2020 master prep question (Ray Holder)

[Dsg319]

Question is written, Where a 2 gang box contains two (2) single-pole switches, unless the box is equipped with permanently installed barriers, voltage between the adjacent switches shall not be in excess if________?
A. 120volts
B. 277volts
C. 480volts
D. 240volts

I was having problems locating the correct article to begin with (I know one should have thought to look under switches lol). BUT when I see the exam books answer they had chosen answer B. (277volts)

When I read the reference they used to get the answer, code language says.

404.8 (B)
A snap switch shall not be grouped or ganged in enclosures with other snap switched, receptacles, or similar devices. Unless they are arranged so that the voltage between adjacent devices does not exceed (300volts), or unless they are installed in enclosures equipped with identified, securely installed barriers between adjacent devices.

Shouldn’t 300volts have been the correct answer rather than 277volts?

 

[infinity]

Yes the NEC answer is a maximum of 300 volts but the test quesion gives you three choices 120, 240, and 277. Like you I would choose 277 but IMO the question is poorly written. In the real world is there a scenario where the voltage between the two adjacent switches can actually be 277 volts?

 

[Dell3c]

yes... you are correct.

 

[Dsg319]

Yes the NEC answer is a maximum of 300 volts but the test quesion gives you three choices 120, 240, and 277. Like you I would choose 277 but IMO the question is poorly written. In the real world is there a scenario where the voltage between the two adjacent switches can actually be 277 volts?

Exactly, real world I would imagine adjacent devices to be 120/277volts.

 

[don_resqcapt19]

That is not really a valid test question, and would likely be tossed out if someone would challenge it. However when taking a test you have to work with what they give you and the answer would be 277.

Where you have a single pole switch from a 208Y/120 volt system next to a single pole switch from a 480Y/277 volt system, there are at least 7 different voltages that could appear between the terminals of the two switches. Four of those 7 exceed 300 volts. (note that some of the 7 would only arise in a very unusual case, the case where both primaries for that transformers that supply the two systems are each supplied from medium voltage primaries)

 

[Dsg319]

That is not really a valid test question, and would likely be tossed out if someone would challenge it. However when taking a test you have to work with what they give you and the answer would be 277.

Where you have a single pole switch from a 208Y/120 volt system next to a single pole switch from a 480Y/277 volt system, there are at least 7 different voltages that could appear between the terminals of the two switches. Four of those 7 exceed 300 volts. (note that some of the 7 would only arise in a very unusual case, the case where both primaries for that transformers that supply the two systems are each supplied from medium voltage primaries)

Wow, could you explain to me the 7 different possible differences. Not that I don’t believe you I just want to know and how. At least the ones that are practicable if you do not mind.

 

[don_resqcapt19]

Wow, could you explain to me the 7 different possible differences. Not that I don’t believe you I just want to know and how. At least the ones that are practicable if you do not mind.

It is vector addition of the voltages based on their phase angles. This works because both systems are wye systems and the XO points are connected together via the grounding and bonding connections. There are really only 3 common ones. These are based on the 30° phase shift that occurs in a delta wye transformer. Those 3 voltages are 183 volts, 302 volts and 386 volts. The 183 would be between A phases of both systems, the 386 would be from A phase to B phase, and the 302 from A to C phases.

 

[mikeames]

WOW if there was a jeopardy game for electricians this would be a a good one.

Where you have a single pole switch from a 208Y/120 volt system next to a single pole switch from a 480Y/277 volt system, how many different voltage possibilities exist between the two?

What is Seven!

I never stop learning here.

 

[Dsg319]

And yet another one that is wrong. My code book says 3/0 aluminum THWN has an ampacity of 155amperes.

I think they must have gotten mixed up with the 90degree column if I was betting.