Cord Cap rating, AC vs DC current & voltage rating

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dvs765

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I have a question that is not directly code related, more electrical knowledge. It is concerning the "Hubbell HBL20444B", generically a cord cap. It is rated at 30 amps, 480 VAC, but it is rated at 20 amps 250 VDC. I believe current is current, but from what I have read, AC ratings are equivalent to DC, but only good up to 30 VDC. WHY? I cannot find literature to satisfy my question. I haven't been in the apprenticeship (ABC), since 2007. Most of my recent studies have been NEC related, this question arose the other day and quite frankly I have no good answer. I am hopeful that there are other electricians that have input on this. This question holds no weight, it is simply a "useless"question that has my curiosity. Thus wanting a better understanding of the electrical field that I have been gainfully employed in since 1998. Any and all help is appreciated. ?
 
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dvs765 said:
I have a question that is not directly code related, more electrical knowledge. It is concerning the "Hubbell HBL20444B", generically a cord cap. It is rated at 30 amps, 480 VAC, but it is rated at 20 amps 250 VDC. I believe current is current, but from what I have read, AC ratings are equivalent to DC, but only good up to 30 VDC. WHY? I cannot find literature to satisfy my question. I haven't been in the apprenticeship (ABC), since 2007. Most of my recent studies have been NEC related, this question arose the other day and quite frankly I have no good answer. I am hopeful that there are other electricians that have input on this. This question holds no weight, it is simply a "useless"question that has my curiosity. Thus wanting a better understanding of the electrical field that I have been gainfully employed in since 1998. Any and all help is appreciated. ?
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It has to do with the fact that when you start to separate current carrying conductors under load, the arc that forms is heating up the conductor ends/tips/contacts. With DC, that arc lasts until the dielectric (resistance) of the air gap is high enough to extinguish it and the higher the voltage, the wider that gap must be. With AC, that arc is inherently extinguished 120 times per second and although it may reform, it gets hard to do that with every millisecond as the conductors separate and the heat energy in that arc is significantly less.

So applying those concepts to a cord and cap, part of the listing process of them involves their ability to not become flaming fire brands when you disconnect under load. Even though technically most will not say they are rated for that, testing authorities all know tht it's going to take place, so it is built in to the standards. Because this has to do with the amount of energy in the arc, that means then by comparison, a given rating of this capability with AC will be either severely limited in DC current, or at the same current will be severely limited in DC voltage. So sometimes you will see what you observed: same current at 30VDC, or less current at a slightly higher voltage like 250VDC. The same is true for contacts, switches etc., anything that breaks a circuit.
 
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