A well-ventilated enclosure allows more rapid cooling and so for a given duty less resistor material is required. Given that the materials inside the resistor enclosure are stainless steels, ceramics and ceramic insulators, all suitable for unprotected outdoor use, we consider that enclosure ratings of IP23 are the right balance between good ventilation and the need to keep out birds and animals. We do also offer IP54 to IP56 enclosures where requested, either with or without filtration.