Here is a good picture of electrical hook from the Vlair 280C manual (part #28021)
The specification in the manual list recommended fuse as 30 Amp.
SPECIFICATIONS
280C AIR COMPRESSOR PART NUMBERS:
• Part No. 28021 / 280C Air Compressor (CE Spec.)
Motor Voltage: 12 volts
Max. Current Consumption: 19 amps
Recommended Fuse: 30 Amps
Motor Type: Perm. Magnetic
Horse Power: 1/4
Max. Working Pressure: 150 PSI
Max. Duty Cycle (@72ºF & 100 PSI): 30%
Minutes On/Off (@72ºF & 100 PSI): 18 On / 42 Off
Max. Restart Pressure: 200 PSI
Max. Ambient Temperature: 158ºF
Min. Ambient Temperature: -40ºF
Auto. Reset Thermal Protection: Yes
Dimensions: 7.8”L x 3.95”W x 6.35”H
Net Weight: 6.95 Lbs.
The specifications in for the 280c in the 150 PSI HIGH-FLOW
AIR SOURCE KIT (part # 20005):
SPECIFICATIONS:
280C AIR COMPRESSOR
Motor Voltage: 12 Volts
Max. Current Consumption: 19 Amps
Recommended Fuse: 25 Amps
Motor Type: Perm. Magnetic
Horse Power: 1/4
Max. Working Pressure: 150 PSI
Max. Duty Cycle (@72°F & 100 PSI): 30%
Minutes On/Off (@72ºF & 100 PSI): 18 On / 42 Off
Continuous Use (@72ºF & 40 PSI): 40 Minutes
Max. Ambient Temperature: 158°F
Min. Ambient Temperature: -40°F
Auto. Reset Thermal Protection: Yes
You might want to contact Vlair on the discrepancy. Both still list the max current @12 volts being 19 Amps. I would think a 30 amp fuse would be the better choice. Of course since, we fuse for the wire the ampacity of the wire should be above that.
I am not a fan of the included wire chart in the manual, as they do not list the temperature rating on the wire that they are specifying. It looks to be inline with 90 C. The Blue Sea circuit wizard is great, just put then appropriate data in. 3% voltage loss is standard for most calculations. Good marine wire is rated at 105 C. I don't expect you were using a pressure swicth in the diagram. In any case the relay should be at least a 40 amp relay, this will minimize the length of you higher current run. The current for the relay should be relatively small in comparison, in most cases the minimal we would use would be 18 awg, just for strength reason. Hope this helps.
-greg