The Briton started last year's race on pole position but McLaren have been off the pace this season and are currently third overall behind leaders Mercedes and Ferrari.
Drivers are allowed three power electronics units per season and the change exceeds the limit, triggering the mandatory penalty.
McLaren said the first unit suffered a terminal failure in China in March, leaving Norris unable to start that race.
The second unit was fitted for the Japanese Grand Prix, also in March, but had to be withdrawn for remedial work after issues in free practice.
It was repaired but then suffered a terminal issue in Monaco practice in June.
"While the power electronics unit we installed in Japan, and have used in every session since Miami, has worked reliably, Mercedes - AMG High Performance Powertrains has since introduced a series of reliability fixes," the team said.
"However, in order to take advantage of these improvements, we must incur a 10-place grid penalty on Lando's car in order to take a new unit.
"We have chosen to do this in Belgium, a circuit where overtaking is relatively more prevalent, as opposed to the following two events in Hungary and Zandvoort (Netherlands)," the team added.
McLaren said they planned to use this fourth unit for the remainder of the season.