Inverter breaker/ fuse question

kansas · March 2015

Yesterday my wife attempted to use a small vacuum cleaner at the cabin. Predictably, the inverter (Sure Sine 300) shutdown. I expected the 3 amp fuse on the AC side of the inverter (recommended by Morningstar) to have blown. Instead, the 30 amp circuit breaker between the battery and the inverter had tripped. The 3 amp fuse if a plug type T-3. The breaker is a Midnight Solar DC 30 amp.

Can anyone help me understand why the circuit breaker tripped rather than the fuse blow?

BB. · March 2015

The little MorningStar 300 watt inverter is rated for something like 600 watts over 10 minutes, using the typical Branch Circuit Design guidelines:

600 watts * 1/0.85 inverter eff * 1/10.5 battery cutoff voltage * 1.25 NEC fuse/breaker/wiring derating = 84 amps

You should have heavier cable and a larger breaker to run the inverter closer to its capabilities.

Get a kill-a-watt type meter for the cabin... It will help answer the "can I run this" questions.

-Bill

mulia · March 2015

kansas wrote: »

Yesterday my wife attempted to use a small vacuum cleaner at the cabin. Predictably, the inverter (Sure Sine 300) shutdown. I expected the 3 amp fuse on the AC side of the inverter (recommended by Morningstar) to have blown. Instead, the 30 amp circuit breaker between the battery and the inverter had tripped. The 3 amp fuse if a plug type T-3. The breaker is a Midnight Solar DC 30 amp.

Can anyone help me understand why the circuit breaker tripped rather than the fuse blow?

Suresine 300 manual recommend a 100A fuse to be installed between battery and inverter, and your 30A is surely way too small in size, that's why it tripped. I believe MS recommend to use 100A more to protect wiring and keep it from short circuit than to prevent it from drawing power higher than its surge rating.

The 3A on AC side will blow when you draw more than 600W from Suresine (I assumed you use Suresine 220V version). Because suresine rated for 600W surge only for a few minutes before it fault.
3Ax220V = 660W

BB. · March 2015

I believe the 3 amp fuse in the AC output is actually, more or less, wrong. If we assume 600 Watt AC ouput, if you choose to install an AC fuse/breaker (I don't think one is needed), it would be:

600 Watts * 1/110 VAC * 1.25 NEC derating = 6.8 Amp minimum branch circuit wiring/breaker/fuse

If you use 14 AWG wiring (minimum) for your cabin's AC wiring--That is good for 15 amps--So a 7-10 amp breaker/fuse would be smaller than required anyway (breakers protect AC wiring, that is why they are there).

-Bill

mulia · March 2015

BB. wrote: »

I believe the 3 amp fuse in the AC output is actually, more or less, wrong. If we assume 600 Watt AC ouput, if you choose to install an AC fuse/breaker (I don't think one is needed), it would be:

600 Watts * 1/110 VAC * 1.25 NEC derating = 6.8 Amp minimum branch circuit wiring/breaker/fuse

-Bill

Yes, that should be like that in case of 110VAC. For my 220V version, 3A is the right one (might be).

600 Watts * 1/220 VAC * 1.25 = 3 A

I think Morningstar needs to made aware of this situation, which I doubt they will. My email received no response for more than 6 months.

BB. · March 2015

Yes, 3 amps is fine for a 220 VAC MorningStar inverter.

By the way, I fixed your last sentence for English--Very readable, but help you with "need" vs "needs" etc.. (not that my English is anything to write home about).

-Bill "crawling back under rock now" B.

kansas · March 2015

I appreciate the comments. I do have a 110 amp fuse (as close as NAWS had to the 100 amp recommended by Morningstar when I ordered my equipment) installed between the battery and the inverter AND a 30 amp breaker between the 110 amp fuse and the inverter. This decision was based on advice not vetted here.

The battery lead to the 110 amp fuse is 4 AWG; the wire between the fuse and the breaker, and the breaker and the inverter, is 6 AWG. My AC wiring is 12 AWG. The breaker has worked well as a disconnect but is obviously limiting the usefulness of the Sure Sine, which, by the way, is 110V. I think I'll change out the 30 amp breaker for an 80 amp, and the AC fuse to something around 6 or 7amp.

For what it's worth, Morningstar has been responsive to my phone calls, less so to my emails.

Bill

BB. · March 2015

Go for the 80-100 amp breaker--If you use the less conservative marine wiring charts--You can get ~160 amps through #4 SAE wire.

http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

-Bill

mulia · March 2015

BB. wrote: »

By the way, I fixed your last sentence for English--Very readable, but help you with "need" vs "needs" etc.. (not that my English is anything to write home about).

-Bill "crawling back under rock now" B.

Thanks Bill, it's very good to improve my English.

For what it's worth, Morningstar has been responsive to my phone calls, less so to my emails.

Unfortunately, it won't work for me. It won't be a good idea to make an international call thousands miles away and I'm doing big mistakes by buying 2 MS products. Can't imagine if there's anything wrong I'll need to make an international call since no email is going to be replied. On the other hand, I'm happy for using another US based company's products, they're very responsive to my email which full of complaints and questions and very helpful, better, all their products I bought are made in USA. Sorry to rant here.

Inverter breaker/ fuse question

Comments

Categories