Washington State Electrical Question of the Month, March 2016

Status
Not open for further replies.

tom baker

First Chief Moderator
Staff member
Each month the Washington State has a question of the month in its Electrical Currents, I got a PM with this question and am reposting it here:

"The subject publication, in Question 3, asks how many volt-amperes are drawn by a 5HP 460v 3-phase air compressor. Their answer was 6311 - calculated by multiplying 480 x 7.6 x 1.73. I contacted Olympia L&I and asked why they used 480v instead of 460v for the calculation. I was referred to Example D3(a) in Inf. Annex D from the 2014 NEC.

I have a difficulty understanding, the NEC notwithstanding, how the above calculation makes sense. Why would the volt-amperes, i.e., HP, change for motor that is operated at a voltage level within its intended operational range (440 to 480 in this case)?"
 

Rampage_Rick

Senior Member
gBKH3cj.gif
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
They are doing a load calculation and it is my opinion that 220.5(A) requires the calculation to be based on the nominal system voltage of 480 and not the nameplate voltage of 460.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
They are doing a load calculation and it is my opinion that 220.5(A) requires the calculation to be based on the nominal system voltage of 480 and not the nameplate voltage of 460.
Fine. In which case the nominal current should also be the full load current at 480, not at 460. The product should remain unchanged.
Just what current ID intended to be shown in the motor HP tables?
If we were allowed to use the motor nameplate numbers we would not change the voltage from the nameplate number but keep the nameplate current unchanged.
 

kevin

Member
Location
Post Falls, ID
They are doing a load calculation and it is my opinion that 220.5(A) requires the calculation to be based on the nominal system voltage of 480 and not the nameplate voltage of 460.

That interpretation may be correct, but it assumes that the motor FLC as determined from Table 430.250 does not vary within the permissible voltage range, i.e., it stays constant at 7.6 amps. For typical motors, at a given frequency, an increase in voltage will result in a decrease in amperage and a decrease in voltage will result in an increase in amperage. That could be very troubling in the latter case, where allowing the calculations not to take into account the higher amperage can possibly result in undersizing the conductor. A more reasonable approach, it seems to me, would be to calculate the load at the Table 430.250 voltage (460) and use that value (6048 v-a) to further calculate the amperage for the actual system voltage. Any thought or comments out there?

For the record, I am the source of the PM to Tom Baker in the OP.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
That interpretation may be correct, but it assumes that the motor FLC as determined from Table 430.250 does not vary within the permissible voltage range, i.e., it stays constant at 7.6 amps. For typical motors, at a given frequency, an increase in voltage will result in a decrease in amperage and a decrease in voltage will result in an increase in amperage. That could be very troubling in the latter case, where allowing the calculations not to take into account the higher amperage can possibly result in undersizing the conductor. A more reasonable approach, it seems to me, would be to calculate the load at the Table 430.250 voltage (460) and use that value (6048 v-a) to further calculate the amperage for the actual system voltage. Any thought or comments out there?
For the record, I am the source of the PM to Tom Baker in the OP.
In the real world, I don't think it makes much difference. I say this for two reasons, first the conductors are sized at 125% of the table full load current and second the table full load current are worst case...that is few motors actually pull the amount of current that is shown in the table.
 

tom baker

First Chief Moderator
Staff member
They are doing a load calculation and it is my opinion that 220.5(A) requires the calculation to be based on the nominal system voltage of 480 and not the nameplate voltage of 460.

Correct Don, the issue of motor voltage on state electrical exams has tripped many electricians up.
This monthly publication from our state electrical department, has a question, and its my opinion that the test questions are similar or the same as those on the actual exam. Maybe its a bonus for those who subscribe to the "Electrical Currents", its a very important publication, and a great way to keep informed. However many electricians don't subscribe to it....


If you are in Washington, take my advice, I have been instructing classes for 20 years...subscribe here http://www.lni.wa.gov/Main/Listservs/Electrical.asp

And then get back issues here http://www.lni.wa.gov/TradesLicensing/Electrical/WhatsNew/Currents/default.asp
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
If you notice the last line in the text above each Tables in 430 you will see that you are allowed to use a range of voltages.

The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges
of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
 

buzzbar

Senior Member
Location
Olympia, WA
Occupation
Electrical Contractor
Correct Don, the issue of motor voltage on state electrical exams has tripped many electricians up.
This monthly publication from our state electrical department, has a question, and its my opinion that the test questions are similar or the same as those on the actual exam. Maybe its a bonus for those who subscribe to the "Electrical Currents", its a very important publication, and a great way to keep informed. However many electricians don't subscribe to it....


If you are in Washington, take my advice, I have been instructing classes for 20 years...subscribe here http://www.lni.wa.gov/Main/Listservs/Electrical.asp

And then get back issues here http://www.lni.wa.gov/TradesLicensing/Electrical/WhatsNew/Currents/default.asp

I've subscribed for many years. It's a great publication, very informative. I also like the monthly "ugly" picture. Shows all sorts of botched electrical installations.
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
In the real world, I don't think it makes much difference. I say this for two reasons, first the conductors are sized at 125% of the table full load current and second the table full load current are worst case...that is few motors actually pull the amount of current that is shown in the table.

The difference is less than .5 amp---close enough I would think--using NOMINAL SYSTEM VOLTAGE is how I generally do it
 
Status
Not open for further replies.
Top