Generator

Location
NJ
Hello, I have question about the method for sizing a generator to feed both 480v and 208/120v loads. I'm connecting to a 480 volt panel with a main, then feeding a step down xfrm to a 208/120 panel. My loads to be served are both 480v and 120v. I made a chart with a 480 column and a 208/120 column and calculated the wattage for each load. Now, my question is: can I just add the wattages together from each column to get the total KW needed. Of course I'm going to give myself a little wiggle room, but I just wanted to check my logic about calculating wattage at 480 for the 480 loads, then calculating the wattage for the 120v loads and adding them together for the total. I'm thinking that because watts are a product of current and voltage (taking into account the 1.723 and p.f. for 3 phase) I should be on the right track. What do you think?
Regards
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yes. The calculation of current in the phase lines can involve square root of three, but power is power and it just adds. Period.

Sent from my XT1080 using Tapatalk
 

BAHTAH

Senior Member
Location
United States
Generator

Hello, I have question about the method for sizing a generator to feed both 480v and 208/120v loads. I'm connecting to a 480 volt panel with a main, then feeding a step down xfrm to a 208/120 panel. My loads to be served are both 480v and 120v. I made a chart with a 480 column and a 208/120 column and calculated the wattage for each load. Now, my question is: can I just add the wattages together from each column to get the total KW needed. Of course I'm going to give myself a little wiggle room, but I just wanted to check my logic about calculating wattage at 480 for the 480 loads, then calculating the wattage for the 120v loads and adding them together for the total. I'm thinking that because watts are a product of current and voltage (taking into account the 1.723 and p.f. for 3 phase) I should be on the right track. What do you think?
Regards
your generator will be sized for your 480V load. The watts on the primary of a transformer is the same on the secondary. Your 480V panel supplies your transformer so you only need to look at your 480V load and then allow for some wiggle room. Be sure to consider the fuel source and altitude of your generator location as they affect the ultimate capacity of the generator.
 

Besoeker

Senior Member
Location
UK
The generator will have a kVA rating which determines maximum current that can be drawn on any phase.
The prime mover, often a reciprocating engine, will have a maximum power. You can't exceed that either.

For the prime mover, you can add powers. For the generator you have to add the currents in each phase taking account of power factors.
 

Besoeker

Senior Member
Location
UK
Yes but you have to take into account the starting kW for each load, sum up them also to decide the size of the prime mover be it diesel or gas.
Starting infers motor starting in which case kVA rather than kW would be the issue and that would impact the generator rather than the prime mover.
 

Sahib

Senior Member
Location
India
Starting infers motor starting in which case kVA rather than kW would be the issue and that would impact the generator rather than the prime mover.
If 'starting' is too much related to 'motors, let us use 'initial' then. So, for example, the initial kW of a set of tungsten lamps is higher than its running, nay,operating kW.
 

Besoeker

Senior Member
Location
UK
So, for example, the initial kW of a set of tungsten lamps is higher than its running, nay,operating kW.
Do you know how many cycles the inrush will last?
Do you think that will have any measurable energy impact on the rotational inertia of the genset?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Bes - Breathe, relax, take a chill pill

Amazingly, Sahib is somewhat correct. Yes, I said that.

Starting motors that are more that half the capacity of the gen will suck the gen right to it's knees. One must pay a bit of attention to that.

Transformer energization is another that will send the gen nutz (engineering definition of the "nutz"). One must pay attention to that as well.

The OP has not given us any indication if the loading is MW, 100s of KW, 10s of KW. So the responses are real general.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
access -
In addition to the excellent comments already posted, you might put in 5% of the xfm kva for xfm losses. However, if you are sizing that close, you are likely already fubared from the motor starting, xfm inrush.

A second item is if you want the gen rated prime, continuous, or standby. This has to co with the hours/month you expect to run the gen, and how close rated output do you expect to be. This mostly matters if you are over 500kw (or so)

Don't let the infighting scare you off - we are interested.

ice
 
Location
NJ
Thank you for the ideas. One thing I thought of was making sure that the loads were balanced at 208/120 panel. The way this is set up is that I have 7, 100 amp, single phase panels being fed from the 208/120 panel, each panel being fed by a 2/100 amp c.b. My calculated load from the 120 volt column is 452 amps. However, I can distribute that load between 3 phase legs. Basically I have a lot of temporary lighting, 30 strings with 10 lampholders each so I calculated each lampholder at 100 watts for a total of 30 kw, divided by 120 volt gives me 250 amps, plus another 202 amps of 120 volt loads. I'm not going to take my 120 volt loads and divide by three because I can't guarantee that the loads are going to be perfectly distributed among three phase legs. However, I think can take some diversity here, or derating of the load (not sure of the right terminology ) at about 70 percent which gives me 316 amps. I'm in the process of drawing out the one-line to see what the actual loads per phase would be in a perfect world. Fortunately I will be the one doing the install so I can make sure I hook everything up right. What do you think about distribution of loads on the 120 side?

p.s. I looked on-line for a calculator or an excel spreadsheet but didn't find anything really useful.
 
Location
NJ
actual values

actual values

On the 480 side I have 2 motors, a sump pump (14 amps) that will run for 20 minutes early in the morning at the same time with the lighting load. The other is a booster pump that runs about 4 minutes every 20 minutes while being used and it is used about 2 hours a day. I have a power-pak that provides temporary 120 volt lighting through a 50 kva xfrm (480 to 240/120) . I put an amp probe on this at the secondary of the xfrm and it was drawing 36 amps, 10 amp and 1.5 amps, respectively. There is a 225 kva xfrm between the 480 panel and the 208/120, the 208/120 panel has a 225 amp main breaker. I know, the xfrm is capable of providing 625 amps at the secondary but is protected at 225 amps.

408v side:
sump 480, 3p....................................14 amp
booster 480, 3p.................................15 amp
Power-pak 480, 3p.................approx. 20 amps

On the 120 side I have:
temp lighting 30 strings at 1000 watts each for 30 kw/120v...........250 amp
fan equipment...............................................................................150 amp
Misc. tool load ................................................................................40 amp
1500 w heater..............................................................................12.5 amp
 

Smart $

Esteemed Member
Location
Ohio
Thank you for the ideas. One thing I thought of was making sure that the loads were balanced at 208/120 panel. The way this is set up is that I have 7, 100 amp, single phase panels being fed from the 208/120 panel, each panel being fed by a 2/100 amp c.b. My calculated load from the 120 volt column is 452 amps. However, I can distribute that load between 3 phase legs. Basically I have a lot of temporary lighting, 30 strings with 10 lampholders each so I calculated each lampholder at 100 watts for a total of 30 kw, divided by 120 volt gives me 250 amps, plus another 202 amps of 120 volt loads. I'm not going to take my 120 volt loads and divide by three because I can't guarantee that the loads are going to be perfectly distributed among three phase legs. However, I think can take some diversity here, or derating of the load (not sure of the right terminology ) at about 70 percent which gives me 316 amps. I'm in the process of drawing out the one-line to see what the actual loads per phase would be in a perfect world. Fortunately I will be the one doing the install so I can make sure I hook everything up right. What do you think about distribution of loads on the 120 side?

p.s. I looked on-line for a calculator or an excel spreadsheet but didn't find anything really useful.
We're still getting sparse information. We need comprehensive information or this is going to be a really long thread if you want a conclusive determination!!!

You mentioned temporary lighting. Is this a temporary power situation for during construction for example or permanently installed? If permanent, full-time main power or utility backup ? legally required or optional standby ? automatic or manual transfer? Motor loads? Transformers, their rating and configuration (some of which you already supplied)?

Essentially you will have to do an Article 220 load calculation (at the very least enumerate all loads and their kVA values for such) if you want to power up everything. Temporary and manual transfer you can get by with just what you want to power up. For permanently installed with automatic transfer, the gennie must be capable of powering the entire calculated load connected by the transfer.
 

Besoeker

Senior Member
Location
UK
Bes - Breathe, relax, take a chill pill

Amazingly, Sahib is somewhat correct. Yes, I said that.

Starting motors that are more that half the capacity of the gen will suck the gen right to it's knees.
Yes. The generator. kVA. The A part which is what I posted earlier. Voltage regulation.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
On the 480 side I have 2 motors, a sump pump (14 amps) that will run for 20 minutes early in the morning at the same time with the lighting load. The other is a booster pump that runs about 4 minutes every 20 minutes while being used and it is used about 2 hours a day. I have a power-pak that provides temporary 120 volt lighting through a 50 kva xfrm (480 to 240/120) . I put an amp probe on this at the secondary of the xfrm and it was drawing 36 amps, 10 amp and 1.5 amps, respectively. There is a 225 kva xfrm between the 480 panel and the 208/120, the 208/120 panel has a 225 amp main breaker. I know, the xfrm is capable of providing 625 amps at the secondary but is protected at 225 amps.

408v side:
sump 480, 3p....................................14 amp
booster 480, 3p.................................15 amp
Power-pak 480, 3p.................approx. 20 amps

On the 120 side I have:
temp lighting 30 strings at 1000 watts each for 30 kw/120v...........250 amp
fan equipment...............................................................................150 amp
Misc. tool load ................................................................................40 amp
1500 w heater..............................................................................12.5 amp
You are doing great. Welcome to being the engineer of record.

You said this is a temporary installation. How long do you expect?

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
6 to 9 months
Repeat question: Are you renting a gen - or buying, or perhaps have several of different sizes available in the yard?

Economics:
6 months 24/7 might be long enough to buy one. 9 months. 40hrs/week - maybe not.

Fuel costs don't matter much. One could run lightly loaded, somewhat below peak efficiency and not care. It is only 6 months.

Longevity:
Running 247, near 100%, may well wear it out.
If it is a rental, so what.

If it is owned, and is expected to be of further use, you may wish to get a prime rated unit or get one rated such that it is only loaded to 80% max.

Just trade offs the EOR gets to resolve.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Sounds like you are expecting some imbalance. That can cause the gen to heat up more than expected. Depending on how much your load calc shows, you may wish to contact the gen mfg and ask if the calculated imbalance is a problem.

ice
 
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