How to size a generators?

Status
Not open for further replies.
P

Prototype1

Member
Location
Kansas City
Occupation
Noob EE
1: I went to Article 430.120 – FLC – 65 A * 1.25 = 81.25 A
2: Table 310.16, look at 75deg C Copper- the AWG at 81.25 Amp is AWG 4.
3: On diagram: The breaker is 150 AF; Table 250.122: 200 is closet to 150: so, AWG 6 G
4: Ch-9 Table-5: At AWG 4- (XHHW)- 0.0814 * 3 = 0.2442
5: Ch-9 Table-5: At AWG 6- (XHHW) – 0.0590 = 0.0590
6: 0.2442 + 0.0590 = 0.3032
7: Ch-9 Table 1: Over 2 conductor: 40%: 0.3032 / 0.4 = 0.758
8: Ch-9 Table 4: RMC article 352-353 – value close to 0.758 (over 2 wires 40%) is 0.794 and
Trade size is 1½, Ground - #6G, #4 AWG
For (50 HP) 65 A.

This is the steps I took to find the ground, conductor, and conduit size using NEC code book 2023. Now, I want to know how to size the generator and breaker. Please help.
I am still new to this NEC code book and single line diagram. I am still confuse on what steps to take.
 
Sizing a generator has nothing to do with sizing the wires and everything to do with the actual loads and how many motors are involved (and do they start in sequence or all at once). And you can't do it just using HP, have to know the voltage. This also involves whether it's optional standby, required, or the prime source of power (see arts 700/701/702/etc).

There are several rules-of-thumb, which I'm sure a few people will drop in here, but most of the generator companies have both guidelines and even calculators on their web sites.
 
zbang said:
Sizing a generator has nothing to do with sizing the wires and everything to do with the actual loads and how many motors are involved (and do they start in sequence or all at once). And you can't do it just using HP, have to know the voltage. This also involves whether it's optional standby, required, or the prime source of power (see arts 700/701/702/etc).

There are several rules-of-thumb, which I'm sure a few people will drop in here, but most of the generator companies have both guidelines and even calculators on their web sites.
Click to expand...
Here is the diagram. Those steps 1 to 8 are for the 50 HP (I want to know if I did them right). Now, I am trying to find the breaker, generator size on the left. I don't what steps I need to take in NEC code book to size the breakers and generators.
 

Attachments

  • Bre1.JPG
    Bre1.JPG
    40.4 KB · Views: 27
Keep in mind the NEC is NOT a design manual. It simply gives you MINIMUM requirements for a safe installation.
240.4 gives some guidelines on breaker sizing but references you to Art 430 for motor where Part V provides guidance.
As zbang notes, there are numerous factors in selecting a generator for motor loads and those are not covered in the NEC>
 
augie47 said:
Keep in mind the NEC is NOT a design manual. It simply gives you MINIMUM requirements for a safe installation.
240.4 gives some guidelines on breaker sizing but references you to Art 430 for motor where Part V provides guidance.
As zbang notes, there are numerous factors in selecting a generator for motor loads and those are not covered in the NEC>
Click to expand...
Does manufacture provide info on selecting and sizing the generator then?
 
Commercial generators that serve buildings can't be treated as infinite buses; you cannot make it the same size as your calculated load. Best thing to do as noted in post #2 is to go to a generator manufacturer website and use their sizing software.

There are a lot of variables that the software allows you to adjust such as voltage dip, frequency dip, load steps, motor starting type (VFD, soft-start, across-the-line) etc. that you will need to know. There is little in the NEC that dictates how to size a generator, only that the generator has adequate capacity and rating to supply its intended equipment (700.4, 701.4, 702.4). I have had good experiences with the Generac and Kohler sizing software.

Couple of notes on that one-line diagram...
  1. Utility transformer size is suspect. 15kVA, single phase, 3W with 480V primary to 120/240V secondary? The FLC of your pump is at 480V,3-phase so these do not align. You need a 480Y/277V, 3-phase, 4W service.
  2. Utility transformer sizes is typically dictated by the utility so not sure where you got that 15kVA from. You just furnish the load letter and they take it from there.
  3. AF is short for amp frame. This is different from AT, which is the actual "Trip" rating. Having AT/AF notations on circuit breakers is typically used for power distribution panelboards, switchboards, and switchgear where OCPD mounting inches and form factors are important.
  4. There are too many unknowns to really help here. What other loads are on this generator? Is it only the 50HP pump? The size of your feeders from the load side of the ATS is sized to 760A (800A breaker) so this leads me to believe that it isn't.
  5. Is that an un-fused disconnect on the line side of your metering? Is that required by the power company?
  6. You can get service entrance ATSs with a circuit breaker on the normal side. Is that the intent here?
 
Prototype1 said:
1: I went to Article 430.120 – FLC – 65 A * 1.25 = 81.25 A
2: Table 310.16, look at 75deg C Copper- the AWG at 81.25 Amp is AWG 4.
3: On diagram: The breaker is 150 AF; Table 250.122: 200 is closet to 150: so, AWG 6 G
4: Ch-9 Table-5: At AWG 4- (XHHW)- 0.0814 * 3 = 0.2442
5: Ch-9 Table-5: At AWG 6- (XHHW) – 0.0590 = 0.0590
6: 0.2442 + 0.0590 = 0.3032
7: Ch-9 Table 1: Over 2 conductor: 40%: 0.3032 / 0.4 = 0.758
8: Ch-9 Table 4: RMC article 352-353 – value close to 0.758 (over 2 wires 40%) is 0.794 and
Trade size is 1½, Ground - #6G, #4 AWG
For (50 HP) 65 A.

This is the steps I took to find the ground, conductor, and conduit size using NEC code book 2023. Now, I want to know how to size the generator and breaker. Please help.
I am still new to this NEC code book and single line diagram. I am still confuse on what steps to take.
Click to expand...
You have an error here. A 1" conduit should take four #4 conductors of most types, which was my red flag. The table area given in the "Over 2 Wires, 40%" column is the area already reduced to 40%. You use that number in the table (.355 for 1" rigid) as is and compare it to the wire area. Since .355 > .3032, a 1" is large enough. Doing it your way (taking the wire area and dividing by .4) you need to look at the last column of the chart which shows the full area of the conduit (1" rigid is .887 which is larger than your .758 calculated full area).

A common factor in starting motors is making sure you have enough current to support the locked rotor amps. A generator may have a "peak" rating which would be the short time it can output a large current for motor starting. Each generator will be different. If this is a test type question, I don't know how they expect you to answer that. A generator sized for just the running load (even with a 125% largest motor factor) probably won't start both of them at once. Need to get generator details. My 15 KVA generator says it will start a "4 ton AC unit". This is a 240V 50A generator (may actually be 60A, but its main breaker is 50A), but it seems it can source a bit over 100A to start a large motor or HVAC compressor.
 
Prototype1 said:
1: I went to Article 430.120 – FLC – 65 A * 1.25 = 81.25 A
2: Table 310.16, look at 75deg C Copper- the AWG at 81.25 Amp is AWG 4.
3: On diagram: The breaker is 150 AF; Table 250.122: 200 is closet to 150: so, AWG 6 G
4: Ch-9 Table-5: At AWG 4- (XHHW)- 0.0814 * 3 = 0.2442
5: Ch-9 Table-5: At AWG 6- (XHHW) – 0.0590 = 0.0590
6: 0.2442 + 0.0590 = 0.3032
7: Ch-9 Table 1: Over 2 conductor: 40%: 0.3032 / 0.4 = 0.758
8: Ch-9 Table 4: RMC article 352-353 – value close to 0.758 (over 2 wires 40%) is 0.794 and
Trade size is 1½, Ground - #6G, #4 AWG
For (50 HP) 65 A.

This is the steps I took to find the ground, conductor, and conduit size using NEC code book 2023. Now, I want to know how to size the generator and breaker. Please help.
I am still new to this NEC code book and single line diagram. I am still confuse on what steps to take.
Click to expand...
I've attached the following link that makes generator sizing calculation manageable and easy to follow, it has three levels depending on the experience on inputting the data. I hope this help.


 
PD1972 said:
Commercial generators that serve buildings can't be treated as infinite buses; you cannot make it the same size as your calculated load. Best thing to do as noted in post #2 is to go to a generator manufacturer website and use their sizing software.

There are a lot of variables that the software allows you to adjust such as voltage dip, frequency dip, load steps, motor starting type (VFD, soft-start, across-the-line) etc. that you will need to know. There is little in the NEC that dictates how to size a generator, only that the generator has adequate capacity and rating to supply its intended equipment (700.4, 701.4, 702.4). I have had good experiences with the Generac and Kohler sizing software.

Couple of notes on that one-line diagram...
  1. Utility transformer size is suspect. 15kVA, single phase, 3W with 480V primary to 120/240V secondary? The FLC of your pump is at 480V,3-phase so these do not align. You need a 480Y/277V, 3-phase, 4W service.
  2. Utility transformer sizes is typically dictated by the utility so not sure where you got that 15kVA from. You just furnish the load letter and they take it from there.
  3. AF is short for amp frame. This is different from AT, which is the actual "Trip" rating. Having AT/AF notations on circuit breakers is typically used for power distribution panelboards, switchboards, and switchgear where OCPD mounting inches and form factors are important.
  4. There are too many unknowns to really help here. What other loads are on this generator? Is it only the 50HP pump? The size of your feeders from the load side of the ATS is sized to 760A (800A breaker) so this leads me to believe that it isn't.
  5. Is that an un-fused disconnect on the line side of your metering? Is that required by the power company?
  6. You can get service entrance ATSs with a circuit breaker on the normal side. Is that the intent here?
Click to expand...
Yes, you are right.
1. I am still not sure what kVA is from the utility (Entergy). It is 480 V, 3 Phase, 4W. (I am trying to find Entergy standards for overhead transformer 800 A.
2. Okay, I will furnish (you mean the closest I get right?).
3. It is distribution panelboards.
4. No, there other pumps too.
5. meter disconnect but still not sure it will be there ( I need to look at standards of Entergy).
 
Not trying to kill the discussion but when I needed a generator sized I put the information out to a few manufacturers and asked them to include quotes including ATS and paralleling gear if needed
 
I got the generator set
suemarkp said:
You have an error here. A 1" conduit should take four #4 conductors of most types, which was my red flag. The table area given in the "Over 2 Wires, 40%" column is the area already reduced to 40%. You use that number in the table (.355 for 1" rigid) as is and compare it to the wire area. Since .355 > .3032, a 1" is large enough. Doing it your way (taking the wire area and dividing by .4) you need to look at the last column of the chart which shows the full area of the conduit (1" rigid is .887 which is larger than your .758 calculated full area).

A common factor in starting motors is making sure you have enough current to support the locked rotor amps. A generator may have a "peak" rating which would be the short time it can output a large current for motor starting. Each generator will be different. If this is a test type question, I don't know how they expect you to answer that. A generator sized for just the running load (even with a 125% largest motor factor) probably won't start both of them at once. Need to get generator details. My 15 KVA generator says it will start a "4 ton AC unit". This is a 240V 50A generator (may actually be 60A, but its main breaker is 50A), but it seems it can source a bit over 100A to start a large motor or HVAC compressor.
Click to expand...
I got the generator set details, Generator Set Model-DS00350D6SRA, Nameplate kW Rating - 350, Engine Model - 3D 8V1600G70S EPA TIER 3 TD, Rated P.F. - 0.8. How do I find size of breaker? The voltage is 277/480 V, 60 HZ, Three phase, Three wire.

The wire, breakers size are not right in the diagram.
 

Attachments

  • 1.JPG
    1.JPG
    40 KB · Views: 7
zbang said:
A genset that size should come with it's own main breaker (what make is that? the model doesn't come up in google). Sizing the motor's breaker would be the same as if it's a utility source.
Click to expand...
I found it. And here is the new modify diagram. Trying to find Entergy standard wire from the transformer(Photo #4). It is confusing. Guys, thanks for help.
 

Attachments

  • 2.JPG
    2.JPG
    63.4 KB · Views: 14
  • 3.JPG
    3.JPG
    51.9 KB · Views: 15
  • 4.JPG
    4.JPG
    170.5 KB · Views: 16
Seemingly you need to verify the "service point". Will the utility supplied wiring stop at the j box or continue to the disconnect ??
Interesting configuration with a 480/277 wye service and a 480 3 phase, wire generator.
Be interesting to see others input on the situation.
 
augie47 said:
Seemingly you need to verify the "service point". Will the utility supplied wiring stop at the j box or continue to the disconnect ??
Interesting configuration with a 480/277 wye service and a 480 3 phase, wire generator.
Be interesting to see others input on the situation.
Click to expand...
 

Attachments

  • 5.JPG
    5.JPG
    50 KB · Views: 15
Status
Not open for further replies.
Top