Author Topic: AC Voltage Rating Of Battery Clamps  (Read 1357 times)

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Offline tommygdawg

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AC Voltage Rating Of Battery Clamps
« on: September 29, 2016, 02:44:44 am »
Hey all,

Just a simple curiosity here. I've noticed when looking for replacement battery clamps (ie for jump starting a car), they're often rated for insane amperages of 800 amps at 12V DC. However, what would their equivalent rating be at 120V or 240V AC? Do they handle this kind of voltage? It leads me to wonder, is there some sort of equation for figuring out how DC and AC relate to each other in terms of ampacity and max voltage?

Thank you!
 

Offline SeanB

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Re: AC Voltage Rating Of Battery Clamps
« Reply #1 on: September 29, 2016, 02:58:13 am »
Still 800A ( though I really doubt that the common cheap cables can actually handle 800A without bursting into flames after 3 seconds), though at any voltage over 50V they will be regarded as a massive safety hazard, as there are very large accessible metal parts in the clamps.
 

Offline tommygdawg

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Re: AC Voltage Rating Of Battery Clamps
« Reply #2 on: September 29, 2016, 03:34:11 am »
Right, but ampacity is still separate from voltage, though they are related. IE, you can have a clamp that is rated for sustained 800A at 12V DC, but it wouldn't still be able to carry sustained 800A at 240V AC, right?

By the way, I'm speaking only of the clamps themselves here and merely the difference in rating for DC vs AC. For the sake of this specific example, I'm not worried about the wires themselves. Though I suppose it doesn't really matter if they're legitimately rated for what the clamps are.
 

Offline Assafl

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Re: AC Voltage Rating Of Battery Clamps
« Reply #3 on: September 29, 2016, 03:52:08 am »
Voltage rating is an attribute of the insulation. Current rating is an attribute of the copper cross section. Hence, if it can carry 800A at 1V, it can carry 800A at 1000V.

But the insulation will go and you'll get a shocker if you touch it.

NB: Don't confuse the voltage drop on the wire with the voltage rating on the wire. Voltage rating is what keeps you safe. I*R is the voltage drop on the wire. Since I is 800A (theoretically) and R stays the same, the voltage drop is the same and so is the power dissipated in the cable (I^2*R).

 

Offline ovnr

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Re: AC Voltage Rating Of Battery Clamps
« Reply #4 on: September 29, 2016, 06:48:14 am »
In fact, your battery clamps will happily pass several gigawatts, all day long! You just need to bump the voltage up high enough.

(At which point you might want to not touch them. And keep them a bit further apart than the usual battery pole spacing.)
 

Offline RGB255_0_0

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Re: AC Voltage Rating Of Battery Clamps
« Reply #5 on: September 29, 2016, 07:06:34 am »
Ampacity also derates based on insulation, specifically the temp rating. NEC uses https://www.copper.org/applications/electrical/building/pdf/copper-wire-install-standard.pdf
Your toaster just set fire to an African child over TCP.
 

Offline Assafl

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Re: AC Voltage Rating Of Battery Clamps
« Reply #6 on: September 29, 2016, 04:40:47 pm »
Ampacity also derates based on insulation, specifically the temp rating. NEC uses https://www.copper.org/applications/electrical/building/pdf/copper-wire-install-standard.pdf

There is the "safety" question and there is the "physics" question. Two completely unrelated questions with diametrically opposite answers.

For the safety (or regulation) question - the answer is no. An 800W cable rated at 12V should never ever be connected to 110V. Ratings should be conservatively followed for safety's sake. Never exceeded.

For the physics question - ampacity and insulation are irrelevant. Carry 800A at 100,000V? NP - Just space them such as they won't arc. You shouldn't do it because of safety. Not because of the cross section of the copper wire. People like Photonic Induction on youtube do this kind of crazy sh*t all the time and get away with it as long as clearances and cross sections are maintained.

BTW - Safety regulations are rarely written with physics in mind. Usually it is an organization that analyzes past accidents (In the case of the National Electric Code it is an organization called the National Fire Protection Association) and decides what is the cost-benefit of different safety margins for different use scenarios. Cross section of cables is something they look at (since too high a gauge will heat up and may cause a fire risk) - but so will thin insulation that falls apart in a conduit and causes a short.
 

Offline amyk

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Re: AC Voltage Rating Of Battery Clamps
« Reply #7 on: September 29, 2016, 08:36:21 pm »
Right, but ampacity is still separate from voltage, though they are related. IE, you can have a clamp that is rated for sustained 800A at 12V DC, but it wouldn't still be able to carry sustained 800A at 240V AC, right?
The current rating is easy to see: there is resistance, and current flow creates heat, thus limiting the maximum current that can flow before things get too hot.

The voltage rating is a bit more subtle --- remember that voltage is always relative, so 12V or 240V relative to what? Others have mentioned it's the voltage across the insulation, which is the only part of the clamp that can sustain a voltage across it large enough without exceeding the current limit.

In some ways, it's like a very low-value resistor: a "250V" 0.01 ohm resistor would have to have 25kA(!) flowing through it before the voltage across its terminals reaches that high, and be dissipating 6.25MW, likely far in excess of its wattage rating.
 


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