MacBook Power Cord Extension
Is it harmless to use the MacBook extension cord accessory which came with the older MacBooks & MacBook Pros on any other MacBook models new or old? In other words, are they wattage/amperage/voltage specific?
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You can use it with any power adapter. The same cord is still available through the Apple Store. See the following from Power Adapter Extension Cable - Apple
The 1.8-metre Power Adapter Extension Cable is an AC extension lead that provides extra length for your Apple power adapter. Use it with MagSafe and MagSafe 2 power adapters, 10W and 12W USB-A power adapters and 29W, 30W, 61W, 87W and 96W USB-C power adapters.