free in the days when they did it free if they found a leak. It was supposed to be 25mm but they didn't have enough with them. I should have made a fuss but didn't know any better then, so I've got about 15m of 20mm mdpe between the pavement and the stopcock (where it goes to 15mm copper).
With a 3 bar main I get about 21 litres/min from the outside tap. Whether it would be any more from that tap with 25mm, I don't know - must do some tests! It's certainly better than it was, and better than my neighbour's, who still has a lead supply.

So for short lengths and normal mains pressure 20mm is OK , but hey, use 25! If you plan on running the whole of a larger house from the mains (ie no cistern in the loft), use 32. The valves will cost more, but that's where much of the resistance is.
Remember to knock off 4-5mm to get the equivalent size copper

Thanks for the reply chrisr, the run from the boundary is approx 15m, I have a cold tank in the loft, one mains cold tap (kitchen) and one mains pressure electric shower (which suffers low pressure at busy periods).
That is all that is off the mains.
I wonder would 25mm mdpe improve the pressure to the shower?

pixey