The total allowable load in amps on a branch circuit by Code is 80 per  cent of the circuit rating. Therefore, a 20 amp circuit shouldn't be  loaded over 16 amps total.  30amp circuit, 24 amp load.
A 30 amp two pole breaker allows up to 30 amps per leg or  phase to conduct in fact provided a "total" of 60 amps. However, 220  volt devices require the rated amperage for each leg.  So a device which requires 220 volt and 60 amps requires a two pole 60  amp breaker and the correct wire size to feed it. (# 6 gauge wire)
 http://www.csgnetwork.com/wiresizecalc.html
http://www.csgnetwork.com/voltagedropcalc.html
 
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