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1) If you have a non-contact voltage tester, the tester will confirm if there if voltage present on the conductors. If you suspect it is a 240 volt circuit, the circuit breaker in the panel will be a double pole type. You can shut OFF all of the double pole breakers in the panel until it is confirmed that voltage is no longer present on the cable using the non-contact voltage tester. If a non-contact voltage tester is not available, you will not have other options but to disconnect power at the electrical panel via the main circuit breaker.
2) If the cable insulation was cut and compromised, you will need a section of wire the same size and type as the existing cable and 2 junction boxes that will need to be accessible. National Electrical Code requires that all boxes be "accessible".
3) Another possibility, if the cable is NM Romex (you would need to confirm this), you could use a Tyco Romex splice kit and then junction boxes would not be required. Keep in mind that the Tyco splice kit is only rated and approved for NM Romex cables and no other types.
Refer to link shown below for the Tyco Romex splice kit sold at Home Depot.
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