It can be used and will protect the 30 amp side...but my directions say not to use it that way permanently, and do not say how long you can use it for just a 30 amp line.
This is from the web site (
www.surgeguard.com):
"Q. Can 50 Amp models be operated on a 120V/30 Amp, 120V/20 Amp, or 120V/15 Amp power source?
A. Operation of these models (using a standard adapter) on a 120V/30A, 120V/20A, or 120V/15A power source is not recommended by the manufacturer. However, the unit will still function properly. If used correctly, this will not void warranty. Keep power use to a minimum and return to a 50 Amp power source as soon as possible."