Some steel mills in Hebei, China's top steelmaking province, have been ordered to curb operations due to power shortages in the region, industrial sources said.

Power grid loading in late July in southern areas of Hebei reached 36.57 gigawatts (GW), the highest on record, as a heatwave spurred electricity demand, according to the southern Hebei division of State Grid Corporation of China.

The grid company - which oversees seven cities in southern Hebei, including the major steel cities of Handan and Cangzhou - has issued an alert on a potential power shortage. It asked industrial companies such as steel mills, ceramic makers and oil field operators to restrict activities during peak hours to ensure power supplies for residential use.

"We have to adjust operation hours in accordance with power-rationing plans, which change every day depending on the power consumption situation," said a manager at a Cangzhou-based steel pipeline company, adding that notices are sent out several hours before power is cut off.

The manager declined to give his name as he is not authorised to talk to media.

Hebei lifted the power shortage alert to "orange", the second-highest in China's four-tier warning system, in seven cities in southern Hebei on Friday, according to a statement issued by Hebei's housing and urban-rural development department.

Under an orange alert, industrial plants can only work from 11 p.m. to 7 a.m., typically off-peak hours for residential use.

Around 30 percent of output capacity has been affected by the rationing, said Gao Jingguang, a manager at Wu'an based-Yuhua Steel, which has capacity of over 5 million tonnes a year.

A manager at an oil field in Hebei, run by China National Petroleum Corp, said they have also received notices on the power rationing.

"We have already gotten used to the power restrictions since it happened last summer as well," said the manager, declining to be named.

Power shortages typically happen when consumption surges over a short period and there is not enough transmission capacity to deliver power to consumers.

State Grid's Hebei division on Monday also scheduled power outages in Tangshan, in Hebei as well and China's top steelmaking city, for the coming three days, according to notices from the power company.