Help with test question

[nholdsy]

Hi Everyone, love the forum so wanted to join and get some opinions on a test question I had and am not sure if I answered correctly.

How many thermal overloads are needed for one 240v single phase motor and one 480v 3 phase motor?

I used table 430.37 and came up with 5 total, 2 for the single phase motor and 3 for the 3 phase motor.

The reason I ask is on both my list and a co-workers list of common incorrect answers from our state masters exams this question was listed. I would love some backup or a correction if necessary. Thanks!

 

[barbeer]

I agree with your answer.

 

[DBack Elec]

Yeah, have to agree 5.

 

[Controls]

Without knowing all the facts. I would say that this a tricky question for me.Assuming that we are speaking about motors that controlled by magnetic starters and no VFDs or other control means used, 480V 3 phase would require 3 thermal overloads. 240V single phase is the tricky one. First, what is the horse power on the motor?. If I am not wrong, if it is 1HP or less, it can be powered by just using a power cord without using any contactors, or a magnetic starter and there may or may not be an internal thermal overload built in with the motor itself.Other possibilities, 240V power with a hot and neutral ( other countries).Someone used 2 hot legs to get 240V ?. May be I am thinking too deep here.However, I have seen many different types of installations that I would have hard time answering this question. I would say answering it with total of 5 would be too easy. I would answer it as 4.May be others could answer it better. I am just brain storming here.Sorry, If i confused you.

 

[billsnuff]

controls, i'm with you. the answer could be two if internal, could be fuses, 3ph 4 wire requires a fuse in the ungrounded....I would start with 430.32 A 1-4 before the chart in 430.37. But i said 5 when i first read it.

 

[Krim]

Without knowing all the facts. I would say that this a tricky question for me.Assuming that we are speaking about motors that controlled by magnetic starters and no VFDs or other control means used, 480V 3 phase would require 3 thermal overloads. 240V single phase is the tricky one. First, what is the horse power on the motor?. If I am not wrong, if it is 1HP or less, it can be powered by just using a power cord without using any contactors, or a magnetic starter and there may or may not be an internal thermal overload built in with the motor itself.Other possibilities, 240V power with a hot and neutral ( other countries).Someone used 2 hot legs to get 240V ?. May be I am thinking too deep here.However, I have seen many different types of installations that I would have hard time answering this question. I would say answering it with total of 5 would be too easy. I would answer it as 4.May be others could answer it better. I am just brain storming here.Sorry, If i confused you.

Controls,
I agree with your explanation above ... it really depends on the equipment one is using ... a 1 h.p. or under could require only a MMS , If a VFD is being used typically the thermal protection is built in , you could even use a fused disconnect switch . The functional scenarios are many and most all would still meet the required protection necessary.

As for the original question by nholdsy " How many thermal overloads are needed for one 240v single phase motor and one 480v 3 phase motor? " My answer would be 5 as he had stated ..." 2 for the single phase (240 VAC) motor and 3 for the 3 phase (480VAC) motor " assuming no other equipment but the OL protection itself .


Carl :smile:

 

[BryanMD]

The Four Disclaimers

The Four Disclaimers

Anyone else a fan of Whad'ya Know? (NPR on wekends)

On it there is a quiz portion with this in the preamble, which applies here:

All questions used on "Whad'ya Know?" have been painstakingly researched, although the answers have not. Ambiguous, misleading, or poorly worded questions are par for the course. Listeners who are sticklers for the truth should get their own shows.

 

[billsnuff]

good point lol :grin:

 

[Krim]

Anyone else a fan of Whad'ya Know? (NPR on wekends)

On it there is a quiz portion with this in the preamble, which applies here:

All questions used on "Whad'ya Know?" have been painstakingly researched, although the answers have not. Ambiguous, misleading, or poorly worded questions are par for the course. Listeners who are sticklers for the truth should get their own shows.

Well I am a stickler for the truth, and believe that that's largely why this forum exists .... to provide a clear understanding (truth) so that no one gets hurt or killed working with electricity .

Carl

 

[Dennis Alwon]

I would say answering it with total of 5 would be too easy. I would answer it as 4

I am not a motor man by any stretch of the imagination. I have rarely hooked up large motors with overloads.-- I think I did one or two in my life. I have never seen a single phase motor with overload but I am sure they exist. My question is how did you come up with 4 overloads. I see three for the 480 volt motor and either 2 or none for the 240 volt motor. How are you getting 4-- I am sure I am not understanding something here.

 

[RB1]

I think the answer is 4. One for the single-phase motor, three for the three-phase motor.

 

[Krim]

...If I am not wrong, if it is 1HP or less, it can be powered by just using a power cord without using any contactors, or a magnetic starter and there may or may not be an internal thermal overload built in with the motor itself.Other possibilities, 240V power with a hot and neutral ( other countries)...

Controls,
Upon going back and re-reading your comment ... I've heard, seen & installed 115 VAC , 1 H.P. motors both with built in thermal protection of which some have used mnftr supplied power cords and without , ie... requiring a manual motor starter switch or contactor with adjustable thermal overload protection , but I have never seen or heard of a 240 VAC motor using a plug in cord or one that uses both a hot & neutral conductor . I am aware of 220 VAC dryer and or range cords of which use 2 hots , a neutral and center ground conductors . Are you sure about 240 VAC and a neutral ?

Carl

 

[Controls]

Krim, you are right 240V hot and neutral statement is not correct. I thought, I remember having single phase motors in Europe wired up that way. I guess, my memory is playing tricks on me.

 

[Krim]

Krim, you are right 240V hot and neutral statement is not correct. I thought, I remember having single phase motors in Europe wired up that way. I guess, my memory is playing tricks on me.

Controls, I don't know much about the European electrical systems, you may be correct on that, like I've said, I wouldn't know. It would be interesting to find out though. FYI the only EU standard I do know is brown = hot, blue = neutral , & green with yellow stripe = equipment ground . Heck I don't even know what voltage that color scheme is for ... 110 vac ?

Carl

 

[Controls]

In the order of L1,L2,L3,Neutral, Ground colors
L1
L2
L3
Neutral
Earth

North America
Black
Red
Blue
White
Green

North America (newer 277/480 installations)
Brown
Orange
Yellow
White
Green

UK until April 2006
Red
Yellow
Blue
Black
green/yellow striped (green on very old installations)

Europe (including UK) from April 2004
Brown
Black
Grey
Blue
green/yellow striped

Previous European (varies by country)
Brown or black
Black or brown
Black or brown
Blue
green/yellow striped

Europe, for busbars yellow green purple – –
As many may already know:Germans label them as it R,S,T fro 3 phase, MP for neutral and SL for ground. Standard voltage set up :220V for single phase and 380V for 3 phase.

 

[Krim]

Controls , note a minor addition to the following :

North America (newer 277/480 installations)
Brown
Orange
Yellow
White or Gray
Green

I also use Black,Red,White & Green for 277 vac lighting ,
am I using the wrong colors guys ?

Carl

 

[BryanMD]

I also use Black,Red,White & Green for 277 vac lighting ,
am I using the wrong colors guys ?Carl

Not by code but by common practice.

I suppose many here have worked in other places besides their home town aside from me...

In MD NC FL LA TX NM CO CA WA MN IA OH NY MA the common colors are in fact common with no exceptions at the panels at least (that I've seen).

240/120 Black/Red/Blue/White/Green
480/277 Brown/Orange/Yellow/Gray/Green

MC whips to 277V lights however can be a complete free for all

Most EC's will *plan* for the BOY colors but inevitably by the end of the job they all seem willing to suspend their intention and use up what is remaining on hand.

 

[Krim]

" MC whips to 277V lights however can be a complete free for all "

Whew , I'm glad you had that in there , that explains why I've always used the previously mentioned colors .
Thanks,
Carl

 

[BryanMD]

Whew , I'm glad you had that in there , that explains why I've always used the previously mentioned colors .Thanks,Carl

Don't confuse my *observations* with any sort of authority ok?

FWIW I am amazed that the NEC doesn't mandate the standard especially when you consider how many other piddling and nanny state rules they are willing to push.

Surely the wire makers would love to force supply houses and EC's to have to inventory/buy more/different kinds of MC... or am I seeing this wrong?

 

[billsnuff]

thermal overloads

thermal overloads

the op said thermal overloads.......100 defines overload and thermal protector, but not thermal overloads that i could find.

430.37 devices other than fuses i agree with three for 3ph and two for
240= 5


430.36 fuses would be four with a 3ph w/gnded cond + two for 240 (120 x 2)= 6

the way the question is phrased depends on the interpretation of the reader....IMO there is not enough information to give an answer...only an opinion