The 10 gauge wire is use for hot water heaters{220volt} on a 30 amp breaker.Each county,state,and city has its oun wiring codes.I dont know about a 40 amp breaker,A fan, air pump,timer,water pump,exhaust fan,400 to 1000 watt light only adds up to about 15 amps.I run all this and a box fan on a 20 amp breaker.If you only need 18 amps use a 20amp breaker.To big of a breaker can cause a fire.Or to small a breaker you trip the breaker all the time.Add up all the amps of your equipment,use a breaker thats a few amps bigger.Check your local ele. codes in your area.I'am not trying to step on anybody toes but electricity is invisible and deadly.