# Fuse size

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Solar Expert Posts: 26
Hello,
The Suresine 115 volt inverter is rated at 300 watts with 600 watts surge capacity. According to the owners manual Morningstar states that a 100 amp fuse should be used on the DC side. I am under the impression that inverter fuses should be sized at 125% of the surge capacity, or 60 amps in this case. Can anybody explain why the larger fuse is recommended? BTW I have emailed Morningstar. Thanks, Mark

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Re: Fuse size

If you assume the inverter can operate down to 10.5 volts, is 80% efficient and the fuse should be sized to 125% of load:

600 watts 1/10.5v * 1/80% * 125% = 89.3 amps...

Round up to next standard size ~100 amps to also account for some other unknowns (like voltage drops through wiring and fuse block).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Fuse size

Yep... 100 A, per page #5 in the manual.

HTH,
Jim / crewzer
• Solar Expert Posts: 26
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Re: Fuse size

Bill,
Thanks for the input.......I've intended to follow the Morningstar recommendations from the start but like to have an explanation/understanding of the fuse ratings compared to the info I have read. Regards!
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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Re: Fuse size

Mark,

I believe that I owe you an apology. I failed to see in your original post that you were inquiring about the rationale for a 100 A fuse, and I thought you were asking about what the fuse size should be.

The NEC Section 690.8 establishes the process for determining the rating for an inverter’s input fuse. The basic formula is [Rated Inverter Power / (Lowest Input Voltage x Efficiency)] x 125%. For your SureSine inverter, the calculation would look like this: [300 W / (10 V x 84%)] x 125% = 35.7 A. The next standard size fuse is 40 A.

WTF?? Well, the inverter contains three 40 A fuses. See manual page 13. I haven’t seen the schematic, so I don’t know why there are three fuses.

I suspect that Morningstar’s recommendation for an external 100 A fuse is some combination of a catastrophe fuse as well as protection for the inverter for when it’s operating at peak power (600 W, 10 minute limit; 10.5 V min due to 4 minute limit). For this scenario, the calculation would look like this: [600 W / (10.5 V x 70%)] x 125% = ~102 A. All things considered, a fuse rated for 100 A is probably “close enough”.

The low voltage values and the efficiency numbers were taken from the inverter’s data sheet. See: http://www.morningstarcorp.com/products/SureSine/info/SI_DataSheet.pdf

Regards,
Jim / crewzer
• Solar Expert Posts: 26
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Re: Fuse size

JIm / crewzer,
Thanks for the response and data. The NEC formula is certainly more precise, so 100 amps it is. Mark