Class T fuse size - Ford Transit USA Forum
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post #1 of 49 (permalink) Old 12-02-2018, 11:06:PM Thread Starter
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Class T fuse size

I'm wondering if someone can tell me the correct amp class T fuse to use for my application. I'm trying to size the fuse from the house batteries to the inverter. I have two 100 amp hour lithium batteries and a 2000 watt inverter about a foot away. I'm using 2/0 cable with a battery switch on the positive side. Total length of cable is maybe two feet. I bought a 300 amp fuse but now am thinking a 200 amp is maybe safer?
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post #2 of 49 (permalink) Old 12-02-2018, 11:46:PM
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I looked at the max discharge amps for the battery, then aimed for a fuse that was below that rating and high enough to easily handle the max load based upon expected use scenario.

Consider your loads and the max amps that might be drawn at any time in normal use, double that and compare it to the max draw on the battery. The max current should be considerably less than the battery's max output. Aim for a happy medium for fuse rating, based upon your setup and calculated current draw for your application.

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post #3 of 49 (permalink) Old 12-03-2018, 12:56:AM Thread Starter
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I looked at the max discharge amps for the battery, then aimed for a fuse that was below that rating and high enough to easily handle the max load based upon expected use scenario.

Consider your loads and the max amps that might be drawn at any time in normal use, double that and compare it to the max draw on the battery. The max current should be considerably less than the battery's max output. Aim for a happy medium for fuse rating, based upon your setup and calculated current draw for your application.
Huh? How do I figure that out? Like the maximum draw my batteries can handle? Worst case draw would be an induction cook top (maybe 1500 watts?) or 1000 watt microwave.
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post #4 of 49 (permalink) Old 12-03-2018, 05:01:AM
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Size fuse for the wire gauge you use. I don't like to load a fuse (or circuit breaker) beyond 80%. For wire length, you have to use both power and return path if using house battery alone for inverter.

Use appropriate wire gauge for component.

http://circuitwizard.bluesea.com/
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post #5 of 49 (permalink) Old 12-03-2018, 05:10:AM
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I went with my inverter manufactures recommendation because the inverter is really what you are trying to protect.

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Last edited by ranxerox; 12-03-2018 at 05:42:AM.
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post #6 of 49 (permalink) Old 12-03-2018, 07:46:AM
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Huh? How do I figure that out? Like the maximum draw my batteries can handle? Worst case draw would be an induction cook top (maybe 1500 watts?) or 1000 watt microwave.
The battery manufacturer will publish specs for the battery. Go to their website and look at the datasheet for the model of battery you have.

As @ranxerox mentioned, if your build has one, an inverter is probably going to be the largest potential load on your system.

If, for example, the inverter is rated for 2000 Watts, using Ohm's Law you would divide 2000 Watts by 12 VDC to get 166 Amps potential draw by this device in normal use. Let's say the battery is rated to deliver up to 600 amps maximum. I would multiply 166 times 2, giving me 333 amps. This is well below the 600 max the example battery can provide, which s good.
(Note: If your potential load exceeds the battery rating you would need multiple batteries wired in parallel with appropriate wire gauge to the inverter.)

A 300 Amp fuse might be appropriate in such a system as that will protect things if a short circuit occurs that draws more than twice the expected current, while still leaving room for the occasional spike over the calculated 166 Amps if there are multiple loads being used simultaneously.

That's a simplified version. If you know you will often be running multiple loads simultaneously you should figure for that scenario's numbers.

I hope this helps.

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Last edited by Travlin; 12-03-2018 at 09:38:AM.
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post #7 of 49 (permalink) Old 12-03-2018, 07:53:AM
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Originally Posted by sportcoupe View Post
Size fuse for the wire gauge you use. I don't like to load a fuse (or circuit breaker) beyond 80%. For wire length, you have to use both power and return path if using house battery alone for inverter.

Use appropriate wire gauge for component.

http://circuitwizard.bluesea.com/
Another way to look at this is to "size wire gauge at the battery for the load supported" and fuse appropriately once this has been determined.

It can be beneficial to go one gauge size larger (smaller number) if the load indicates it will be running at the upper range of the gauge the BlueSea calculator provides. Length of the cable coming off the battery figures into this as well, so, knowing how long that run will be is part of the calculation if the batteries are placed some distance from the inverter.

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Last edited by Travlin; 12-03-2018 at 09:39:AM.
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post #8 of 49 (permalink) Old 12-03-2018, 11:43:AM
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Originally Posted by Travlin View Post

If, for example, the inverter is rated for 2000 Watts, using Ohm's Law you would divide 2000 Watts by 12 VDC to get 166 Amps potential draw by this device in normal use.
That is assuming that inverter's are 100 percent efficient at inverting power, They are not, It is more in the realm of 75 to 95 percent efficient depending on the quality of the inverter.

https://www.e-education.psu.edu/eme812/node/738

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post #9 of 49 (permalink) Old 12-03-2018, 12:36:PM
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I have a Magnum 1000 watt inverter and the mfg. recommended a 150 amp type T fuse. What does your inverter mfg. recommend? Battery is a single 255 amp-hr 8D AGM. Same system installed on the sold Sprinter as in the current Transit. 10 years with system.

The 1000 watt Magnum will run a "600" watt Westinghouse microwave (950 watts actual)

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post #10 of 49 (permalink) Old 12-03-2018, 12:42:PM
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Most LiPO4 batteries can do 1C, meaning they are continuously rated to output their capacity in 1 hour. This output is cumulative per battery, so two 100Ah 1C batteries can output a combined 200A continuous. Surge capacity is often 2C, or double the capacity rating, for a short time such as 30 seconds. 1/2 second surge capacity can be 3 or 4C. You really need to plan for the surge capacity of your inverter + other 12v loads, not just the published watt rating.


The 2000W Victron inverter I'm using actually has a surge rating of 3000W (~250A). I plan to over build my system to handle at least this surge rating continuously. I have two 1C 100A Battle Born LiPO4 batteries. I'll wire the batteries and distribution with at least 1/0 wire (~300A) and a 300A T fuse. This way the system can handle the inverter surge for up to 30 seconds, then the BMS will shutdown the batteries if necessary. The 300A fuse will protect the wire and batteries in case of a short, well before the BMS 1/2 sec surge protection kicks in.
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Last edited by Inkog; 12-03-2018 at 12:44:PM.
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