# Xantrex PROWatt SW 600 Inverter fuse size & surge question

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**94**✭✭✭
So the inverter is 600 watt inverter (runing from 12VDC batteries), 540 watt continuous, 1200 watt surge (as its says). The manual says "DC Fuse ratings" "ALS fuse 80A, Class T 80A", ok so it says to use a 80 Amp fuse. Two questions, its says later on the max DC input is 60 Amps. Is it really more and thats why it says to use a 80 Amp fuse? Question two; Is the 1200 watt surge (AC) rating for like when a refrigerator start up, "Start up surge"? I hope so! Because I'm counting on it. If so, the 80 Amp DC fuse is only going to let 960 DC watts come form the batteries to the inverter, so how could it power 1200 watt AC surge? or am I missing something?

The main question is I need to know if 80A fuse seems right? I need to order them ASAP when I know for sure what I need.

The main question is I need to know if 80A fuse seems right? I need to order them ASAP when I know for sure what I need.

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## Comments

30,520adminFuses and Breakers have a time element to their design... A typical fuse may take minutes to hours to trip at rated current. If you draw 10x rated, it will probably trip in fraction of a second.

With NEC (and fuse/breaker ratings) it is assumed/defined that you will only operate at 80% of the circuit wire/fuse rating (1/0.8 = 1.25). So, if your system were designed for 60 amp continuous:

- 60 amps * 1.25 NEC Factor = 75 amp minimum rated fuse/circuit

Or, round up to 80 amps (next standard size).The back of the envelope calculation:

- 600 watts * 1/10.5 volts cuttoff * 1/0.85 typical inv eff * 1.25 NEC factor = 84 amps

So, it is reasonably close to the 80 amps that the manual required.Starting surge currents are typically only a few seconds in length, so most fuses/breakers will not trip very quickly with 2x rated current.

I have to leave right now--But see if you can find a document like this:

http://www.mgl.com.hk/mgl/documents/Ferraz_PDF/A-GPUS/A3T&A6T.pdf

It shows nominal trip time vs current.

-Bill

17,615✭✭Don't panic!

Trust in the company and use an 80 Amp fuse.

There's lots of things to consider when sizing fuses, not the least of which is the NEC derating. 60 Amps * 1.25 = 75 Amps, nearest equivalent being 80 Amp.

Otherwise you end up doing calculations like maximum power * minimum input Voltage and wondering about how long the particular fuse will hold if it is over current (and by how much) and you might put in a fuse which will protect it against exceeding 1200 Watts / 10.5 Volts but that's not really the size you want to use for

continuousoperation.As for start-up surge ... that's more tricky. First problem being electric motors aren't easy to predict in their start surges and this is not something that can be easily measured because it happens so quickly. That said, the second problem is how long the inverter can maintain its surge rating as sometimes the motor start-up does

nothappen that quickly. Personally I don't like to depend on surge ratings for anything other than "whoops! Everything turned on at once!"Another problem (as if you needed more to worry about) is the ability of the battery bank to handle the surge current demand. This is especially problematic on 12 Volt systems as the ratio between the DC side and the AC side is approximately 10:1. That 1200 Watts can be 120 Amps DC @ 10-ish Volts. It is at least 100 Amps. If the wiring offers too much resistance, the Voltage at the inverter can drop below its minimum and the thing will shut down. The length of the battery wires will affect this too, as will imperfections in connections.

Slight differences on the DC side make big differences on the AC side:

80 Amps (fuse rating) @ 12 Volts nominal (lowest you really want the battery to get) = 960 Watts but at 12.5 Volts it's 1000 Watts and at 13.8 it's 1100 Watts. Use the biggest wire you can fit & afford for the DC, use the 80 Amp fuse, and hope the start-up isn't well above the inverter's maximum.

94✭✭✭Thanks, so the like 0 awg to link the (4) batteries, and batteries to inverter. So there is never too fat of a wire for any DC wiring? (or just batteries?)

10,300✭✭✭✭that's a yes and no answer for electrically the bigger the better as it presents a lower resistance. now an extremely large cable can be a problem to maneuver around physically and may prove more difficult to make the connections at times.