Motorists in Surrey are being warned of disruption as a 143-tonne transformer is moved from Clacket Lane services on the M25 to Weybridge.
The transformer, on a vehicle over 68m (223ft) long and 5.3m (17ft) wide, will travel at 12mph, National Grid said.
It left Tilbury Docks in Essex at 19:00 BST on Friday, travelling over the Dartford Crossing before stopping for 24 hours at Clacket Lane services.
Disruption is expected in Surrey until about 16:00 on Sunday.
National Grid said the transformer would be transported on the motorway as far as junction 11 where it would continue on the A320 until the junction with the A245.
It will follow the A245 and B383 on Woodham Lane before turning into Pinewood Grove.
A short section of Pinewood Grove is being closed to allow the transformer to be moved to a smaller vehicle capable of manoeuvring the smaller roads towards the substation in Parkside in West Weybridge, National Grid said.
It is due to arrive at about 16:00.
A National Grid spokesman said: "We've carefully planned the route with our specialist haulage contractors and local highway authorities to minimise the disruption to road users.
"The vehicle will also have a police escort to help manage traffic and make the process as smooth and safe as possible."
He said National Grid had written to residents affected by road closures and parking restrictions apologising for the inconvenience.
Surrey Police have advised motorists to plan ahead if travelling on the M25 on Sunday.