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Old 15th February 2004, 07:33 PM   #1
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Default Current Ratings at different voltages

Looking at several different types of components (switches, relays, fuse holders, cable, etc.); I wondered what the current ratings were. Most were rated at 230v AC. if the components are used at a low voltage, and with DC instead of AC, what effect would this have on the current rating?

Say I have a fuse holder, and is rated at x amps at y volts, AC. what would the rating be at z volts DC? How can I work this out? For cable, I can look at the thickness, but without actually testing them to destruction, how can I find the current rating of other components?

I have some fuse holders rated at 13A 230V AC, and I have some 15A fuses. Used at say, 12v DC, will the fuse holder melt before or after the fuse?

Any help/advice appreciated.
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Old 15th February 2004, 07:52 PM   #2
sreten is offline sreten  United Kingdom
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for current interruption devices, generally 250VAC devices
are rated at 160VDC, the difference due to the zero voltage /
current crossing nature of AC voltage/current.

For thermal ratings e.g. cables voltage is relatively immaterial,
current counts, AC RMS or DC makes no difference.

Fuse holders again voltage is usually immaterial,
but you obviously cannot use them for 3kV.

sreten.
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Old 15th February 2004, 08:06 PM   #3
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Quote:
Originally posted by sreten
for current interruption devices, generally 250VAC devices
are rated at 160VDC, the difference due to the zero voltage /
current crossing nature of AC voltage/current.

For thermal ratings e.g. cables voltage is relatively immaterial,
current counts, AC RMS or DC makes no difference.

Fuse holders again voltage is usually immaterial,
but you obviously cannot use them for 3kV.

sreten.
So you are saying that the current rating is approximately correct for any (sensible) voltage?
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Old 15th February 2004, 08:38 PM   #4
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The voltage rating is for the voltage it is guaranted to not Arc-Over (has to do with the spacing between parts).

The current flowing through a part generates heat, so the current rating is the maximum current that the part can handle without getting to hot (above a specified limit, such as to meet a UL/CSA/CE rating). So this means that curent rating is the max for any voltage, doesn't matter except there is a difference between DC and AC current ratings.

Note that switches and relays are spec'd for resistive or inductive loads as these behave differently.
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Old 15th February 2004, 08:47 PM   #5
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Quote:
Originally posted by crown300
The voltage rating is for the voltage it is guaranted to not Arc-Over (has to do with the spacing between parts).

The current flowing through a part generates heat, so the current rating is the maximum current that the part can handle without getting to hot (above a specified limit, such as to meet a UL/CSA/CE rating). So this means that curent rating is the max for any voltage, doesn't matter except there is a difference between DC and AC current ratings.

Note that switches and relays are spec'd for resistive or inductive loads as these behave differently.
I understand voltage ratings. My question refers to components like the one in the image, which are rated for a current at a certain voltage, in this case 230V. What will the rating be when this component is used with 12V?
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File Type: gif fuse.gif (5.4 KB, 66 views)
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Old 15th February 2004, 09:20 PM   #6
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20 Amps. The voltage doesn't matter. 20 Amps flowing through the fuse holder will cause it to heat up some. It will probally take 30 Amps for a while, but run to much Amps through and it will melt.
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Old 15th February 2004, 10:37 PM   #7
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Quote:
Originally posted by crown300
20 Amps. The voltage doesn't matter. 20 Amps flowing through the fuse holder will cause it to heat up some. It will probally take 30 Amps for a while, but run to much Amps through and it will melt.
does the fact that the voltage is AC or DC matter?
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Old 15th February 2004, 10:51 PM   #8
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It should not matter in the case of that fuseholder.

BUT..........

if it has printed on it 20 Amps AC, I would derate it for DC current.
(some relays will have an AC amps and a DC amps rating on them)

(I looked at two Fuse web sites but it seems that they are reducing the ammount of information they carry thesedays)

Are you going to be running close to 20 Amps?
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Old 15th February 2004, 11:03 PM   #9
sreten is offline sreten  United Kingdom
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A fuse holder, unlike a switch or relay does not need derating
for DC voltages as opposed to AC voltages, as arc over is not
an issue.

(Except at 3kV.)

sreten.
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Old 15th February 2004, 11:43 PM   #10
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Quote:
Originally posted by crown300
It should not matter in the case of that fuseholder.

BUT..........

if it has printed on it 20 Amps AC, I would derate it for DC current.
(some relays will have an AC amps and a DC amps rating on them)

(I looked at two Fuse web sites but it seems that they are reducing the ammount of information they carry thesedays)

Are you going to be running close to 20 Amps?
what do you think would be a sensible DC rating?

most websites seem to have less and less information on them, unless you pay. not this one though

no, probably 15A maximum. (worst case) the voltage will be between 12v and 40v DC. i wasnt looking at that particular fuse holder, it was just an example.
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