test question

[CONTROL FREQ]

recepts, heat and motors are not continous, ie no 125%. Also, you left out largest motor.

Out of my realm... I guess. I'm an industrial guy, and I have always been taught my whole life "Unless there are mechanical interlocks to prevent all these things from EVER being at FLA at the same time---you'd better make sure you protect them as if they will be"... kind of a better safe than sorry mentality I suppose.

 

[CONTROL FREQ]

recepts, heat and motors are not continous, ie no 125%. Also, you left out largest motor.

You are right. Sticking with "my line of thinking", your largest AMPACITY motor would've been 55 Amps, not 70. This would bring your total "possible" FLA to 360 multiplied by 1.25, you would end up with 450 Amp (500) service protection. I'm not a scholar by any means, but I truly hope you post the REAL explanation when you do find out. It seems like they want you to make it harder than it is. I am always open to learning something new.

Mike

 

[treyp3]

You are right. Sticking with "my line of thinking", your largest AMPACITY motor would've been 55 Amps, not 70. This would bring your total "possible" FLA to 360 multiplied by 1.25, you would end up with 450 Amp (500) service protection. I'm not a scholar by any means, but I truly hope you post the REAL explanation when you do find out. It seems like they want you to make it harder than it is. I am always open to learning something new.

Mike

450 is a standard breaker size. I think it goes like this:
2hp=24a= 24
5hp=56ax125%= 70
10hp=50a= 50
lighting= 50ax125%= 62.5
recept= 42.5
appliance=35ax125%=43.75
heat= 104
added up = 396.75
The book takes the 5hp motor as 56a x 250% = 150,and comes up with 476.75 total amps. Pretty sure that part is wrong witch is what Im trying to pin down.

 

[augie47]

I completely forgot to figure 430.52 into my calculations, but even when I do, I only come up with a 331 amp OCP (next size 350) so obviously my thinking is way of base from theirs.

 

[Dennis Alwon]

I completely forgot to figure 430.52 into my calculations, but even when I do, I only come up with a 331 amp OCP (next size 350) so obviously my thinking is way of base from theirs.

That's where they got the 250%.... Thanks Gus-- I still say you are closer to the reality then we were.

 

[Dennis Alwon]

I completely forgot to figure 430.52 into my calculations, but even when I do, I only come up with a 331 amp OCP (next size 350) so obviously my thinking is way of base from theirs.

That's where they got the 250%.... Thanks Gus-- I still say you are closer to the reality then we were. But does the 250% need to be in the calc?

 

[Little Bill]

That's where they got the 250%.... Thanks Gus-- I still say you are closer to the reality then we were. But does the 250% need to be in the calc?

The way I was taught to do the calculations, you wouldn't use the Table 430.52 ratings in your load calculations. That would be for branch circuit only. As someone else posted you don't add amps to get your load/ocpd. Not a good question, IMHO!

 

[CONTROL FREQ]

I came up with 500A by rounding up to the next standard size, but not using 250%. Just 125% of the 5Hp motor since it draws 6A more than the 10Hp 230V motor.

Frankly, IMO it's a lame question because your not supposed to calculate service size based on ampacity, you should always use VA, KVA, etc.

AMEN, I can see challenging students, but when you mess with them to the point of discouragement, what gives? Why would they say ?120/240 service?, then throw in a 1? 230v motor? Seems silly unless that?s the whole point of the lesson. All my literature says to fuse 230v 1? 10 HP motor @ 42.5A, 120v 1? 5 HP motor @ 30A, and 120v 1? 2 HP motor @ 22A, under load respectively. I understand the importance of the math here, but everything comes with a ?nameplate? one way---or another. In my honest opinion people who post here, should have the common courtesy of AT LEAST being Mike Holt customers. I spent my tax refund awhile back, just to UNDERSTAND things that you just can?t learn in ?on-the-job seminars? I?m still going through it all (in my own sweet time). But I HAVE NOT EVER yet ran into a DUMBSHTUFF question like this in any of the Holt tests I?ve taken. JUST SAYIN?

 

[treyp3]

AMEN, I can see challenging students, but when you mess with them to the point of discouragement, what gives? Why would they say ?120/240 service?, then throw in a 1? 230v motor? Seems silly unless that?s the whole point of the lesson. All my literature says to fuse 230v 1? 10 HP motor @ 42.5A, 120v 1? 5 HP motor @ 30A, and 120v 1? 2 HP motor @ 22A, under load respectively. I understand the importance of the math here, but everything comes with a ?nameplate? one way---or another. In my honest opinion people who post here, should have the common courtesy of AT LEAST being Mike Holt customers. I spent my tax refund awhile back, just to UNDERSTAND things that you just can?t learn in ?on-the-job seminars? I?m still going through it all (in my own sweet time). But I HAVE NOT EVER yet ran into a DUMBSHTUFF question like this in any of the Holt tests I?ve taken. JUST SAYIN?

Who is discouraged? Im not too bothered by va kva. Your gonna divide kva by voltage in the end anyway, what size 75 degree F wire would you use for 50kva. Also, Thompsom Prometric asks a lot of odd questions in a hundred different ways. You should check 430.248 for single phase fla., that is were your test questions will be coming from. Ill agree that this book is not to great, its Stallcups by the way, and I promise that if I dont pass on my first attempt Ill try Mike Holt. N.E.F.B.A. alumnus thank you.

 

[kingpb]

Gus I don't think you can calculated the lighting load at 120v then divide by 240 in the end.

Also the 10hp is not the largest motor-- is it-- I thought it was the largest amp motor.

Lighting is 50*1.25= 62.5 amps
Recep......................42 amps
25000Kva.................104 amps
35 amp appliance.......35 amps
50 amp motor............50 amps
55 amp motor.............70 amps
24 amp motor.............24 amps

add them up and 387.5

This is exactly how I came up with the 500A because 388 x 1.25 = 485, i.e. round up to 500A.

Did the question ask for the minimum size? Because remember the code is minimum requirements. You can install whatever size service you want over that, just may be wasted money.

 

[Dennis Alwon]

This is exactly how I came up with the 500A because 388 x 1.25 = 485, i.e. round up to 500A.

Why would you add 125% at the end when it was already added in originally.

 

[kingpb]

The way I understand it, is that the 125% was added to the largest motor load. Then, this determines total load, which is the 387 and change.

The rounding up to 400A for next size main service breaker. But, the main breaker cannot be loaded to more than 80% of the rating (320A), we have 387A, so multiply by 125% (inverse of 80%) and you get 483A, rounded up to standard size 500A.

 

[treyp3]

The way I understand it, is that the 125% was added to the largest motor load. Then, this determines total load, which is the 387 and change.

The rounding up to 400A for next size main service breaker. But, the main breaker cannot be loaded to more than 80% of the rating (320A), we have 387A, so multiply by 125% (inverse of 80%) and you get 483A, rounded up to standard size 500A.

I cant find "80% of the rating" in the code book. Do you know the article?

 

[Dennis Alwon]

The way I understand it, is that the 125% was added to the largest motor load. Then, this determines total load, which is the 387 and change.

The rounding up to 400A for next size main service breaker. But, the main breaker cannot be loaded to more than 80% of the rating (320A), we have 387A, so multiply by 125% (inverse of 80%) and you get 483A, rounded up to standard size 500A.

I added 125% for continuous lighting, for motors etc. There is nor reason to add 125% at the end, IMO

 

[kingpb]

It really is a bad question, because it does not give you enough information. A lot of assumptions have to be made. First big assumption is that this is the load all on one phase since it is asking to size a breaker. Second, assume it is not residential since the lighting load is specified as continuous
Continuous
50a lighting load (Demand factor doesn't apply since it is stated as continuous)
35a appliance load (1 appliance, non-residential, so no demand factor applies)
25 kva heat (assume 240V and it is fixed electric space heat so the amps per phase 104A)
Assume Continuous since not stated
motor load 230v 1 phase 10HP (50A)
motor load 120v 1 phase 5 HP (56A x 1.25 = 70A)
motor load 120v 1 phase 2HP (24A)
Non-continuous
42.a receptacle load (demand factors have been applied)

(50 + 35 + 104 + 50 + 70 + 24)*1.25 + 42 = 458A, round up to 500A

 

[marti smith]

210.20 (A) through (D) was as close as I could find....