How to properly size fuse for Victron Phoenix 12/1200 inverter?

[Johnny Doe]

Hi,

I have recently bought a Victron Phoenix 12/1200 inverter and want help finding correct size for a external fuse on the positive wire, there is already a internal 200A MIDI 32V fuse inside the inverter.

I'm planning on using a AWG2 (~35mm2) wire to my Victron Phoenix 12/1200 inverter which is recommended in the manual if wire length is longer than 1,5 meters. My wire will be much shorter, but I want to minimize power losses.

Datasheet
Manual

Inverter specifications:
Voltage = 12V
Continuous power = 1000W
Peak power = 2200W
Max efficiency = 91%

1) What is the max current a 2 AWG wire can handle? I have seen everything from 125-200A on the internet.

2) Should I size my fuse for cont. power or peak power?

(1000 / 12) * 0,91 = 75,83A
75,83 * 1,25 = 94,78A

(2200 / 12) * 0,91 = 166,83A
166,83 * 1,25 = 208,54A

3) What kind of fuse should I use? Mega, ANL or T fuse? Why?

 

[FilterGuy]

'm planning on using a AWG2 (~35mm2) wire to my Victron Phoenix 12/1200 inverter which is recommended in the manual if wire length is longer than 1,5 meters. My wire will be much shorter, but I want to minimize power losses.

Datasheet
Manual

I would always go with at least the recommendation in the manual (or larger).

This resource can help you calculate a minimum size for the inverter wire:

Calculating Inverter DC Wire & Fuse Size

Note: Click on the Orange 'Download' button at the top of the page to get the paper. When designing a system, some of the most critical connections are the big wires to the inverter. The best advice is to do what the inverter manual says, but...

diysolarforum.com

1) What is the max current a 2 AWG wire can handle? I have seen everything from 125-200A on the internet.

The question is incomplete. The current rating of the wire is based on a combination of the gauge and the thermal rating of the insulation. I like to use this chart for determining the wire size to use.

https://nwm2.wpenginepowered.com/wp-content/uploads/ABYC-Unbundled-Current-Rating-Chart.jpg

I always use Marine grade wire with 90deg C rating so 2 AWG can handle up to 180A (outside of an engine compartment. (If the setup is going to be someplace that gets very hot, I would use the lower rating even if it is not an engine compartment.)

2) Should I size my fuse for cont. power or peak power?

(1000 / 12) * 0,91 = 75,83A
75,83 * 1,25 = 94,78A

(2200 / 12) * 0,91 = 166,83A
166,83 * 1,25 = 208,54A

I generally fuse for continuous power.

Also, note that the wire size should be chosen to handle *more* than the rating of the fuse. So, if you size the fuse for continuous 94,78A, you will round that up to a 100A fuse and the wire must be sized to handle at least 100A.

3) What kind of fuse should I use? Mega, ANL or T fuse? Why?

In a simplified view of things, there are 4 key ratings for fuses

1. Trip value. This is what everyone thinks about when choosing a fuse.
2. Trip Speed. How fast will the fuse blow when the current hits the trip value? We don't typically talk about this on the forum, but it can be important. (There are all sorts of complexities around this that I won't go into here)
3. Voltage rating. Almost any fuse you choose will work for a 12V system. However, many DC fuse types are only rated for 12V and 24V systems.
4. Amper Interrupt Capacity (AIC). When there is a dead short, the current from a LiFePO4 battery can go to 10 or 20 thousand amps before the fuse blows (particularly on 48V systems with many batteries in parallel). Most fuses don't have AIC ratings this high so technically they should not be used with LiFePO4 systems. Of the fuses you list, the only one with a 20KA AIC rating is the T-Class fuse.

The general practice on the forum is to have a T-class fuse on the battery and then use whatever is convenient elsewhere in the system. To tell the truth I have never really understood the logic of this. If a T-class is needed on the battery, why wouldn't it be needed elsewhere?
Note: T-class fuses are fast-blow fuses. They can't handle sustained surges above their rating without blowing.

Victron documentation and Victron equipment recommend Mega Fuse. Victron even has 48V-rated Mega fuses. This is very different than the people on the forum that insist on T-class fuses. Victron is a very high-quality supplier and I give their recommendations a *lot* of credence and I have never heard a rationalization of the two positions. I *think* the difference is between theory and practice. In theory, the current could go to 10 or 20KA, but in practice, the resistance of everything in the circuit like the wire, crimps, shunts, and even fuses will keep the max current down to something more manageable.

I might get beat up for saying this, but for a 12V system I would feel comfortable with a Mega Fuse or an ANL.

 

[John Frum]

I prefer 105C rated insulation because then you can use a 200 amp fuse for 35mm2 wire.
Class t fuses are expensive and not required for a 12 volts nominal LFP based system.
Both ANL or MRBF have sufficient breaking capacity at the required voltage for this application.
I prefer MRBF fuses because they can be mounted on the battery terminal in a special carrier.
This just about eliminates the chance of an un-protected short.

MRBF Terminal Fuse Block - 30 to 300A - Blue Sea Systems

Easily and economically satisfies ABYC 7 circuit protection rule by mounting on a 3/8 battery post, battery switch or bus bar.

www.bluesea.com

 

[Pi Curio]

I would always go with at least the recommendation in the manual (or larger).

This resource can help you calculate a minimum size for the inverter wire:

Calculating Inverter DC Wire & Fuse Size

Note: Click on the Orange 'Download' button at the top of the page to get the paper. When designing a system, some of the most critical connections are the big wires to the inverter. The best advice is to do what the inverter manual says, but...

diysolarforum.com

The question is incomplete. The current rating of the wire is based on a combination of the gauge and the thermal rating of the insulation. I like to use this chart for determining the wire size to use.

https://nwm2.wpenginepowered.com/wp-content/uploads/ABYC-Unbundled-Current-Rating-Chart.jpg

View attachment 119059

I always use Marine grade wire with 90deg C rating so 2 AWG can handle up to 180A (outside of an engine compartment. (If the setup is going to be someplace that gets very hot, I would use the lower rating even if it is not an engine compartment.)



I generally fuse for continuous power.

Also, note that the wire size should be chosen to handle *more* than the rating of the fuse. So, if you size the fuse for continuous 94,78A, you will round that up to a 100A fuse and the wire must be sized to handle at least 100A.


In a simplified view of things, there are 4 key ratings for fuses

1. Trip value. This is what everyone thinks about when choosing a fuse.
2. Trip Speed. How fast will the fuse blow when the current hits the trip value? We don't typically talk about this on the forum, but it can be important. (There are all sorts of complexities around this that I won't go into here)
3. Voltage rating. Almost any fuse you choose will work for a 12V system. However, many DC fuse types are only rated for 12V and 24V systems.
4. Amper Interrupt Capacity (AIC). When there is a dead short, the current from a LiFePO4 battery can go to 10 or 20 thousand amps before the fuse blows (particularly on 48V systems with many batteries in parallel). Most fuses don't have AIC ratings this high so technically they should not be used with LiFePO4 systems. Of the fuses you list, the only one with a 20KA AIC rating is the T-Class fuse.

The general practice on the forum is to have a T-class fuse on the battery and then use whatever is convenient elsewhere in the system. To tell the truth I have never really understood the logic of this. If a T-class is needed on the battery, why wouldn't it be needed elsewhere?
Note: T-class fuses are fast-blow fuses. They can't handle sustained surges above their rating without blowing.

Victron documentation and Victron equipment recommend Mega Fuse. Victron even has 48V-rated Mega fuses. This is very different than the people on the forum that insist on T-class fuses. Victron is a very high-quality supplier and I give their recommendations a *lot* of credence and I have never heard a rationalization of the two positions. I *think* the difference is between theory and practice. In theory, the current could go to 10 or 20KA, but in practice, the resistance of everything in the circuit like the wire, crimps, shunts, and even fuses will keep the max current down to something more manageable.

I might get beat up for saying this, but for a 12V system I would feel comfortable with a Mega Fuse or an ANL.

I've ordered a marine grade 2 AWG wire with 70C insulation rating, I see that the chart has only 60C and the next one is 75C.

How can I guess what would be the max A of the wire at 70C?

I've already ordered the remaining component which is Victron's shunt, but I'm unable to find the information on which T class fuse to go with. There are a lot of charts with all sorts of ratings, some go as low as 115A for 2AWG wire at 75C.

Maybe it's just my lack of understanding or experience with solar, but it's impossible for me to determine what class T fuse size should I go with when there are so many conflicting wire max amperage charts for the same wire gauge out there.

At this point, I really don't know what is the actual 2 awg wire rating, let alone what fuse size. And class T fuses aren't cheap.

 

[FilterGuy]

I've ordered a marine grade 2 AWG wire with 70C insulation rating,

I have never seen 70C marine grade wire. Got a link to it?

 

[Pi Curio]

I have never seen 70C marine grade wire. Got a link to it?

Oceanflex Extra Flexible Tinned Copper PVC Battery Cable - 35mm² 240A

Oceanflex 240A 35mm² extra flexible, tinned copper PVC marine grade battery cable. Particularly suitable for corrosive environments.

www.12voltplanet.co.uk

 

[Pi Curio]

I have never seen 70C marine grade wire. Got a link to it?

Guess, you can't tell the max current this wire could carry either?

Heh, the last thing about solar I thought would be difficult to figure out is the wire ampacity. What is the reason there are so many variations in a specific wire gauge ampacity is beyond me. From barely 90 amp up to 200A+ for 2AWG wire. Other than obviously, insulation rating.

 

[Tomthumb62]

Guess, you can't tell the max current this wire could carry either?

Heh, the last thing about solar I thought would be difficult to figure out is the wire ampacity. What is the reason there are so many variations in a specific wire gauge ampacity is beyond me. From barely 90 amp up to 200A+ for 2AWG wire. Other than obviously, insulation rating.

My understanding for the range of ratings is it has everything to do with the insulation. The metal wire can carry the load, but if the insulation catches fire that’sn no good. So if whatever chart you’re viewing doesn’t list what kind of insulation (aka temperature rating) the chart refers to, then the chart is nearly meaningless and should be ignored.

 

[Pi Curio]

My understanding for the range of ratings is it has everything to do with the insulation. The metal wire can carry the load, but if the insulation catches fire that’sn no good. So if whatever chart you’re viewing doesn’t list what kind of insulation (aka temperature rating) the chart refers to, then the chart is nearly meaningless and should be ignored.

Agreed, good info.

I have a much better understanding of the topic these days.

Thanks for chiming in, truly appreciated.

 

[FilterGuy]

The other complicating factor is that different standards bodies require different margins of safety. The US NEC charts are very conservative so for any given gauge and insulator rating, they will have a much lower current rating than the ABYC (American Boating and Yacht Council).

I have never looked at Ampacity charts for EU standards, but it is likely they will be different than either the NEC or ABYC.

Speculation follows:
In a house, space and weight don't matter much but if you have ever done boat wiring, you know that there is never enough space for that wire you want to run.... so it is easy to see why the ABYC would opt for smaller wires where they can. However, because they are less conservative, they have a different spec for inside the engine compartment where it is really hot.

 

[Pi Curio]

The other complicating factor is that different standards bodies require different margins of safety. The US NEC charts are very conservative so for any given gauge and insulator rating, they will have a much lower current rating than the ABYC (American Boating and Yacht Council).

I have never looked at Ampacity charts for EU standards, but it is likely they will be different than either the NEC or ABYC.

Speculation follows:
In a house, space and weight don't matter much but if you have ever done boat wiring, you know that there is never enough space for that wire you want to run.... so it is easy to see why the ABYC would opt for smaller wires where they can. However, because they are less conservative, they have a different spec for inside the engine compartment where it is really hot.

Yea, back then when I asked the question all these charts and different ratings were confusing.

 

[KITROBASKIN]

J. Doe thanks for asking this question. You have helped others, including me.

I guess another element is to consider getting a quality fuse from a reputable company.