Generator Sizing

[Grouch1980]

Hi all,
I'm adding a new standby generator to an existing 3 story townhouse. The existing electrical service to this townhouse is 600 amps at 208 volts 3 phase (yes, it's oversized ). The owner wants to backup the entire townhouse on a new generator that will be installed on the roof.

I have the electrical bills for the townhouse for a year's worth.

My question is, what is best... sizing the generator based on the electrical bills, or based on the full 600 amp service? The usage on the bills is way less than 600 amps. A generator based on 600 amps would be way oversized.

 

[LarryFine]

Base it on a realistic worst-case load calc.

 

[augie47]

and..... after he sees the cost you can discuss selected circuits

 

[Grouch1980]

Base it on a realistic worst-case load calc.

Makes sense... something in between the worst-case bill and the full 600 amp service.

 

[Grouch1980]

and..... after he sees the cost you can discuss selected circuits

to be expected .

Thanks guys!

 

[ron]

I would do it based on demand readings from the utility meter.

Some residential meters will show kWh for the overall month (or billing period), but that is not usefull.

If they have a demand charge from the utility, use that. If not, ask them to log into their utility website account, and most will have a way to export the energy usage data in either 5 minute intervals or 15 minute intervals for the whole year.

Then you you multiply the 5 minute data by 12, or the 15 minute data by 4, and that will give you a really good idea of the actual demand load to size your generator.

 

[Jpflex]

Hi all,
I'm adding a new standby generator to an existing 3 story townhouse. The existing electrical service to this townhouse is 600 amps at 208 volts 3 phase (yes, it's oversized ). The owner wants to backup the entire townhouse on a new generator that will be installed on the roof.

I have the electrical bills for the townhouse for a year's worth.

My question is, what is best... sizing the generator based on the electrical bills, or based on the full 600 amp service? The usage on the bills is way less than 600 amps. A generator based on 600 amps would be way oversized.

You didn’t provide data on the yearly usage. Cost per kilo watt hour?

If going off of half 600 ampere 3 phase service at 208 volts =

600 i/ 2 x 208 E x 1.732 = 108,077 KVA generator or at least a 100 KVA generator (our company used a 300 KVA 3 phase delta to supply several lab rooms, blacksmith shop, pilot plant)

108,077 VA / 208 / 1.732 = 300 i ampere panel

 

[electrofelon]

I would do it based on demand readings from the utility meter.
.

This right here ^^^^. I'm assuming the utility has a demand meter if it's 600 amp 3 phase, but perhaps not if it's residential. Definitely an odd service for a dwelling.

 

[Grouch1980]

I would do it based on demand readings from the utility meter.

Some residential meters will show kWh for the overall month (or billing period), but that is not usefull.

If they have a demand charge from the utility, use that. If not, ask them to log into their utility website account, and most will have a way to export the energy usage data in either 5 minute intervals or 15 minute intervals for the whole year.

Then you you multiply the 5 minute data by 12, or the 15 minute data by 4, and that will give you a really good idea of the actual demand load to size your generator

This right here ^^^^. I'm assuming the utility has a demand meter if it's 600 amp 3 phase, but perhaps not if it's residential. Definitely an odd service for a dwelling.

Yes, I do have the demand readings on the utility bills, as well as the KWH readings.

The building has 2 meters. One 400 amp Trans-s Con Edison meter cabinet, and a 200 amp regular meter. The 400 amp meter feeds the residence, and the 200 amp meter feeds what's labeled as 'commercial'... but years ago they combined the 2 spaces into one townhouse. It still has the same meter setup however. All at 208 volts, 3 phase.

So you guys recommend base it on the worst case demand reading? With some extra capacity of course.

 

[ron]

Yes, I do have the demand readings on the utility bills, as well as the KWH readings.

The building has 2 meters. One 400 amp Trans-s Con Edison meter cabinet, and a 200 amp regular meter. The 400 amp meter feeds the residence, and the 200 amp meter feeds what's labeled as 'commercial'... but years ago they combined the 2 spaces into one townhouse. It still has the same meter setup however. All at 208 volts, 3 phase.

So you guys recommend base it on the worst case demand reading? With some extra capacity of course.

The demand reading is as close as you will get to NEC 220.87

 

[electrofelon]

Yes, I do have the demand readings on the utility bills, as well as the KWH readings.

The building has 2 meters. One 400 amp Trans-s Con Edison meter cabinet, and a 200 amp regular meter. The 400 amp meter feeds the residence, and the 200 amp meter feeds what's labeled as 'commercial'... but years ago they combined the 2 spaces into one townhouse. It still has the same meter setup however. All at 208 volts, 3 phase.

So you guys recommend base it on the worst case demand reading? With some extra capacity of course.

So what are the demand readings? If there are two meters then adding the two demand figures will probably over count a bit because they probably didn't occur at the same time but that would be a good figure to go by.

 

[Grouch1980]

So what are the demand readings? If there are two meters then adding the two demand figures will probably over count a bit because they probably didn't occur at the same time but that would be a good figure to go by.

I'll post it tonight... out and about now, i'll get them tonight.

 

[Grouch1980]

So what are the demand readings? If there are two meters then adding the two demand figures will probably over count a bit because they probably didn't occur at the same time but that would be a good figure to go by.

Meter 1 (labeled 'Residential'): the highest demand reading is 1.62 kw.
Meter 2 (labeled 'Commercial'): the highest demand reading is 12.40 kw.

Is my math off? I'm calculating 38.9 amps, by adding both KW's and dividing by 208 volts 3 phase. And with a power factor of .8 accounted for... 48.7 amps.

 

[electrofelon]

Meter 1 (labeled 'Residential'): the highest demand reading is 1.62 kw.
Meter 2 (labeled 'Commercial'): the highest demand reading is 12.40 kw.

Is my math off? I'm calculating 38.9 amps, by adding both KW's and dividing by 208 volts 3 phase. And with a power factor of .8 accounted for... 48.7 amps.

Since the "residential" meter is instrument metered, there is probably a multiplier...?

 

[Grouch1980]

Since the "residential" meter is instrument metered, there is probably a multiplier...?

Actually both meters have a multiplier that shows up on their bills. Here's the bill for the residential meter attached. Do I multiply the 8.40 kw demand by 40? But then I'll be over 600 amps. How is this calculated?

 

[Grouch1980]

Meter 1 (labeled 'Residential'): the highest demand reading is 1.62 kw.
Meter 2 (labeled 'Commercial'): the highest demand reading is 12.40 kw.

Is my math off? I'm calculating 38.9 amps, by adding both KW's and dividing by 208 volts 3 phase. And with a power factor of .8 accounted for... 48.7 amps.

Sorry... it was late... I had it backwards... Residential's highest demand reading is 12.4 kw. Commercial's highest demand reading is 1.62 kw. I had the values flipped.

The attachment that I sent above is one of the residential bills... the demand shown on the bill is 8.4 kw. I believe though that IS the actual power draw for the townhouse... the 'reading diff' of .21 is multiplied by the 'multiplier' of 40... giving you 8.4 kw on the far right column. which translates to 23 amps at 208 volts, 3 phase.

So the 12.4 kw highest demand reading, on the residential meter, translates to 34 amps. With a power factor of .8 it's about 43 amps.

Please let me know if I'm off somewhere, or if this is correct.

 

[electrofelon]

Interesting. Cheap electricity rate but high demand rate. What the hell are they doing in there where they use 10,000 KWH /month?

Ok so I get about 58 amps adding the eo demand together. You don't need to do anything with power factor as three-phase generators are already rated at .8 power factor. I would just maybe look at some other bills to see if that figure holds on different months.

 

[Birken Vogt]

Where I am from, you have to look at maximum demand over an entire year, or something like that.

This is a very small demand for such a large house and service. I would scrutinize it further.

 

[Grouch1980]

Interesting. Cheap electricity rate but high demand rate. What the hell are they doing in there where they use 10,000 KWH /month?

We have a lot of characters in NYC. god knows what they're doing in there.

Ok so I get about 58 amps adding the eo demand together. You don't need to do anything with power factor as three-phase generators are already rated at .8 power factor. I would just maybe look at some other bills to see if that figure holds on different months.

I got 39 amps. 1.62 kw + 12.4 kw = 14.02 kw. 14.02 kw * 1000 / (208 * 1.732) = 38.9. How'd you get 58?

 

[Grouch1980]

Where I am from, you have to look at maximum demand over an entire year, or something like that.

This is a very small demand for such a large house and service. I would scrutinize it further.

I have the bills for an entire year... all of 2023. The 2 values I provided in post #13 are the maximum demands... I just had them flipped.