Generator sizing

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genesparky

Member
Location
Kingman Az.
We have a 100KW emergency generator that we need to replace due to permit issues. When it was installed we had much more load than we do now. My boss wants to know how small we can go and get by. We have 3 480 volt 3 ph. 15 hp. motors and a 25 KVA single phase tx. that feeds a small load center. If I do the math:
21A x480 =10080
x .80 PF= 8064
x1.73 =13950 va
=13.9 KVA for each motor, do I multipy by 3 and add the 25 for the tx and get 66 KW?

I must be missing something
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
You need to consider the starting currents (locked rotor amps) of the motors as well as their running currents. Also, will all three motors start at the same time, or will they be staggered/sequenced?
 

W6SJK

Senior Member
Yes, you need to consider starting current. Compare the running load with the steady state capacity of the generator, and compare the starting load with the surge capacity of the generator. The mfr rep or vendor will usually do the calcs using a program, for free.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Won't the 1-phase transformer load two phases higher than the other one? I would think that 25 KVA load would have to be calculated on just two of the phases and not spread across all three.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Also, I'm not sure that's the proper use of the power factor.
(others will have to confirm or refute this)


Edit: I think it should be 21*480/.8 instead of 21*480*.8
 
Last edited:

bwyllie

Senior Member
Location
MA
I would suggest contacting your local generator rep and give them the loads. they should have programs that will give you the correct size generator based upon the loads.
 

genesparky

Member
Location
Kingman Az.
The motors will only run one at a time, however for now we are starting them across the line. There is a lot of jogging involved but thankfully only one at a time. Hardworkingstiff, not sure what you mean with 21? I do think we will get the sales rep. involved, I would like to be "snowproofed" though.
 

JMPE

Member
Make sure that you provide the correct motor nameplate information to be inputed into the program, this would be ideal.
 

marketman

Member
gen sizing

gen sizing

I'll give it a shot...I have the Onan sizing program.
I plugged in the three motor loads, in three steps (1 15hp each step)for a non simultaneous start. You really need the NEMA code letter A-thru V (very important). I assumed Nema code letter G, starting across-the-line.
The 25 kva single phase load was entered as a receptacle type load with .9 power factor and entered into step 1, motors followed in steps 2, 3, 4. If a deviation is req'd from these assumptions, it changes everything.

You never mentioned fuel type so:

Nat. Gas = 85GGHG with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 75GGHF with PMG excitation 125 deg temp rise

You get a slight reduction from the 100 kw unit you had previously. Very important to note... you stated the motors will not start simultaneously. Once you need to start two or more simultaneously you need a much larger unit, approximately 125 kw, such as would occur with manual jogging.

You could look at the 25 kva load more closely with regard to diversity, if you are interested in downsizing further. For arguments sake I removed the 25 kva load entirely and ran numbers again (therefore just considering the motor portion of the load):

Nat. Gas = 60GGHE with shunt excitation 125 deg temp rise
Diesel = 50DGCA with shunt excitation 125 deg temp rise
Propane = 60GGHE with shunt excitation 125 deg temp rise

So purely for arguments sake you need a genset with capacity rating somewhere between these two size groups, based on the diversity of the 25 kva single phase load.

If your looking for a reduction from the full load size group, Solid State soft start type starters on the motors would help, although I'm not sure how practical that would be, with consideration to starts per hour, the amount of jogging or bumping could exceed the duty rating of an otherwise addequately sized starter.... buts lets entertain the notion that you can use them, therefore motors loaded in 3 steps, with the 25 kva single phase load in step 1. I assumed the starters would be set to a maximum inrush of 300% of RLA, with length of ramp up set accordingly:

Nat. Gas = 70GGHF with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 75GGHF with PMG excitation 125 deg temp rise

Remember the last group is utilizing soft Starts, not across the line, WITH the 25 kva single phase load included.

Well that was fun, I hope it helps.

MM
 
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marketman

Member
gen sizing

gen sizing

I'll give it a shot...I have the Onan sizing program.
I plugged in the three motor loads, in three steps (1 15hp each step)for a non simultaneous start. You really need the NEMA code letter A-thru V (very important). I assumed Nema code letter G, starting across-the-line.
The 25 kva single phase load was entered as a receptacle type load with .9 power factor and entered into step 1, motors followed in steps 2, 3, 4. If a deviation is req'd from these assumptions, it changes everything.

You never mentioned fuel type so:

Nat. Gas = 85GGHG with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 70GGHF with PMG excitation 125 deg temp rise

You get a slight reduction from the 100 kw unit you had previously. Very important to note... you stated the motors will not start simultaneously. Once you need to start two or more simultaneously you need a much larger unit, approximately 125 kw, such as would occur with manual jogging.

You could look at the 25 kva load more closely with regard to diversity, if you are interested in downsizing further. For arguments sake I removed the 25 kva load entirely and ran numbers again (therefore just considering the motor portion of the load):

Nat. Gas = 60GGHE with shunt excitation 125 deg temp rise
Diesel = 50DGCA with shunt excitation 125 deg temp rise
Propane = 60GGHE with shunt excitation 125 deg temp rise

So purely for arguments sake you need a genset with capacity rating somewhere between these two size groups, based on the diversity of the 25 kva single phase load.

If your looking for a reduction from the full load size group, Solid State soft start type starters on the motors would help, although I'm not sure how practical that would be, with consideration to starts per hour, the amount of jogging or bumping could exceed the duty rating of an otherwise addequately sized starter.... buts lets entertain the notion that you can use them, therefore motors loaded in 3 steps, with the 25 kva single phase load in step 1. I assumed the starters would be set to a maximum inrush of 300% of RLA, with length of ramp up set accordingly:

Nat. Gas = 70GGHF with PMG excitation 125 deg temp rise
Diesel = 80DGDA with PMG excitation 125 deg temp rise
Propane = 75GGHF with PMG excitation 125 deg temp rise

Remember the last group is utilizing soft Starts, not across the line, WITH the 25 kva single phase load included.

Well that was fun, I hope it helps.

MM

an added note: The natural gas downsizing from ATL to SS starters looks like it was achievable with an extended stack and full single phase output incorporated into the genset. Those type of things cost money, this is where a generator sales engineer could be helpful.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
genesparky said:
Hardworkingstiff, not sure what you mean with 21?

The OP stated:
If I do the math:
21A x480 =10080
x .80 PF= 8064
x1.73 =13950 va

It seems to me that the math is incorrect. If the motor is pulling 21A, I believe you don't multiply by the PF to get the running amps, I believe you are supposed to divide by the PF.

I think the calculations as shown above would grossly undersize a generator.

Actually, now that I look at this again, I'm not sure what to do w/the PF.
 

W6SJK

Senior Member
PF does not get involved if you already know the amps, and volts. A x V x SQRT(3) = VA or A x kV x SQRT(3) = kVA

If you want to know the kW, multiply the kVA above, by the PF.
 
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hardworkingstiff

Senior Member
Location
Wilmington, NC
sparkie001 said:
PF does not get involved if you already know the amps, and volts. A x V x SQRT(3) = VA or A x kV x SQRT(3) = kVA

If you want to know the kW, multiply the kVA above, by the PF.

Are generators rated in KW or KVA?

If KW, then is it based on a unity PF?

In the case of the OP, then you would need to multiply the KW rating of the generator by the PF in order to get the KVA the generator would be able to deliver?
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
sparkie001 said:
Genset specs are usually published as kW/kVA with the kVA at 0.8PF, e.g. 80kW/100kVA@0.8PF.

I'm confused again. Didn't we move from KW to KVA because we needed a standard that PF did not confuse?

I would think the Genset would be rated 80KW @ .8PF / 100KVA (no PF adjustment).

Edit: The power factor is the ration of KW/KVA. So an 80KW load operating at a .8 PF will require 100KVA.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
To me, that's a difference (100Kw to 80 Kw) not worth the expense, unless I'm missing something.
 

W6SJK

Senior Member
I'm gonna go out on a limb here and mention that if memory serves me... the kW capacity determines the engine HP needed and the kVA capacity determines the winding size to handle the "extra" amps. Am I all wet? :)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
hardworkingstiff said:
Larry, I don't understand your comment. Can you elaborate?
From the OP:
genesparky said:
We have a 100KW emergency generator that we need to replace due to permit issues. When it was installed we had much more load than we do now. My boss wants to know how small we can go and get by. We have 3 480 volt 3 ph. 15 hp. motors and a 25 KVA single phase tx. that feeds a small load center. If I do the math:
21A x480 =10080
x .80 PF= 8064
x1.73 =13950 va
=13.9 KVA for each motor, do I multipy by 3 and add the 25 for the tx and get 66 KW?
And from a recent response:
hardworkingstiff said:
I would think the Genset would be rated 80KW @ .8PF / 100KVA (no PF adjustment).

It looks like the discussion involves replacing a 100Kw with an 80Kw. Doesn't look like enough of a reduction to warrant the cost.
 
Last edited:
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