Breaker sizing

[kisa2418]

I have a question for an exam prep question. here is the question word for word

"A 7 1/2hp 3 phase motoer has a temp rise of 38c and is being protected by instantaneous trip circuit breaker. system voltage is 230 volts and the moter is code letter B. What size breaker is needed?"

A.70
b.80
c.90
d.100

The answer is 90amps

Im not sure if its right I have tried to figure it out with no luck any help would be greatly appreciated.

 

[augie47]

motor

motor

90 would not be my answer..so I'll have to wait and see who can verify that so we can both learn

 

[davidr43229]

A 7 1/2 hp motor has a fla of 22 amps. Most
designers however, assume a starting torque of less than
150 percent when using Class B induction motors to
start and run loads.
NEC art 430.52 says you can go 800% (max) for instantaneous trip breakers. I know breakers have a 40 degree C rating.
Just my $.02

 

[cpal]

is there more to the question ??

is the breaker sized for running protection or branch circuit and short circuit protection??

 

[charlie b]

NEC art 430.52 says you can go 800% (max) for instantaneous trip breakers.

But it also says you can go to 1100% for a Class B motor, which is the case in this problem. 1100% of 22 is 242 amps, and that is not one of the choices. So I am stuck. Also, I do not know whether the 38 degrees in the statement of the problem is relevant at all.

 

[kisa2418]

I went through all off those options as well. I have written the question word for word Im not sure it some of the information is irrelvant. I have noticed that exam questions sometimes have too much info.

Here is how I tried to figure it out

first I used table 430.251(b)

a 7 1/2 hp motor at 230v design b motor FLC is 127 amps

That is already too much

so then I tried table 430.7(b)

it states that code letter b motors have an amp/hp of 3.15-3.54

3.54X7 1/2=26.55

38c motors requires seperate overload protection no more than 125%

26.55X125%=33.19

so neither of these outcomes is anywhere near 90 amps.

I am stumped

 

[jcole]

I was taught in school that you use the tables for ground fault and short circuit protection and the nameplate rating for overload protection. Since the question does not give the nameplate rating I assume it is talking about ground fault and short circuit protection. Temp rise has to be figured in when doing overload protection for continuous duty per 430.32. This does not apply here. Locked rotor code letters are needed when sizing the horsepower rating of motor controller and disconnect per 430.110 and table 430.251B. This does not apply here. So IMHO the only info needed is 7.5 hp and 230v. Per table 430.250 motor amps would be 22. 22 X 800% = 176a. Next size up per 430.52 would be 200a. Obviously this is not one of your answers. Sorry I couldnt help. I would like to know the outcome of your question. I would bet the question is worded wrong or the right answer isnt one of your choices. What did your teacher say? Is it out of a book? If so what book?

 

[chris kennedy]

here is the question word for word

"A 7 1/2hp 3 phase motoer has a temp rise of 38c

IMO a meteor will have a temp rise in the thousands of degrees when increased drag is incountered. This could prove catastrophic.

 

[Jraef]

...snip

a 7 1/2 hp motor at 230v design b motor FLC is 127 amps

Huh??? 127A? What chart are you looking at?
127A is more like 50HP at 230V. You slipped a cog there my friend, but your subsequent calcs were closer to the truth on FLC.


But read the question again everyone, they never asked for Overload protection, they asked for the instantaneous circuit breaker sizing.

Their answer it technically incorrect however, and in fact they don't have the correct answer available in the choices. Here's why:

Instantaneous (i.e. Mag-Only or MCP or ETI) circuit breaker "sizes" are based upon the FLC carrying capacity of the breaker, NOT the magnetic trip setting. So the answer would be to size the breaker for 125% of the motor FLC (to cover the conductor rating per 430.22.A) then make sure the Instantaneous trip adjustment range falls within the rules in 430.52.C.3 and/or exception 1. So the breaker "SIZE" would be 22A x 1.25 = 27.5A so a 30A rated breaker, with mag trips adjustable to cover 8 - 13 times FLC, so 176 to 286A (which ends up being 6-8x the breaker rating). But again, when you go to BUY the breaker, you would select a 30A mag.- only breaker, not a 90A. A 90A breaker will end up with mag. trip settings too high for the necessary ratings per 430.52.C.3.

So whomever did your test question is misinformed.

Oh and by the way, the temperature rise and code letter of the motor are just red herrings. They don't factor in to the breaker sizing.

 

[tryinghard]

Instantaneous (i.e. Mag-Only or MCP or ETI) circuit breaker "sizes" are based upon the FLC carrying capacity of the breaker, NOT the magnetic trip setting.

I have never bought or used instantaneous breakers, sounds like they have a duel purpose. I understand your explanation as these type of breakers are sized according to the overload protection needed then adjusted for motor start up. Does this type of breaker provide overload protection as well?

 

[kingpb]

An Instantaneous trip only breaker is a magnetic only breaker, i.e. no thermal element, which means it has no long time or short time trip functionality. It looks similar to a molded case circuit breaker. They are used in conjunction with motor overloads, NOT in place of overloads. They would be considered analogous to a fuse, except they can be reset after tripping.

Design Letters and Code Letters are not the same thing. The Code letter in the question relates to Table 430.7(B). As indicated by 430.7(A)(8), by providing a Code Letter, it means the motor is NOT a polyphase wound rotor motor, as this information "shall" be omitted from the nameplate if it is such. Therefore, since it is provided, the maximum Locked Rotor Current is calculated using the Code Letter as follows:

LRC = 7.5Hp x 3.54KVA/Hp x 1000/(230 x 1.73)
LRC = 66.7 A

In selecting the MCP device, as stated earlier, it is based on the full load (continuous) current of the motor, therefore the 22A x 125% = 27.5A would apply, and selection of a 30A MCP would be appropriate. Now, since it is a Code Letter B, the adjustable rating would need to give you some range that the LRC falls between. In looking at the Cutler Hammer Catalog, I would select the device with a lower end of 45A and an upper end of 150A (HMCP).

Obviously, other manufacturers may have different ranges, so you will need to consult your favorite to see what is available that meets your needs.

Regardless, I cannot see how 90A was the answer, I would have selected 70A (for the setting of the MCP). I would agree with others that think the person writing the question may have used some improper terminology thus leading to answer E - None of the Above.

 

[eric stromberg]

--------------------------------------------------------------------------------

I have a question for an exam prep question. here is the question word for word

"A 7 1/2hp 3 phase motoer has a temp rise of 38c and is being protected by instantaneous trip circuit breaker. system voltage is 230 volts and the moter is code letter B. What size breaker is needed?"

A.70
b.80
c.90
d.100

The answer is 90amps

Im not sure if its right I have tried to figure it out with no luck any help would be greatly appreciated.
-------------------------
I don't see how 90 Amps can be the answer. Table 430.250 shows 22 Amps for this motor. For an instantaneous Short-Circit/Ground-Fault device, Table 430.52 shows either 800 or 1100%, depending on the design letter. It doesn't really matter because 22 * 8 = 176 and 22 * 11 = 232. Respectively, 240.6 and 430.52(C)(1)exception no. 1 allow us to go to 200 and 250 Amps. Neither of these is among the answer choices.

On a more philosophical note, the given answer (90) doesn't even make any intuitive sense. An instantaneous device must be rated for higher than inrush or else the motor won't start. The inrush on this motor might be around 22 * 6 = 132 Amps. The instantaneous breaker would have to be above that rating.

Now, let's assume that the test question meant to say that the motor was a 460 Volt motor. In this case, the FLA is given as 11. 11 * 8 = 88. Next size up is 90. This makes sense.

Eric

 

[davidr43229]

A "typo" on a test question?, How unheard of.... I agree with your reasoning.
Just my $.02

 

[LarryFine]

This makes sense.

I'm sorry, Eric, but that is not allowed in this forum. Please refrain from doing it again.

 

[eric stromberg]

I'm sorry, Eric, but that is not allowed in this forum. Please refrain from doing it again.

A thousand pardons, i beg of you. I'll go back and re-read the forum rules.

 

[nyerinfl]

38c motors requires seperate overload protection no more than 125%

26.55X125%=33.19

so neither of these outcomes is anywhere near 90 amps.

I am stumped

Stealing your 33.19 answer as I don't want to do the math again, since it's for an instantaneous CB, wouldn't the next step be to size it 250%, which would give 83A, next size up it and you have a 90A CB.

 

[eric stromberg]

Short Circuit/Ground-Fault protection and Overloads

Short Circuit/Ground-Fault protection and Overloads

Stealing your 33.19 answer as I don't want to do the math again, since it's for an instantaneous CB, wouldn't the next step be to size it 250%, which would give 83A, next size up it and you have a 90A CB.

The question is not entirely clear, so some assumptions must be made. The facts:
three phase motor, 7.5 HP
Motor temp rise of 38C

Assumptions:
The question asks for the size of "Breaker." This would be short-circuit/ground-fault protection. Since this is a three phase motor, the assumption is that this motor is using a starter that has separate overload (heater/relay) and short circuit elements.

Since the temp rise is given as less than 40degC, we are allowed to use 125% of nameplate current in order to size the overload element. But this is not a part of the problem. If they had wanted this, they would have had to have given us the nameplate current, which they did not. Further evidence that what they wanted was the S.C./G.F. protection.

So, assuming the motor is 460 V, 7.5 Amps. The FLA (430.250) is 11.
The Motor branch circuit is 125% of this, or 13.75 Amps. We can use just about any size wire for this, but let's just use 12 AWG.

The overload relay has to be dialed to a setting of 125%, or the same 13.75 Amps.

The short-circuit/ground-fault protection, since an Inst. Bkr. is being used, is 11 * 8 = 88, next size up, 90 Amps. (another assumption here, that the motor is not Design B)

You can see that the branch circuit conductors are really protected by both the overloads and the SC/GF devices. The overloads are set to the same ampacity that must be carried by the conductors.

 

[eric stromberg]

Wow, maybe i should read first...

Wow, maybe i should read first...

Stealing your 33.19 answer as I don't want to do the math again, since it's for an instantaneous CB, wouldn't the next step be to size it 250%, which would give 83A, next size up it and you have a 90A CB.

OK. After typing all of that, i went back and read your original question. Which i did not answer.

The 250% is only for inverse time breakers. It is column 4 in Table 430.52

Since the problem statement askes about an instantaneous, we must use column 3.

 

[nyerinfl]

OK. After typing all of that, i went back and read your original question. Which i did not answer.

The 250% is only for inverse time breakers. It is column 4 in Table 430.52

Since the problem statement askes about an instantaneous, we must use column 3.

I read Instantaneous, but my mind said Inverse Time.

 

[billk554]

so you instantaneously inversed your answer?