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What size class T fuse do I need ?

CabinBill

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Nov 2, 2021
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I recently purchased a Growatt 5000 watt inverter and 6 48 Volt 100 ah E G4 batteries. I was wondering what size T class fuse I should use heading towards the inverter. Thank you for any replies !
 
5000W / 48V / 85% efficiency = 125A max draw from the batteries so you need to wire accordingly (1AWG or bigger would be good). 125A * 125% = 156A fuse size. I would round that up to a 175A Class T fuse for the main battery fuse.
 
Maybe take into consideration the charging/discharging amperage (in/out) of the battery ?
 
5000W / 48V / 85% efficiency = 125A max draw from the batteries so you need to wire accordingly (1AWG or bigger would be good). 125A * 125% = 156A fuse size. I would round that up to a 175A Class T fuse for the main battery fuse.
Is .85 a good efficiency number for a growatt inverter? I thought they were closer to 95%…
Also, what about surge capacity? Is 25% over inverter rating ok for surge? Or is surge handled by the inverters capacitors? I never know this answer, and the inverter manuals don’t cover it…
Some inverters rate their surge capacity at 3X rated for over 20Seconds… do we size everything for that short bump an ampdraw?
I wish the manuals would cover this info…

I just bought a 12kW LF inverter… so, 85% seems wise, but should I also upsize everything for the 30sec 36kW? Maybe just the fuse? Fuse and wiring? Who’s to say?
 
I would keep it tight at 150 amps. Class T can pass 2x rating for about a minute before it opens. Plenty of time to provide surge performance. Might even go 125 amp if the utilization is expected to be closer to 4000/4500 watts max or less.

ClassT_Fuse.jpg


https://www.bluesea.com/products/5112/Fuse_A3T___Class_T_110_Amp
 
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I'm planning the other direction. 400A class T fuse on each 280Ah battery for battery dead short protection, which will feed into a combiner box breaker with appropriate breakers that actually have a higher Aic than class-T for primary overload protection and service disconnect. The fuse is strictly for preventing battery damage, and I'd prefer my resettable breakers trip, as that's a lot cheaper.
 
400A class T fuse on each 280Ah battery
Hopefully you have at least 4/0AWG wire otherwise the fuse is too big. Also, what size BMS do you have on each battery? A 400A fuse on a battery with a 100A (or even 200A) BMS seems rather oversized.
 
Is .85 a good efficiency number for a growatt inverter? I thought they were closer to 95%
I have no idea what the efficiency of a Growatt inverter is but 85% seems like a good conservative value when working out the details. I'd rather end up with a slightly oversized wire and fuse to avoid hot wires or overstressed setups.
 
Yes, I'm using 4/0 wire. I haven't bought the BMS or the fuse yet, though the former will probably be 200A continuous rated. I might go with 300A fuse as sufficient, but I had seriously considered a contactor based BMS. Thanks to @upnorthandpersonal's tests, we know a class-T will protect a MOSFET BMS from a dead short, and my theory is I have enough other protection that the battery fuse doesn't really have anything to do except protect from a wiring/wire moving short mishap. I'm not expecting a significant difference in blow time from a 200A vs. 400A fuse in a dead short situation.
 
I believe you are using 600Ah total LiFePO4@48v, with 6 packs in parallel? 4/0 wire is fairly sturdy stuff. Your impedance is very low with the potential of some large fault currents.

Have you given any thought to fusing each battery pack? The reason this may be worth looking at is if one cell fails the remaining healthy cells will try to dump their energy into the failed cell. This could go critical in an instant and you may not want to be nearby. I would imagine this is a large $ investment and a failure could potentially take all the cells out.

You may want to consider some kind of inrush limiter on this setup. This has been discussed very well on other threads so I wont go into it here. You will need it to protect your fuse and BMS. I have a growatt 5kW and am using a 200A fuse with a 125A breaker with an inrush limiter as well.

Sounds like a very nice setup. Good luck.
 
@Roswell Bob Yes, 16S 280Ah*2. My plan is a 300-400A class-T on each pack (400A means in an emergency I can drain anything the pack can safely put out, if I even can find the loads to do so), that feeds through BMS to a 175 or 250A breaker in the combiner box, which can also be my battery string servicing disconnect. Then each inverter has its own breaker in the combiner box, as do the SCCs. And thanks: yes, I do need to add a precharge setup, and despite some comments helping others with them the past couple days I hadn't really gotten to thinking through mine yet.

ETA: I'm not the OP here; my system is somewhat confused in the discussion as I'd offered my plans for OP to consider. I appreciate the comments on mine, too, as they're helpful.
 
@Roswell Bob Yes, 16S 280Ah*2. My plan is a 300-400A class-T on each pack (400A means in an emergency I can drain anything the pack can safely put out, if I even can find the loads to do so), that feeds through BMS to a 175 or 250A breaker in the combiner box, which can also be my battery string servicing disconnect. Then each inverter has its own breaker in the combiner box, as do the SCCs. And thanks: yes, I do need to add a precharge setup, and despite some comments helping others with them the past couple days I hadn't really gotten to thinking through mine yet.

ETA: I'm not the OP here; my system is somewhat confused in the discussion as I'd offered my plans for OP to consider. I appreciate the comments on mine, too, as they're helpful.

In the event of a short circuit I would think the fuse will clear way before the breaker you plan on using. Are you intending to use a mosfet based BMS?

I am using a NTC Thermistor and a contactor for precharge. Here is thread with pic of circuit. I've used these on Large industrial 480V inverters.

I posted some simulation data below in this thread. I'm not sure how accurate it is because I don't have good capacitor internal resistance.
 
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