(In paragraph 2, removes reference to price rises for retail customers being backdated to Jan. 1 and clarifies phasing of the increase)
LUSAKA, May 10 (Reuters) - Zambia's Energy Regulation Board has approved a 75 percent increase in the price of electricity for retail customers in 2017, acting Chairman Francis Yamba said on Wednesday.
The first 50 percent will be effective from May 15 and the remainder of the rise will be implemented on Sept. 1.
In January last year Zambia's president reversed a sharp increase in electricity tariffs aimed at generating revenue for investment in additional supply, saying the hike had ended up hurting the poor.
Yamba said Zesco Ltd was still negotiating a proposed electricity price increase with mining companies with whom it had long-term agreements.
Zambia plans to introduce a flat electricity tariff of 9.30 U.S. cents/kilowatt hour (kWh) backdated to January for mining companies, instead of individually negotiated rates that have averaged 6 U.S. cents/kWh, the state power firm said on April 4.
Energy Minister David Mabumba said in April some mining firms opposed the rise and talks with them were continuing.
Mining companies operating in Zambia include Canada's First Quantum Minerals, Swiss commodities giant Glencore and London-listed Vedanta Resources.
In 2015 and 2016, Zambia experienced a critical shortage of power of up to 600 megawatts (MW) due poor rainfall and growing demand and supply has remained shaky, according to Zesco data. (Reporting by Chris Mfula; Editing by James Macharia and Edmund Blair)