60 amp 12 volt fuse = 150 amp 32 volt fuse? Roughly?

softdown · September 2019

Trying to confirm this equation a bit. #MyDodgyMath

softdown · September 2019

Thinking my math was probably worse than dodgy. Maybe 60 amp 12 volt ~ 20 amp 32 volt? Someone here knows.

BB. · September 2019

Certainly power=voltage*current, so,

12v * 60a = 720 VA

720 VA / 32 volts = 22.5 amps

However, you need to understand the current source (how much current it can supply) and the loads (how much current they will take).

Fuses and breakers are there to protect the wiring from overheating and catching fire.

Bill

icarus · September 2019

Fuses (and breakers) are there to protect the wiring from over current heating. They need to be sized according to amperage regardless of voltage. Amperage (current) is amperage...voltage is voltage. Drop just an example, a #12 awg wire will handle safely 20 amps and therefor that is the “normal” fuse size for it, regardless of the voltage.

Keep in mind that fuses need to be sized for the smallest wire size in the circuit that it is protecting, so if you have a piece of #12 feeding into a box and coming out with #14 the correct fuse size is 15 amps for the #14 not 20 for the #12. There are exceptions for 120 vac plugin devices with limited cords, like lamps that are wired with #16 and even #18 wire. That said, the classic fire situation is a lamp cord coiled up under a rug, so it runs warm. Branch circuit fused with 20 amps, but the warm wire keep heating up with the load and lights itself on fire. Old fashioned standing lamps may have had 3 or 4 hundred watt bulbs around the perimeter with one large 150 watt bulb in the centre, drawing maybe 5 amps from 110 vac, add the light weight lamp cord, a bit of resistance in the connectors, and pile a rug on it an voila...fire!

Tony

softdown · September 2019

I like to be prepared and have been stocking up on popular Mega fuses that are used in BMWs, Jaguars, golf carts and many other places. They are rated "32 volt" with most being between 100 amps and 250 amps. They are as small as 60 amp and as large as 500 amp. They are also called "slow blow" and are rated to handle a quick "pop" without disconnecting. I currently use them on my battery strings and camper control box.

Now - they mostly seem too large for almost any realistic application. It is also harder to find them in less than 100 amp sizes. Here is an example: https://www.ebay.com/itm/Blue-Sea-5105-MEGA-AMG-Fuse-200AMP/352795910688?epid=1000083973&hash=item5224466220:g:JyMAAOSwTW1dg8Mq

I'd hazard a guess that 200 amps is the average size.

Then again - I suspect most cautionary purchases fail to pan out. Though we may never know if such a purchase silently saved the day.

Proper prepping is almost unlimited in potential expenses. Especially considering that a thorough prepper must consider a couple dozen different disaster scenarios. Which lead me to the conclusion that I can give 50 reasons to prep and 100 reasons not to. But if one if ideally situated for prepping - then why not? I guess.

mike95490 · September 2019

Fuses are absolute limits, they are not power devices. A 40A fuse will blow at 40A @ 3 volts, or 40A at 200 volts.

The voltage rating on a fuse only indicates what voltage it will not arc/flash over at. (pretty much the same with circuit breakers)

And there are tolerances & known delays, 40A may take 20 seconds to let loose, but 45A goes out in 3 sec. 38A may take a week to melt the fuse

mvas · September 2019

softdown said:
60 amp 12 volt fuse = 150 amp 32 volt fuse? Roughly?
Trying to confirm this equation a bit. #MyDodgyMath

Thinking my math was probably worse than dodgy. Maybe 60 amp 12 volt ~ 20 amp 32 volt? Someone here knows.

No, not even "roughly", the answer is never.

The fuse blows from HEAT = I^2 x R, over a period TIME.
From an overload point of view, the fuse has no idea what Voltage is across the load.
The fuse could care less about the voltage, until it is time to break the current flow and extinguish the arc.

You should never use a fuse in circuit that exceeds the Voltage rating or the Amp rating of the fuse.

softdown · September 2019

Seems that the 32 volt rating may be almost superfluous at times. It would take a lot of current to "arc/flash over". It is more about the amps - not the volts. According to some - not so according to others.

mvas · September 2019

mike95490 said:

Fuses are absolute limits, they are not power devices. A 40A fuse will blow at 40A @ 3 volts, or 40A at 200 volts.
The voltage rating on a fuse only indicates what voltage it will not arc/flash over at. (pretty much the same with circuit breakers)

And there are tolerances & known delays, 40A may take 20 seconds to let loose, but 45A goes out in 3 sec. 38A may take a week to melt the fuse

I disagree.
Typically, a 40 amp fuse should never blow with only 38 amps flowing.
In fact, a 40 amp fuse should never blow at 40 amps either.
Where did you get 20 seconds?

On average, it take about 16 Minutes to blow a 40 amp automotive fuse with a whopping 50 amps flowing.

A 40 amp fuse is not designed to blow at 40 amps,
it is designed to allow 40 amps to flow,
and at some amperage above 40 amps, with enough time, it will eventually blow.
This is called the i2t time-current characteristic of the fuse.

mvas · September 2019

softdown said:

Seems that the 32 volt rating may be almost superfluous at times. It would take a lot of current to "arc/flash over". It is more about the amps - not the volts. According to some - not so according to others.

When selecting a fuse for a DC application ...
a) The voltage rating of the fuse must never be exceeded.
b) The amperage rating of the fuse must never be exceeded.

Suggestion otherwise is very bad engineering.
Both the voltage rating and the amperage rating of a fuse, is important when trying to break a DC arc, even at 12 volts.

BB. · September 2019

More or less, any voltage over ~12 VDC has the potential to sustain an Arc.

The AC and DC rating voltages for a fuse are there to ensure that an Arc will not be sustained if a fuse (or breaker) pops. "Arc Flash" (basically explosive arcs in higher voltage/higher current situations that releases heat in the ~30,000F+ and hard UV radiation for AC voltages at ~400 VAC and above???).

And there is the current a fuse will pop (which is a current vs time--High current, the faster a fuse will pop). And the minimum current at which a fuse will not open (I use 80% of rating for "typical" devices--Although, there are different rating methods for different products).

The AIC Amp Interrupt Current is the maximum current from the "supply" -- If you exceed that source current, then the fuse/breaker cannot manage that energy and will self destruct/sustain an arc/etc.

This link has (probably) more than you would ever want to know about fuses/breakers/overcurrent protective devices:

https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

-Bill

mike95490 · September 2019

mvas said:

Typically, a 40 amp fuse should never blow with only 38 amps flowing.
In fact, a 40 amp fuse should never blow at 40 amps either.
Where did you get 20 seconds?

On average, it take about 16 Minutes to blow a 40 amp automotive fuse with a whopping 50 amps flowing.

A 40 amp fuse is not designed to blow at 40 amps,
it is designed to allow 40 amps to flow,
and at some amperage above 40 amps, with enough time, it will eventually blow.
This is called the i2t time-current characteristic of the fuse.

There are dozens of specs for fuses. Speed, tolerance, voltage. You can get rapid blow , slow blow, thermal delay.......

As fuses age, their blow characteristics change. I've dealt with mil-spec gear and replaced many, many fuses and have seen all sorts of surprising things, including what you mentioned. I've seen fuse holders melted, and fuses never blow, because automotive fuse systems are garbage.

60 amp 12 volt fuse = 150 amp 32 volt fuse? Roughly?

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