we have sales center, included 4 Air Condition Package units (34, 34, 30 and 30 KVA) , lighting Load is 40 KVA, Small Power (Sockets) 36 KVA and I would to sizea generator set (prime) for all the previous loads , Please i need helping to how can sizing the requred unit.
Generator Sizing
- Thread starter TMMB
- Start date
- Status
sinnadurai sripadmanaban
Member
You should not take the arithmatical sum of the load. You should apply diversity and load factors to assess the load. Others except you donot know those factors as it depends on the loads working at the same time(simultaneouisly)
broadgage
Senior Member
- Location
- London, England
220/380 suggests a European system in which case the regulations of the country concerned apply, not the NEC.
As regards sizing the generator, this depends on the actual loads to be supplied.
If all the loads listed might run together, then the generator needs to be sized for the simple sum of the loads.
If each A/C unit serves a part of the building, then in hot weather all might run together and this needs to be allowed for.
If on the other hand the A/C units are duty/standby, then the total load will be less.
As regards lighting, it would be reasonable to assume in a workplace that all or virtually all of the lights will be on together. Therefore the generator should be able to supply 100% of the lighting load. Note however that this should be 100% of the actuall installed load, not 100% of the lighting circuit ampacity. The actual lighting load may be determined by mesurement, or by counting fixtures and determining the load of each.
General purpose outlets may be heavily loaded, or hardly used, the best approach is to size the generator to supply perhaps 120% of the measured load, not the circuit ampacity. A margin of say 20% should be allowed in case the load at the time of measuring was lower than normal, you dont want the generator to be overloaded because 4 laser printers and 4 coffee machines that were idle when the load was measured, are used together in a blackout.
Another valid approach in some cases is to consult the utility bill for the premises, if this includes 1/2 hour billing or maximum demand billing, then sizing the generator according to the worst 1/2 hours load can be valid.
A whole years bills at least should be consulted since heating or A/C loads vary with the seasons.
This approach wont work with very large loads used for a few minutes, a load of 600KVA used for only 15 minutes will need a 600KVA generator, despite showing as a maximum demand of 300KVA on half hourly billing.
For offices and shops it works well.
Add a margin of say 20% in case the maximum demand increases perhaps due to hotter or colder weather in the years following.
As regards sizing the generator, this depends on the actual loads to be supplied.
If all the loads listed might run together, then the generator needs to be sized for the simple sum of the loads.
If each A/C unit serves a part of the building, then in hot weather all might run together and this needs to be allowed for.
If on the other hand the A/C units are duty/standby, then the total load will be less.
As regards lighting, it would be reasonable to assume in a workplace that all or virtually all of the lights will be on together. Therefore the generator should be able to supply 100% of the lighting load. Note however that this should be 100% of the actuall installed load, not 100% of the lighting circuit ampacity. The actual lighting load may be determined by mesurement, or by counting fixtures and determining the load of each.
General purpose outlets may be heavily loaded, or hardly used, the best approach is to size the generator to supply perhaps 120% of the measured load, not the circuit ampacity. A margin of say 20% should be allowed in case the load at the time of measuring was lower than normal, you dont want the generator to be overloaded because 4 laser printers and 4 coffee machines that were idle when the load was measured, are used together in a blackout.
Another valid approach in some cases is to consult the utility bill for the premises, if this includes 1/2 hour billing or maximum demand billing, then sizing the generator according to the worst 1/2 hours load can be valid.
A whole years bills at least should be consulted since heating or A/C loads vary with the seasons.
This approach wont work with very large loads used for a few minutes, a load of 600KVA used for only 15 minutes will need a 600KVA generator, despite showing as a maximum demand of 300KVA on half hourly billing.
For offices and shops it works well.
Add a margin of say 20% in case the maximum demand increases perhaps due to hotter or colder weather in the years following.
- Location
- Massachusetts
Call a generator vendor, provide them with the load information and they will determine the size needed. Going to more than one vendor would be appropriate to see if they come up with the same results.
It is not as easy as saying the totaled connected load is 150 Kva so I will use a 150 kva generator.
You need to make sure the generator is large enough to maintain voltage and frequency stability when a motor load comes on.
It is not as easy as saying the totaled connected load is 150 Kva so I will use a 150 kva generator.
You need to make sure the generator is large enough to maintain voltage and frequency stability when a motor load comes on.
Thanks for all
From my previouse calculations in other project, we calculate as follow:
Starting load for the largest Pckage unit + the sum of running load of the rest of package units+ lghting loads + sockets load
and the result will multiply by 1.2 for spare ( 80% loading of the generator)
Is this way correct or not?
Please advice
From my previouse calculations in other project, we calculate as follow:
Starting load for the largest Pckage unit + the sum of running load of the rest of package units+ lghting loads + sockets load
and the result will multiply by 1.2 for spare ( 80% loading of the generator)
Is this way correct or not?
Please advice
Ragin Cajun
Senior Member
- Location
- Upstate S.C.
Forget rules of thumb!
Get Generac's sizing program and do it right and sleep tight.
RC
Get Generac's sizing program and do it right and sleep tight.
RC
This method seems like a good beginning. The size of a generator needs to have a capacity of 2 to 3 times the FLA of the motor to start it. What happens if you have to start up after an interruption. All the motors will start at the same time. You need to make sure that does not happen. I also think you should get with a vendor to make sure your calculations are correct.
sinnadurai sripadmanaban
Member
For an already energised building,calculation may be misleading. Go to an electrical contractor or genset supplier who has a datalogger or power analyser and record the V,I,W,Var,VA,pf,kwh,THD etc for a 24hr period and ask the genset manufacturer or an experienced engineer to recommend the size of generator required.Don,t be mislead by max demand or Amp drawn.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
The issue that has not been specifically mentioned is inrush current. To account for this you have to develop a duty cycle for loading.
Upon energization of the bus with the engine generator there are loads that will draw more than running load, i.e. transformer and motors. You have to account for these in starting sequence. Try to add stagger start to the the loads, if possible.
Something to watch for is the starting load requirement is so high, that the EG is much larger than needed for normal running load. If the load on the generator is not at least 30% of its standby rating, you will get what people have termed "wet stacking" or "slobbering". To counteract this a load bank has to be added.
Some mentioned having manufacturer size. This is helpful, but remember they are in the business to sell equipment. Often times they will be high on the very conservative side.
Upon energization of the bus with the engine generator there are loads that will draw more than running load, i.e. transformer and motors. You have to account for these in starting sequence. Try to add stagger start to the the loads, if possible.
Something to watch for is the starting load requirement is so high, that the EG is much larger than needed for normal running load. If the load on the generator is not at least 30% of its standby rating, you will get what people have termed "wet stacking" or "slobbering". To counteract this a load bank has to be added.
Some mentioned having manufacturer size. This is helpful, but remember they are in the business to sell equipment. Often times they will be high on the very conservative side.
different generator company's have different requirements. ng and lpg generac generators allows 150% of full load for starting current get the larges l.r.a. from you a/c units and multiply it by 150% thats is a start. when supplying a office with power, computers and fluorecent lights watch your power factor. Ups's require 3 to 4 times there size. check with the company you plan on getting the generator from they may be different. you can under size the generator and the power factor is so bad nothing will work. when picking a generator diesel vs natural gas look at the the maintenance cost. also output vs fuel which gets the most power with the least fuel usage.
- Status