Generator question

[lcthebeast]

I'm not sure if this against terms of service, I was wondering for informational purposes.
I was working on a job with a generator like this
Multiquip Ultra Silent diesel generator model DCA-25USI2C
(I had nothing to do with its settings)

The 25kVA generator was running in 3 phase 120V/208V, Is this running 120 Amps @120V across all legs, or 120 Amps @120V per leg?

If it was put into single phase 120V/240V, Is this running 208 Amps @ 120V across all legs, or 208 Amps @120V per leg?

Also are my calculations even correct, i just used some online calculator?

 

[Sony]

The rating is per leg, not total, so 120 amps per leg on 3 phase would be 360 amps total output. Same for Single Phase it would be 208 amps per leg so 416 amps total. Thats about as detailed as I can get without violating the CB TOS. I don't know the calculations so I can't verify the numbers.

 

[mstaylor]

Hopefully you had a generator tech or electrician to do the tie-ins. By the way, that is a good generator to work with. Good on noise and very dependable.

 

[lcthebeast]

thank you, my boss did in fact tie-in power.

 

[derekleffew]

Single Phase: 208.3A @120V; Three Phase: 69.4A @208V.

More information at Power Calculator for Generators: Convert kVA to kW, kW to kVA, kW to HP.

 

[mstaylor]

thank you, my boss did in fact tie-in power.

I usually end up being the gen tech. I am the company electrician whether in in a building or tieing to a generator.
My big complaint on the bigger ones that also are 480v capable is the rental company doesn't have it on the right setting.

 

[TimMiller]

lots of rental companies run generators at 480 on construction sites to power the building. They are also ran at 480 when they are running the big rental AC units, for party tents and such. Thats why whenever you do any tie in check your voltage. I once was borrowing a generator and i fired it up (voltage switch was set at 120/240) and i was getting 180 a leg no matter what i did with the voltage adjustment knob. Called my friend who owned it and his first response was thats why all my guys keep burning up drill motors, and my electrican said its fine. Also whenever tieing into any panel watch our for that nasty single leg of 208 that will kill off gear.

 

[mstaylor]

Absolutely you have to be careful. The generator in the OP, if I remember correctly has two sets of taps so you can't accidently hook to too high a voltage, but I lot of times they don't have the tie-in hardware on both taps so you have to move everything. Also, I believe there is a switch inside to change also.

 

[TimMiller]

I was once setting up for an event and a generator rental company dropped off a generator with a 50A cord and a spider box. A group of guys from the were standing around looking real confused at it. I needed power and asked them if they would like me to hook it up for them. They said yes please they had no idea. I checked the voltage and it was set on 480. I called the generator company and raised hell, telling them if they dropped off a generator to a church with a cable and a spider box, and they told you they didnt know anything about it, why didnt you at least set the proper voltage before somebody gets killed. So moral of the story ALWAYS CHECK THE VOLTAGE SELECTOR SWITCH BEFORE STARTING GENERATOR, AND METERING THE POWER AFTERWARDS.

 

[FMEng]

How about some electricity 101. For a three phase source that is 120 VAC to ground, the phase to phase voltage is always 208 V.

To compute the total load presented to a three-phase source, we do not just add volts times amps of the three hot legs.

The formula is:
P=V*I*1.73,
where V is the phase to phase voltage; I is the current of each leg assuming a balanced load. 1.73 is the square root of 3.

 

[FMEng]

lots of rental companies run generators at 480 on construction sites to power the building. They are also ran at 480 when they are running the big rental AC units, for party tents and such. Thats why whenever you do any tie in check your voltage. I once was borrowing a generator and i fired it up (voltage switch was set at 120/240) and i was getting 180 a leg no matter what i did with the voltage adjustment knob. Called my friend who owned it and his first response was thats why all my guys keep burning up drill motors, and my electrican said its fine. Also whenever tieing into any panel watch our for that nasty single leg of 208 that will kill off gear.

What he's referring to is the "wild leg" of a 240 V, three phase service. This happens with a delta connection with a corner ground. This why it's is good practice to always measure the phase to phase and phase to ground voltage whenever you connect to an unfamilar source. This means making six measurements. If we call the three hot legs A, B, and C here is what we want to measure:

A to B (208) [240]
B to C (208) [240]
A to C (208) [240]
A to Neutral (120) [120]
B to Neutral (120) [120]
C to Neutral (120) [208] !!

In parenthesis () I have put the nominal readings for a typical wye connection. The numbers in brackets [] represent what you might find on a corner grounded delta. Smoke would be released from the dimmer or amplifier powered by the C leg from the grounded delta.

Use a digital voltmeter. A Wiggy solenoid type voltage tester is a dangerous tool that should be avoided.

 

[mstaylor]

What he's referring to is the "wild leg" of a 240 V, three phase service. This happens with a delta connection with a corner ground. This why it's is good practice to always measure the phase to phase and phase to ground voltage whenever you connect to an unfamilar source. This means making six measurements. If we call the three hot legs A, B, and C here is what we want to measure:

A to B (208) [240]
B to C (208) [240]
A to C (208) [240]
A to Neutral (120) [120]
B to Neutral (120) [120]
C to Neutral (120) [208] !!

In parenthesis () I have put the nominal readings for a typical wye connection. The numbers in brackets [] represent what you might find on a corner grounded delta. Smoke would be released from the dimmer or amplifier powered by the C leg from the grounded delta.

Use a digital voltmeter. A Wiggy solenoid type voltage tester is a dangerous tool that should be avoided.

I have never seen a generator run a wild leg system. I'm not saying they don't exist but I have never found one. I have worked in buildings with it when I was doing electrical maintainence.