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I have a DC inverter that's rated for 3kw sustained with a 6kw peak (for up to 5 seconds).

Per the Inverter manual, the manufacturer recommends a battery fuse rated for "at least 350 amps".

How can I determine what size fuse would be ideal in order to accomplish the following:

  1. Never blow at 3kw/250amps (inverter's nominal rating)
  2. Allow surge currents of up to 6kw/500amps for up to a few seconds
  3. Blow pretty darn quick after the 6kw/500amps threshold

I was considering a 400amp fuse, but I'm not sure how to determine if it fits the bill without trying. Frankly, I'd rather have a breaker with similar characteristics since these fuses are upwards of $50 a piece, but it is what it is.

This is a photo of the fuse: http://ecx.images-amazon.com/images/I/71FMkor8t7L.SX355.jpg

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  • \$\begingroup\$ Follow the manufacturer's instructions: Slow-blow of 350 amps or more. If your fuse is rated at 400 amps slow-blow, do you think it fits the description? If not, why not? Do you think the genny manufacturer is trying to mislead you? If so, why? \$\endgroup\$ – WhatRoughBeast Apr 30 '16 at 0:55
  • \$\begingroup\$ @WhatRoughBeast Yes, I think 400 amps is ">= 350 amps", as mentioned in the manual. I don't think they are trying to mislead me, but the directions left room for interpretation by not listing an upper limit. Also, they didn't mention whether it should be a slow blow or not, they only listed an amp rating. A 350amp fast blow fuse, however, wouldn't be able to sustain a 5 second 500amp surge (as I understand it), so I deduced that it needs to be a slow blow fuse. However, 350amps is neither at the continuous rating of 250amps nor is it at the surge limit of 500amps - it's ambiguous! \$\endgroup\$ – user1142433 Apr 30 '16 at 1:44
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    \$\begingroup\$ My first thought here would be to contact the manufacturer for clarification and a part recommendation. \$\endgroup\$ – Ian Bland Apr 30 '16 at 1:57
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    \$\begingroup\$ Also, since this is the battery fuse, I would think that the fuse is primarily for fault protection as opposed to than overcurrent protection (this distinction is made here in the UK electrical regs, dunno about other countries). Hence the "at least". It's thus basically there to blow if you get a gross overcurrent i.e. from a short, fault in the inverter etc. Does the inverter have protection circuitry to detect/protect against overcurrent drawn by the load, perhaps? \$\endgroup\$ – Ian Bland Apr 30 '16 at 2:24
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Firstly, don't buy critical components from Amazon or Ebay or any other general online retailer. When you need a component that has to meet specific requirements, you need a datasheet. And when you need a datasheet, you need to buy the component from a reputable distributor.

Here's a fuse that I found on Digikey that might be close to what you need: 0298400.ZXH (I didn't actually compare it against the requirements you mentioned, so don't assume it's the right one). The datasheet has these figures that you'll need to study to determine if it meets your requirements:

enter image description here enter image description here

If this part fails your requirements, you can use the parametric search tool (which any good electronics distributor will have) to find more fuses and compare their datasheets.

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You have another answer that advises not to buy from Amazon. Your description illustrates why. A Class T fuse is not a slow-blow fuse as stated in the amazon description it is a fast blow fuse. If you buy from Amazon at least try to find the manufacturer's specifications and check them. There is still some risk that you will get a counterfeit part.

If the manufacturer doesn't specify a fast blow fuse, don't buy one. They are more expensive than standard fuses. The fuse is not going to be fast enough to protect the inverter from damage due to an output fault. It is very difficult to protect power electronic circuits with fuses. The best designs have electronic protection. In some cases, the manufacturer will recommend a specific fuse, but that is not to protect the electronics circuit from damage. It is used to prevent the equipment from exploding or starting a fire if it fails. It is probably required by the UL listing.

The inverter manufacturer probably put a high minimum on the battery fuse because a standard fuse of that ratting can withstand the inverter's capacitor-charging surge current when it is first connected to the battery.

You should select it according to the battery manufacturer's recommendations. If they recommend less than 350 amps, get a different battery.

Added re fuse speed

It appears that most fuses that are not specified as "slow blow" may be considered to be "fast blow." The Class T JJN fuse is probably more expensive than you need because it is rated for a higher voltage than you need. Look at ANN and CNN fuses. They are rated 125 V AC / 80 V DC.

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  • \$\begingroup\$ Wow. That's crazy. Google shows dozens of pages with "Class T Slow Blow" fuse listings. Thank you! \$\endgroup\$ – user1142433 Apr 30 '16 at 18:20

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