Current draw of the block heater isn't near 20 amps. I would guess it is about 500W which is about 4 amperes. I'm saying that because I had a 1200W tank type heater on another MB and in an hour it had the entire engine so that it felt warm to the touch, almost as if it had been shut off a short time ago. The original block heater isn't any where near that capacity.

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