Met with a customer who wants me to bring more circuits into his emergency generator backup panel so that his normal load is at least 30% of the generators capacity. The generator looks really old and says it does 486amp at 120/208 which is what he has. My understanding was to get the total combined load from all the 3 phases to equal at least 146amps. I accomplished this and now he is wondering if I need to hit 146amp on each individual phase. I told him that I would check, but I thought it would be all combined because that is the total load the generator is producing altogether. What are your thoughts, I'd like to know if I'm wrong.
Generator Testing
- Thread starter aptman3
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zbang
Senior Member
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- Roughly 5346 miles from Earls Court
Need to? You don't need to load it at all. If in fact the generator is rated for 486a, that gives you 162a/phase, not 146. What's the KW and KVA ratings and the output breaker size? If the calculated or measured load is greater than 162a/phase, there will be a problem. Might be worth a call to the manufacturer or their local rep.
- Location
- Lockport, IL
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- Retired Electrical Engineer
It absolutely does not! It gives you 486 amps / phase, if we must use that manner of expression. I prefer to avoid the whole "amps per phase" terminology, for it creates more confusion than it resolves. In any event, if you have a balanced current of 486 on each phase, then the "total current" (another phrase I don't like) is 486 amps.
The 146 was derived by taking 30% of 486, which is what the customer wanted for the panel's load.
- Location
- Lockport, IL
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- Retired Electrical Engineer
iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
edited to add; I'm a slow poster. So added to what charlie said:
First you (all) really need to get rid of this concept of adding the current magnitudes from each phase. That practice is of no use in 3 phase - the number is meaningless.
If the generator nameplate says the FLA is "486A", then, if the gen is fully loaded, that is what you would measure with an ammeter on each phase.
Again, if the nameplate does in fact say 208/120, 3 phase, 486A, (and probably .8pf) then the gen is likely a 140kw machine. So, if the owner wants the machine loaded (engine power loading) to 30%, that is 42kw. For a resistive load (1.0pf), the current will be about 117A. Please note this is the current that will be measured on each of the three legs.
iceworm
First you (all) really need to get rid of this concept of adding the current magnitudes from each phase. That practice is of no use in 3 phase - the number is meaningless.
If the generator nameplate says the FLA is "486A", then, if the gen is fully loaded, that is what you would measure with an ammeter on each phase.
Again, if the nameplate does in fact say 208/120, 3 phase, 486A, (and probably .8pf) then the gen is likely a 140kw machine. So, if the owner wants the machine loaded (engine power loading) to 30%, that is 42kw. For a resistive load (1.0pf), the current will be about 117A. Please note this is the current that will be measured on each of the three legs.
iceworm
Last edited:
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
whoops - goofed post
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Charlie- Rereading the original, I see your point, it's not clear the size of the generator- ~150kw or ~50kw. That's why I asked about the actual ratings. The problem here is that there's a "400a/phase" (144kva) generator and a "400a total" (48kva) generator (both assuming 120/208). In some places, it's very common to talk of a generator by total current, so a "1200 amp" generator is 400 amps per phase (or 144kva). I really wish people wouldn't to that, but I can't stop them.
It's also what happens when I read without benefit of sufficient coffee.
It's also what happens when I read without benefit of sufficient coffee.
- Location
- Lockport, IL
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- Retired Electrical Engineer
Well, no there isn't. Are you saying that you have seen machines labeled both ways by their respective manufacturers?
I wish I could stop them as well. Here, at least, I try to break forum members of that habit. So just for the record, may I infer that you are aware that "400 amps per phase" does not equate to "1200 amps total"?
I've never heard of anyone adding up the combined loads of each phase to come up with the load of a gen before... 1200 amp is 1200 on each phase... 400 amp 3pole breakers dont take 1200 amps to trip do they?
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
No, I didn't. I said that some people call them such. Just as some people call a single phase 120/240v service "two phase". (I should have read the use of an exact number as FLA, not a WAG.)
Yes. Blindly adding currents will get you into trouble if you don't know the connections. However, if I have three 48kw single phase L-N loads on a 120/208v feed, that sounds like about 400a/phase to me. OTOH, with L-L loads, then no.
As I mentioned, in some areas of the country and in some industries, they do add up the current because they assume single-phase L-N loads. For instance, if you call a movie/video production house for a "1200 amp generator", you'll probably get one that's rated for about 400a/phase. And if you ask for a 150KW genny, they'll say "How many amps is that?" I've been round those more than once.
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