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Discussion Starter · #1 ·
So I'm planning on using a Kat's 13150 block heater to provide cabin heat through the standard heater core on my Miata. I'll be connecting it to the traction pack (45 * TS 180Ah cells - 144V nominal). As the heater is 1500W, it should pull c. 10-11A. I assume that as well having the correct amp rating (say 15A) it also needs to be rated correctly for the voltage (so a 15A auto blade-type fuse rated for 12V ain't going to cut it).

So can anyone suggest what type of fuse I should use?

Thanks
 

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Discussion Starter · #2 · (Edited)
I think I can answer my own questions after a bit of research.

First, yes I do need a fue rated for the correct voltage. When a fuse blows a plasma arc can form across the gap caused and if the voltage is high enough (i.e. the fuse is not rated correctly) it will maintain that plasma which will allow current to continue to flow. So I need a fuse rated for at least 144VDC - realistically 250V or higher.

Now as for the amp rating. Well the Kat's heater produces 1500W when running on 120V AC. Using P = V*I that means is draws 12.5A. Now P = I^2*R too, so that tells me that the internal resistance of the heating element is 9.6ohms (1500/(12.5*12.5)). My multi-meter confirmed that value by measuring directly.

But I'm running the heater on 144V DC. V=I*R says that I'll actually be drawing 15A. Which also means that my '1500W' heater will actually be producing 2160W. I think that means it's heat the water 44% faster! (I'm pretty comfortable that the heater element will be able to cope with the additional load - someone yell if you disagree!)

So lastly what make of fuse? Well I was planning on using some Ferraz Shawmut A30QS600 fuses between the elements of my pack - they are rated at 250V and 600A and it seems a popular option on many EV parts sites . That type of fuse comes in smaller amperage levels including 30A (A30QS030) which is what I think I need here.

So I think I'll call http://www.fusesunlimited.com/ on Monday and order those (together with the fuses and traction pack, charger and DC-DC converter) at the same time. Has anyone here used these guys? Any feedback or other suggestions for fuse suppliers.

Thanks
 

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standard 'slo-blo' fuses are available up to about 30a at 250v. You'll want to go higher than you expect to run because of the initial in-rush to the heater. I know that my ceramic heater pulls less than 10 amps once hot, but the first second spikes up to about 35amps as the elements heat up.
 

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Discussion Starter · #4 ·
standard 'slo-blo' fuses are available up to about 30a at 250v. You'll want to go higher than you expect to run because of the initial in-rush to the heater. I know that my ceramic heater pulls less than 10 amps once hot, but the first second spikes up to about 35amps as the elements heat up.
Dan,

Thanks - but I 'think' resistive elements in a water heater have different characteristics than ceramic heater cores.

I've measured the resitance of the element at 9.6 Ohms - physically it is impossible to pump more than 15A through that with 144V (Mr Ohm's law says that). Now my fully charged pack voltage will likely be 162-171V - but even then that's 17.8A max so I think a 30A fuse is the right size.

Someone please yell of I've got this wrong!

Thanks
 
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