The National Grid delivery left Goole in East Yorkshire on Thursday and is expected at Cilfynydd, near Pontypridd, on Saturday morning.
Lorry and cargo will only be able to travel at 30mph (48km/h) on motorways and 10mph (16km/h) on A roads.
It will travel along the A48 from Newport and up the A470 and A472.
The special heavy haulage vehicle will have a private escort and National Grid said it expected the delivery would cause disruption until it is delivered at about 10:00 GMT.
It is 25m (82ft) long, 5m (16ft) high and 3m (10ft) wide.
National Grid project engineer, David Shaw, said: "There are likely to be some delays on the motorway and on smaller, narrow roads close to the substation.
"I'd like to thank people for their patience in advance of our work."
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