understanding motors

[starbright28]

My dad and I were trying to figure out how the engineer was calculating different items for the exhaust fans on the roof.

here is the given information.
1/4 HP
single phase
115 Volts
It's a roof exhaust fan

Using Table 430.248 - full load currents in amperes, single phase alternating current motors.

from that table I get 5.8 amps.

Now here is how i was taught.

You take the 5.8 amps times 1.25% to get your total amps. (which is 7.25 A)

Then you take your total amps times your voltage - which I use 120v. (which is 870 watts)

This is the way the engineer did it:

he took the value from the chart - 5.8 amps. Then he took 115V times the 5.8 amps to get 667 watts.

He listed the 667 watts on the panelboard schedule as so, with a power factor of 0.8 percent.

Is this correct?? Are you not supposed to take the 125%? What voltage are you supposed to use, the 115V or 120V?

Please help.

 

[charlie b]

This is a good question, and it shows that you are thinking your way through the code book. Admirable, and not at all an easy task.

I would have done it the same way as your engineer did it.

First of all, when you are calculating load, you don?t apply the factor of 125% to every motor. Sizing the conductors to a motor and sizing its overcurrent protection system is another matter. But a key purpose of the panel schedule is to demonstrate that the demand load is within the rating of the panel. For that purpose, you only have to apply that 125% factor to the largest motor.

Secondly, the values of current correspond to the voltage level at the top of the column. As an exercise, try taking the value of current from the 115 volt column, then multiplying it by the factor (115/200). You will see that this gives you the value in the 200 volt column. Multiply that value by (200/208), and you will get the value in the 208 volt column. So if you wish to use the value of 120 volts, instead of 115 volts, you are permitted (but not required) to use 5.55 amps, instead of 5.8 amps. That will, of course, bring you back to the calculated load of 667 watts.

 

[starbright28]

Charlie

I'm glad I ask. I was taught that no matter what you take 125% of the motor. The problem I gave is for that 1 and only motor.

So there are instances that you don't take the 125%? What are those?

This may be why I have been not getting the questions right on my test (if I have gotten them wrong that is, which I'll never know).

So - I'm still curious. why use the 115 V for that motor? for 3 phase it is usually what your voltage is, correct?

 

[charlie b]

Much depends on the nature of the question. If you are calculating load, then you have to add an extra 25% to the load associated with the largest motor. If there is only one motor, then you include a total of 125% (i.e., the 100% value obtained from the table plus the added 25%) for that one motor.

As to using 115 or 120, I don't think that a test question whose "right" answer turns on that point would be a fair question. Take a look at the text that appears above the tables (e.g., Table 430.248) and the title of the table. It includes a statement that the currents apply at the rated motor voltage (which is always lower than the rated voltage of the distribution system). It also says these values can be used at any system voltage between (for your example) 110 and 120 volts. So what value of voltage should you use (i.e., 110 or 115 or 120), when calculating the load value for your test question? I would use the value of 115 volts. But the person who wrote the question might have used the value of 120. Neither of us could say that the other was wrong, or that the other's answer would result in a code violation.

 

[starbright28]

Well I asked my question because I wanted to know if the engineer correctly put down the right amount on the panelboard schedule.

We were trying to find the load - or watts and Va to show on the panelboard schedules. When I figured out how he did it, I didn't think it was right and wanted to ask.

We usually only put the roof top exhaust fans 1 to a circuit.

so when the mechanical department specifies the exhause fan that has a 1.4hpo, single phase, 115v. and you go to the chart and see the 5.8 on the chart in the code book.

My question is this. do i take this 5.8 times the 125%? or just do I leave it be and then put the value to the panelboards?

Maybe that helps in the direction that I am seeking.

 

[charlie b]

You need to consider the panelboard schedule as a whole. Start by using 100%, not 125%, for every motor powered from that panel. Add all the receptacle loads and other loads. Then before you finish, go back and look for the single largest motor that is powered from that panel. Add 25% of that motor's load to the total load on the panel.

Some people have developed Excel files that do the math automatically for you. Those spreadsheet files that I have seen and used all share the characteristic that the extra 25% for the largest motor appears in a separate line at the bottom of the sheet. That is, the 25% does not appear on the same line(s) as the motor itself. Also, you don't wind up with 125% of every motor on the whole panel, but rather only one motor: the largest.

Does that help explain the load calculation process for motors?

 

[starbright28]

Okay. I am beginning to understand

when we have a project, we will just on the panelboard put the load for the motors. at the beginning i am not to worry about taking the 125% of that motor, unless it becomes the largest motor on the panelboard.

so. for our panelboard we also list power factor, and he put 0.8 for the power factor. i don't think he took the 125% rule for hte largest motor on the panelboard.

but for a test sake, and you only have the one motor and you are trying to find like a wire size for it, you then take the 125% regardless, because it is one motor for that problem.

Am I interperting this correctly?

 

[charlie b]

Am I interperting this correctly?

You are getting close. But let me point out a pure coincidence:
(1) When calculating load, you multiple the largest motor by 125%
(2) Dividing by a power factor of 0.8 happens to be the same as multiplying by 125%.
(3) Those two facts are not related to each other in any way.

Next, I will agree that, for the purposes of a test, where the test is asking about the calculation of loads, if there is but one motor you take 125% of the load associated with that one motor. Simply put, if there is but one, then it is the "largest."

Let me also point out, as I hinted at earlier, that the process of calculating load and the process of selecting a wire size are not the same process. In some cases, both processes wind up applying a factor of 125%. But that is for different purposes. So if you want to get into sizing wires and overcurrent protection for motors, let me suggest we first settle any remaining confusion and answer any remaining questions related to calculating load.

 

[starbright28]

Hi Charlie

I got busy yesterday and didn't get a chance to respond.

I think I understand but I also think I don't. When it comes to the test questions I understand, but if I want to take over my dad's business one day, I want to understand corectly how to figure this stuff out.

So, what I originally asked was just 1 exhaust fan. There was more exhaust fans for this building, but all are on their own circuit. My question is, how do you calculate for each exhaust fan the load to put on the panel? at this point we don't worry about figuring out wire size. that is up to the electrician (but we'll get to that next). I have the same problem with roof top units as well.

if you want, instead of this forum, maybe we can discuss over phone or email. pm me to get that info.

 

[charlie b]

Actually, I prefer to ?discuss? such things on the open Forum. That way, if I am right, others might learn as well. And if I am wrong, someone is sure to let me know. :grin:

Let?s work with a simple example.

• You have a 42-circuit panel; 30 breaker positions are used, and the other 12 are spares. The panel is a MCB, rated for 120/208 volts and 100 amps.
• You have five motors, all of the same type you mentioned in the first post (1/4 HP, 115 volts, single phase). Each is on its own circuit, so that accounts for 5 of the 30 used positions.
• For the moment, let us not be concerned about the other 25, except to say that none of them supply motors, and the total load on the other 25 breaker positions is 20,000 watts.

There are two ways I can think of to complete the calculation of the panel?s total load.

Method 1:

• Motor #1 equals 667 watts (as calculated in your first post).
• Motor #2 equals 667 watts.
• Motor #3 equals 667 watts.
• Motor #4 equals 667 watts.
• Motor #5 equals 834 watts (here, I have multiplied by 125%).
• The other 25 circuits add up to 20,000 watts.
• Grand total: 23,502 watts (or equivalently, 65 amps).

Method 2

• Motor #1 equals 667 watts.
• Motor #2 equals 667 watts.
• Motor #3 equals 667 watts.
• Motor #4 equals 667 watts.
• Motor #5 equals 667 watts.
• The other 25 circuits add up to 20,000 watts.
• Subtotal: 23,335 watts.
• Add in 167 watts (this is 25% of the largest motor)
• Grand total: 23,502 watts (or equivalently, 65 amps).

 

[steveng]

thanks charlie, you helped me too

 

[starbright28]

understanding a bit more

understanding a bit more

Charlie

I am understand a bit more. I do understand in your simple problem.

now take a look at our actual panelboard and let me know what you think. i can give you the horsepower/load if you need it.

if i understand that everything on this panelboard was calculated correctly and then maybe i can gain a better understanding on how to do this for the future.

thanks

 

[charlie b]

I won?t have time to check the math on your panel schedule, Amanda. But I can offer a few comments.

First, it has more information than I normally put on a panel schedule. I generally include the load in terms of VA, and do not also show the power factor and watts associated with each load. I also don?t show the panel's overall power factor or the power angle. It?s not in any way an evil thing to show, but I think it unnecessary.

Secondly, we have discussed the basis of assigning 667 VA for a 1/4 hp, 115 volt motor. But your schedule also has a couple of 1/6 hp, 115 volt motors, and I do not get the same value for their VA load. From Table 430.248, that motor draws 4.4 amps. Multiplying 4.4 times 115 gives me 506 VA. The panel schedule shows 445 VA.

Third, there is no accounting for the additional 25% load of the largest motor. In this case, the largest motor appears to be the 1/4 hp fan. So you need to add 167 VA somewhere on the panel schedule. But the format of this particular panel schedule does not appear to have a place for inserting that additional 167 VA. In this case, there is no real significance (other than completeness and accuracy), since that 167 VA equates to another half amp on a panel that is only loaded to about 17% anyway. But a Plan Reviewer might make an issue of it anyway, and you will encounter larger motors that might have a significant influence on the total load.

 

[starbright28]

Hi Charlie

I wasn't looking for math checks - but just a general looksy to see what he came up with, and probably what i need to fix. Good thing this isn't final drawings yet.

You sure have taught me a few things with the question I posed. Now to explain this to my dad... because he was on my way of thinking - using the 125% for each motor, and not thinking about the rest of the panel. So I'll "teach" him now.. I keep him updated that way. (and he doesn't buck my ideas like the engineer here does).

thanks again. we'll tackle doing wire sizing next for these units just so i understand how to do them.

 

[benaround]

Charlie,

I got to tell you, You're the Man, it's you and all the other special folks

that make this place Priceless!!