Continuous power vs surge power for the fuse and the wire sizing?

[JanC]

Hi,
can someone please explain what determines the size of the cable and the size of the fuse from the battery to the inverter. I have a 24v battery bank and I'm planning to buy a 2000w inverter with a 4000w surge power. People usually reference the continuous power when deciding on the wire and the fuse but it still confuses me. The maximum power that this wire and a fuse might encounter is 4000w. So logically you should calculate based on this number. But I heard an explanation that this surge power lasts for so short that it wouldn't blow a fuse. I also heard that some inverters might be able to deliver longer lasting surges that might trip the fuse. Another statement I heard is that some types of fuses will trip immediately after going above the limit and some don't.

Can someone please clear this out for me. I'm planning to power a fridge with this inverter plus some extra low consumption devices, so in a way I rely on this surge capacity of this inverter because it might happen in my situation and I want to size everything correctly.

I'm probably buying this inverter and it doesn't say how long can it handle the surge power. https://www.xunzel.com/inicio/xj-2000w-24v/?lang=en

Thank you

Jan

 

[Supervstech]

The inverters with a double capacity surge, RARELY have that surge beyond a handful of milliseconds… no point in doubling the wire size for the minuscule time needed to supply the surge.

Even top shelf LF inverters with iron core transformers and a 30 second surge going three times the continuous demand rating likely wouldn’t benefit with sizing the wires for the surge rating.

That said, oversizing for the lowest loss and least resistance gives you the most efficiency out of your battery bank… and handles surge the best.

 

[JanC]

The inverters with a double capacity surge, RARELY have that surge beyond a handful of milliseconds… no point in doubling the wire size for the minuscule time needed to supply the surge.

Even top shelf LF inverters with iron core transformers and a 30 second surge going three times the continuous demand rating likely wouldn’t benefit with sizing the wires for the surge rating.

That said, oversizing for the lowest loss and least resistance gives you the most efficiency out of your battery bank… and handles surge the best.

I understand that within such short period of time it won't overheat but what about the fuse sizing? Imagine I put a 100A fuse on the wire and there comes a situation where the appliances need 150A for a few miliseconds. Wouldn't it trip the fuse? As I said I heard that some fuses would trip, others might not. So considering my example, is it still okay to size according to continuous power?

 

[John Frum]

Hi,
can someone please explain what determines the size of the cable and the size of the fuse from the battery to the inverter. I have a 24v battery bank and I'm planning to buy a 2000w inverter with a 4000w surge power. People usually reference the continuous power when deciding on the wire and the fuse but it still confuses me. The maximum power that this wire and a fuse might encounter is 4000w. So logically you should calculate based on this number. But I heard an explanation that this surge power lasts for so short that it wouldn't blow a fuse. I also heard that some inverters might be able to deliver longer lasting surges that might trip the fuse. Another statement I heard is that some types of fuses will trip immediately after going above the limit and some don't.

Can someone please clear this out for me. I'm planning to power a fridge with this inverter plus some extra low consumption devices, so in a way I rely on this surge capacity of this inverter because it might happen in my situation and I want to size everything correctly.

I'm probably buying this inverter and it doesn't say how long can it handle the surge power. https://www.xunzel.com/inicio/xj-2000w-24v/?lang=en

Thank you

Jan

The following assumes you have no other loads except the inverter.
2000 ac watts / .85 conversion factor / 20 volts low cutoff = 116.55011655 inverter amps
116.55011655 inverter amps / .8 fuse headroom = 147.058823529 fuse amps

That means minimum 4 awg pure copper wire with 105C insulation and a 150 amp fuse of sufficient breaking capacity for the battery chemistry and voltage.
If you have an inverter/charger with programmable disconnect you can use 24 volts for the volt cutoff but the result won't change much

The other consideration in wire sizing is voltage drop.
This calculator will help.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

Concepts for further investigation:
80% rule
inverter low voltage disconnect
over-current protection breaking capacity
dead short ampacity by battery chemistry and nominal voltage

UPDATE: recalculated for 2000 watt continuous inverter

 

[Supervstech]

The following assumes you have no other loads except the inverter.
4000 ac watts / .85 conversion factor / 20 volts low cutoff = 235.294117647 inverter amps
235.294117647 inverter amps / .8 fuse headroom = 294.117647059 fuse amps

That means minimum 2/0 awg pure copper wire with 105C insulation and a 300 amp fuse of sufficient breaking capacity for the battery chemistry and voltage.
If you have an inverter/charger with programmable disconnect you can use 24 volts for the volt cutoff.

The other consideration in wire sizing is voltage drop.
This calculator will help.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

Concepts for further investigation:
80% rule
inverter low voltage disconnect
over-current protection breaking capacity
dead short ampacity by battery chemistry and nominal voltage

But I don’t think sizing this way is needed on a 2000W inverter with a 4000W 5 millisecond surge…

 

[Supervstech]

I understand that within such short period of time it won't overheat but what about the fuse sizing? Imagine I put a 100A fuse on the wire and there comes a situation where the appliances need 150A for a few miliseconds. Wouldn't it trip the fuse? As I said I heard that some fuses would trip, others might not. So considering my example, is it still okay to size according to continuous power?

I wouldn’t use a 100A fuse on a 2000W 24V inverter. Specked out, has 147.05… amps, so a 150A fuse.
Fuses don’t blow exactly at the rated value, they have a time factor some more than others. The surge could be double this value, so 300A, but for such a short duration, it is meaningless.

 

[Zwy]

But I don’t think sizing this way is needed on a 2000W inverter with a 4000W 5 millisecond surge…

I agree, no reason for it. 2000W on 24V is 83A. 6 AWG would handle it with VD under 2% for 10 feet. I'd bump it up to next sizes of 4 AWG or 2AWG, put in a 100A fuse and forget it.

 

[RCinFLA]

Typical single phase induction motor startup surge lasts for about half a second.,

If inverter shuts down for high surge current you may need larger cable. It depends how quick the inverter reacts to DC input voltage slump.

Since inverter AC output voltage is regulated, the more DC input voltage slumps the greater the DC current draw will be to make the required AC output power. This compounds the problem. Also, inverter conversion efficiency drops the lower the DC input voltage slumps further increasing the battery current draw.

 

[John Frum]

But I don’t think sizing this way is needed on a 2000W inverter with a 4000W 5 millisecond surge…

Please explain why not.

 

[RCinFLA]

it doesn't say how long can it handle the surge power. https://www.xunzel.com/inicio/xj-2000w-24v/?lang=en

Thank you

Jan

High frequency inverters rely on high voltage filter capacitor and saturation limit on battery to HV DC converter ferrite transformer.

It does not have enough filter capacitance to survive for more than about 1 millisecond before HV supply drops below voltage necessary for peak of sinewave output. Although this allows them to spec an overload, in practice, 1 millisecond of surge capability is pretty much useless.

Even a resistive heating element takes more current when first turned on when it is cold. It takes 50-100 milliseconds to heat up causing its resistance to increase, dropping its current draw. An incandescent light bulb has an initial surge current until its filament resistance rises when it glows hot.

 

[Supervstech]

I agree, no reason for it. 2000W on 24V is 83A. 6 AWG would handle it with VD under 2% for 10 feet. I'd bump it up to next sizes of 4 AWG or 2AWG, put in a 100A fuse and forget it.

Now, don’t think 83A is the most you will see sustained on a 24V 2000W inverter… when battery voltage drops, amps go up.

 

[robby]

I size my fuses based on what the max current the inverter will draw during its max normal operational specs .If it’s drawing more than that for several seconds then something has gone wrong. I would prefer the fuse blows rather than a fire starting.

 

[Zwy]

Now, don’t think 83A is the most you will see sustained on a 24V 2000W inverter… when battery voltage drops, amps go up.

I'm pretty certain it would be best to derate that inverter as it doesn't have a fan for cooling. I'd rather use a fuse to protect the system from thermal meltdown. I would not size according to the peak rating of 4000W as this unit might be lucky to handle that for about 2 ms.

There is no way that inverter will sustain 2000W continuous.

I have noticed with cheap inverters, usually as voltage sags, the amps actually decrease.

 

[JanC]

I'm pretty certain it would be best to derate that inverter as it doesn't have a fan for cooling. I'd rather use a fuse to protect the system from thermal meltdown. I would not size according to the peak rating of 4000W as this unit might be lucky to handle that for about 2 ms.

There is no way that inverter will sustain 2000W continuous.

I have noticed with cheap inverters, usually as voltage sags, the amps actually decrease.

This is a pretty expensive inverter. It costs 600€. How do you know it doesn't have a fan? On this picture it seems like there is a fan.

 

[Zwy]

This is a pretty expensive inverter. It costs 600€. How do you know it doesn't have a fan? On this picture it seems like there is a fan.

Must have confused it with another thread with a 1500W inverter. Carry on.

I run a 2200W Giandel, which is similar, in the back of my cab on my truck camper rig. It's only purpose is to provide 120V to the rear of the rig in order to charge the 24V bank thru the Growatt if I ran into a few days of limited sun or if the Growatt were to somehow break down on a trip. I do not believe in running full output sustained. I derate most of the cheaper stuff. Usually only run the Giandel at around 800W (30A charging on the 24V bank). Not often I need it, it's one of those just in case. I would not rely upon it for all the needs in my camper due to it is 12V.

I did run 2/0 cable to it from the second alternator due to the voltage drop with the 12V. I think the run from from alternator to inverter location was 20 feet.

 

[JanC]

The following assumes you have no other loads except the inverter.
2000 ac watts / .85 conversion factor / 20 volts low cutoff = 116.55011655 inverter amps
116.55011655 inverter amps / .8 fuse headroom = 147.058823529 fuse amps

That means minimum 4 awg pure copper wire with 105C insulation and a 150 amp fuse of sufficient breaking capacity for the battery chemistry and voltage.
If you have an inverter/charger with programmable disconnect you can use 24 volts for the volt cutoff but the result won't change much

The other consideration in wire sizing is voltage drop.
This calculator will help.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

Concepts for further investigation:
80% rule
inverter low voltage disconnect
over-current protection breaking capacity
dead short ampacity by battery chemistry and nominal voltage

UPDATE: recalculated for 2000 watt continuous inverter

Thanks for the calculation. I'm planning to build my system like on the diagram I attached. No bus bars. Does it mean I need to use three T-class fuses if I have 3 pairs of cables connected to the battery terminals? I have a 24v 100ah lithium battery

 

[Alice2021]

Only read the continuous power when we choose inverter, for your 2000W inverter, 200amp fuse is OK.
For the cables, please choose short and thick cable as you can ,in order to pass high current appliance. 3AWG within 3 ft, 0AWG within 6ft, the thicker the better