Resistance for high amperage T-class fuses are typically between 0.25 and 1 millohm, depending on current they are carrying.
This scared me a little bit, and as this thread is the only place on the internet where the resistance / power loss of a class T fuse is mentioned and I wanted a definitive answer to that question, I asked Bussman directly. My main motivation was to compare the power loss of a
600A Class T (JJN-600) to a an
NH3 630A (630NHG3B). The NH3s are bulkier, but have a higher DC voltage (250V), interrupt rating (120kA) and are significantly cheaper. Needless to say that if the class T fuse indeed had a resistance of 1 mΩ (dissipating a scary 360W at nominal current) my choice was going to be very easy.
Bussman's answer to my query was that a JJN-600 at rated current and 25°C ambient temperature had a resistance of 0.097 mΩ (min. 0.09mΩ, max. 0.104mΩ). Which leads to a nominal voltage drop of 58mV (max 62mV) and a power loss of 35W (max 37.5W).
From the datasheet, NH3 will dissipate 46W at rated current and 25°C ambient temperature. Which means a voltage drop of 73mV and a resistance of 0.115mΩ. To compare it with the JJN and assuming the resistance is roughly the same at 600A, the power loss at 600A would be about 41W.
I had no reply from the 2nd manufacturer of class T fuses I know of (Ferraz Shawmut).
These numbers make sense as given the NH has better IR and voltage ratings it is likely to expose a slightly higher resistance due to its internal construction. I think it is not absurd to extrapolate these number to other fuse ratings, with power losses of class T fuses slightly lower than the equivalent NH fuse as provided in the
datasheet.
As I am on a boat, Wh and (mostly) space are precious, I think I will go with Class T even if they are 3 times more expensive.
EDIT: from another datasheet I can't find again and that was reporting power losses at 80% nominal current (which is generally the circuit's 'design' current), they were around 2/3 of the power loss at nominal current. If the extrapolation to a class T stands, a JJN-600 would be likely to dissipate something in the vicinity of 25W at 500A. With a few reasonable assumptions (fuse temperature at nominal current), it should be relatively easy to evaluate the order of magnitude of power losses for any current.