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# 3-Phase Generator Question

Discuss 3-Phase Generator Question in the Commercial Electrical Advice area at ElectriciansForums.net

C

#### Current Sniffer

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Hi all,

Got an interesting generator related issue here………………. I’m currently helping a friend with a 100KVA geni and I’m after some advice. I’m trying to work out what the maximum current draw would be…………..

So if O\P Amps = KVA / (O\P Volt / 1000) = 100/(240/1000) = 416.66 Amps

But as it’s a 3-phase geni should I be using 415V and not 240V??

The reason I want to know is because I want to add a supply breaker to each of the x3 distribution panels. So assuming my above calculation is correct that’s 138.88Amps per phase or 100amp to be safe.

Many thanks
Steve
Deepest Somerset

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#### ian.settle1

##### -
Mentor
Arms
Wrong voltage! you should be using 230/400 volts for your calculations

S

#### Shakey

Hi all,

Got an interesting generator related issue here………………. I’m currently helping a friend with a 100KVA geni and I’m after some advice. I’m trying to work out what the maximum current draw would be…………..

So if O\P Amps = KVA / (O\P Volt / 1000) = 100/(240/1000) = 416.66 Amps

But as it’s a 3-phase geni should I be using 415V and not 240V??

The reason I want to know is because I want to add a supply breaker to each of the x3 distribution panels. So assuming my above calculation is correct that’s 138.88Amps per phase or 100amp to be safe.

Many thanks
Steve
Deepest Somerset
Hi Steve

i generally do this by assuming an evenly distributed load

so thats 33.3KVA per phase, at 230V = 144A per phase (and most gennys will comftably handle about 10A per phase difference in loading per 50Kva rated output)

BUT that is at unity power factor (ie 1)

which you are only going to get with a purely resistive load

in reality, you will be running at about 0.8 p.f

which gives the genny output at 80Kw of true (or useable!) power

so thats 26.66kw per phase @ 230V = 115A per phase

this is important because if you were using the VA figure of 144A BUT running at 0.8pf then you would be overloading the genny

So yeah, 100A Ib (and In) would be playing it safe

unless of course you are feeding thyristor controlled circuits and we start taking harmonics into account

erm... rather not

T

#### [email protected]

Rather beasty generator set you got on your hands! can i buy some power from you? wat are you running by the way?

C

#### Current Sniffer

Hi Shakey,

Great, many thanks for your help!!

The geni is being used to run several thousand light bulbs (most of which are 11W energy efficient) x3 small 3-phase motors and a few music amplifiers plus associated music kit. So, on the whole it’s a resistive load but I would be interested in ensuring I’m allowing for the correct PF I guess I need to find out the PF rating on the 3-phase motors to get a nearer true rating??

Topgeza, hope the above is of interest to you………… and by the way its mounted in the back of a commercial trailer which forms part of a moving vehicle which is 100 foot long!!

S

#### Shakey

Hi Shakey,

Great, many thanks for your help!!

The geni is being used to run several thousand light bulbs (most of which are 11W energy efficient) x3 small 3-phase motors and a few music amplifiers plus associated music kit. So, on the whole it’s a resistive load but I would be interested in ensuring I’m allowing for the correct PF I guess I need to find out the PF rating on the 3-phase motors to get a nearer true rating??

Topgeza, hope the above is of interest to you………… and by the way its mounted in the back of a commercial trailer which forms part of a moving vehicle which is 100 foot long!!

because of the amount of resistive load your p.f should be pretty good, obviously the motors, being inductive will add Xl to the circuit, and assuming you are running speakers as well, these will also reduce it it.

most decent motors will be above 0.8 pf, so with the resisitve effect of the lamps you could be hitting a p.f of 0.9 or above

still, if you work on the o.8 pf you will be well inside the genny maximum anyway.

I used to use a lot of 25kva units and we used to put them on a load bank at over 40kva and they were chugging away happily.

oh and 100kva is a 'chunky' genny?

nah! i worked on a 1.2MW unit a while ago. Loaded it up the max, and the generator itself started dripping molten metal out the bottom..... guess the windings were knacked anyway......now THAT's a real genny

C

#### Current Sniffer

Cool, Thanks Shakey....................... agreed 100amp per phase looks about right, I’m going to purchase some 16mm 4-core 3-phase arm cable. Just need to make sure the load is balanced correctly, need to keep the neutral current nice and low.

Agreed 100KVA is nothing you want to see a half Mega Watt, now that’s juicy!!

S

#### Shakey

Cool, Thanks Shakey....................... agreed 100amp per phase looks about right, I’m going to purchase some 16mm 4-core 3-phase arm cable. Just need to make sure the load is balanced correctly, need to keep the neutral current nice and low.

Agreed 100KVA is nothing you want to see a half Mega Watt, now that’s juicy!!
ooo, carefull with the neutral, that will only matter if you are running pure three phase balanced loads such as the motors

assuming your lamps are single phase, obviuosly the same current will be running in the neutral as in the line conductors

(sorry if thats egg sucking)

C

#### Current Sniffer

Yea, that’s cool mate, your right the bulbs are obviously only single phase so we’re going to see quite a bit of current flow in the neutral. I’m assuming that if the overall current flow (worst case) is going to be equivalent to that flowing in P1+P2+P3, hence allow for 100A current rating ie 16mm dia cable. I’m planning to split the load over x3 100amp 3-phase distribution boards each of which I want to balance as closely as possible.
Its probably a bit over the top but I’d rather not risk under rating the cable and distribution board.

R

#### rumrunner

Hi ,shakey,
i have never understood this neutral current thing properly ,if you have the time to try and explain it a bit better,i for one stand to learn something ,much appreciated
atvbitwww

S

#### Shakey

Hi ,shakey,
i have never understood this neutral current thing properly ,if you have the time to try and explain it a bit better,i for one stand to learn something ,much appreciated
atvbitwww
Ok Rum, do my best!

with single phase, current flows out on the line, back on the neutral - job done.

ok imagine three phases and a neutral.

L1 drawing 10A
L2 drawing 5A
L3 drawing 15A

all that current goes out....so its gotta come back, so we now have 30A flowing back through the neutral, which would have to be sized accordingly. (this is a simplistic analogy of course)

of course, in practice, we take a Neutral cable out with each phase cable if we are splitting it into three single phase supplies, as this chap is doing with the lamps, so it doesnt matter

ok, imagine supplying a balanced three phase motor.

each phase is 120degrees apart. (their sine waves i mean)

so when L1 is peaking, L2 is at adifferent point in its 360 degree angular cycle , as is L3

now at this point we go into vectors, and i was on the 'Bow last night so i aint going there

so: the three phases, when 'added' together, produce a net result of zero (in VERY simple terms)

what this means is some current is flowing out on L1, a little bit is flowing in L2, and some is flowing back in L3 (and in reality this relationship is shifting between the three phases as they move through their relative 360degree cycles)

SO: there is no resultant current to flow back though the neutral, and it becomes a 'dead' conductor

SO: with balanced three phase motor supplies we often dont bother putting the neutral in (because it will just get bored)

of course, if the load was unbalanced (say two of the phases draw 10A and the third draws 8A) THEN the resulant current difference would flow back through the neutral!

ok, thats me done Rum, hope this helped

oh, and i just made it all up, of course

R

#### rumrunner

yes that helped ,i understand that now ,i knew some of it but did,nt know why , i am really pleased you answered again in such a clear way shake ,you should write a book
atvbitwww

M

#### MacSparky

I agree with rum on that shakey, if you write a book in the manner you answer these posts then i'll definately have a copy.....and i'm sure you wouldn't go short on orders

T

#### [email protected]

Hi ,shakey,
i have never understood this neutral current thing properly ,if you have the time to try and explain it a bit better,i for one stand to learn something ,much appreciated
atvbitwww
Rum runner,

The way i could only properly explain this is if you could imagine 3 houses, all single phase supplies, and each house is connected to a phase, so 1 connected to brown, 1 to black and 1 to grey!

Each house pulls slightly different currents to each other, and house 1 is running at 20 amps, house 2 at 22 amps and house 3 at 24 amps.

So if you think about it, the number common to all three houses is 20, so all the houses have at least 20 amps of load. The neutral does exactly wat is says basically it neutralises any excess unbalanced currents. the current in the neutral would be 6 amps.

Hows that?

R

#### rumrunner

hi ,
Well done ,the neutral ,"neutralises the current" what a player you are topgezza ,ill neverforget that
thanks

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