The incandescent rating of a relay is often 1/5 or so of the amperage rating. The reason being that an incandescent lamp is close to a dead short when it is first turned on and it pulls a huge amount of current until it warms up. This turn-on current burns the relay contacts and can weld the contacts shut if the load is high enough.
A relay that can switch a 12A heating load is usually rated for 200W incandescent load.
I don't know the rating of the Patterson's relay, but if the fuse rating is 5A then I would guess they expect a 200W maximum incandescent lamp load and the relay contacts are rated at 10-12 amps.
If you have a 300W+ lamp then you should use a remote contactor - a very heavy duty relay: http://www.mcmaster.com/#general-purpose-relays/=m4xint