The wide prong is neutral on 15 amp US power outlets.

Is there a reason it was designed this way, or could the narrow slot just as easily have been neutral instead? Was this design decision arbitrary?

  • There's really no such thing as arbitration in engineering otherwise a lot of people would get hurt using the products we design. I think it would be best if refer to NEMA Standards to kind of point you in the right direction. – KingDuken Sep 21 at 14:42
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    @KingDuken Tell Ben Franklin before he arbitrarily decides the direction of "current"... – Elliot Alderson Sep 21 at 14:49
  • @ElliotAlderson Well, at least we've been consistent LOL :) – KingDuken Sep 21 at 14:52
  • without a definitive source - I suspect that polarized plugs came along after non-polarized versions. It makes sense to make one prong wider, forcing the use of polarized receptacles, also forcing the Neutral only to the dedicated neutral slot. – Chris Knudsen Sep 21 at 14:56
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    @king arbitrary decisions become standards all the time. – Passerby Sep 21 at 15:48
up vote 22 down vote accepted

I think the logic is that it's more important that the neutral wire of some appliance never be connected to hot, than it is for the hot wire not to be connected to neutral.

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    I like this answer. Instead of thinking of the polarization problem as "let's make a plug that can go in only one way" to instead "let's make sure the neutral can never go in the hot." – rrauenza Sep 21 at 16:55
  • There is no actual need for such a requirement. Many sockets don't have a fixed polarity (like CEE 7/3, type C, type F) – asdfex Sep 21 at 18:10
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    It had also crossed my mind that a tinier hole for HOT makes it just a little more difficult to stick a butter knife into it. So if you want to make one larger and one smaller, you'd go with the larger one being neutral since that one is more likely to allow more of the butter knife to enter into it. This is my own theory based upon decades of watching little 1 1/2 yr to 2 yr old children playing on the floor, where the wall output is EXACTLY at their eye level when they are on their butts in diapers. I wouldn't have gone the other way as a designer, for sure, from only two options to choose. – jonk Sep 21 at 19:21
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    @asdfex While I personally would not rely on these things, it may make the appliance more resilient against accidents if it has a light bulb socket where one terminal is easier to touch, or if it has a single-pole switch. – wrtlprnft Sep 21 at 20:49
  • @asdfex The 7/3 plug is grounded as is type F which largely obviates the need for a polarized plug anyways. Type C is being phased out for having no notion of protective grounding, so is a poor example. – user52386 Sep 21 at 22:33

I don't know a definitive historical answer. From an electrical perspective there's no reason to prefer one over the other, at least once the connection is made.

There are two potential reasons I can think of:

1) If the plug is only partially inserted (or has been slowly falling out), the wider prong may be more likely to make contact. If only one of the two is connected, it's safer for it to be the neutral one.

2) If a child happens to pick up a paperclip or some other bit of metal and decides to stick it in the plug, there's a chance it will only fit in the larger of the two slots, or maybe psychologically they would tend to choose the larger of the two slots. Don't rely on this!

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    1 doesn't make sense. The width would not make more contact. If that was the case, then it would be longer. And that's also why the ground pin on a grounded connector is longer than the hot/neutral. – Passerby Sep 21 at 15:54
  • Indeed, the ground pin (on grounded outlets) is a little longer, so that it's the first to make contact, and the last to lose it. I believe when polarization became a "thing" and the decided to make one of the current prongs wider, they decided against making the "hot" one any more susceptible to foreign objects. So the neutral got bigger. – CCTO Sep 24 at 16:51

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