Wiring 3 phase Generator

[EasyE]

I have 120/208 Wye at my shop and am wiring in a generator with ATS. This may be a silly question but....

Is it important to match the phases to the incoming utility phases? I will not be running any motors while the generator is running. Only single phase loads.

If I have to match the phases what is the easiest way to do this?

 

[Fasthotrod]

When we start looking at the utility, an ATS, and a generator, we have to make some assumptions about the utility and which wire is A-phase, B-Phase, and C-Phase. In truth, it doesn't really matter what you call them, as long as the phases are in the same phase rotation.

You can get a phase rotation meter to verify this. You want to connect the phase rotation meter to the utility side and see what direction it's rotating, then when you run the wiring from the main disconnect to the ATS, you can match the rotation there.

Clockwise: A-B-C, B-C-A, and C-A-B.
Counterclockwise: A-C-B, C-B-A, and B-A-C.

It's good practice to physically verify that the generator A-phase, B-phase, and C-phase are connected to the ATS exactly as the diagrams show, and verify that it's rotating clockwise. Then connect the utility side to the ATS with the phases in a clockwise rotation, that way there is no confusion in the future during testing/troubleshooting.

Hope this helps.

Mark

 

[MBfreak]

Before even worrying about phase rotation being the same, you absolutely have to have an approved switch over installation that totally prevents that you backfeed the genarator power out on the utility network.
In Europe, approved fail safe mechanical interlocks are mandated.
Sinners, if caught, are prosecuted with hefty fines.
Utility linesmen have died from backfed "dead lines". The lines should have been shortcircuited and grounded b4 working on them, but sometimes mistakes happen.
Also the other way around, your generator will thank you.

Ola

 

[Norcal]

Before even worrying about phase rotation being the same, you absolutely have to have an approved switch over installation that totally prevents that you backfeed the genarator power out on the utility network.
In Europe, approved fail safe mechanical interlocks are mandated.
Sinners, if caught, are prosecuted with hefty fines.
Utility linesmen have died from backfed "dead lines". The lines should have been shortcircuited and grounded b4 working on them, but sometimes mistakes happen.
Also the other way around, your generator will thank you.

Ola

If the OP is using a automatic transfer switch, ATS, there will be no backfeeding issues.

 

[EasyE]

Before even worrying about phase rotation being the same, you absolutely have to have an approved switch over installation that totally prevents that you backfeed the genarator power out on the utility network.
In Europe, approved fail safe mechanical interlocks are mandated.
Sinners, if caught, are prosecuted with hefty fines.
Utility linesmen have died from backfed "dead lines". The lines should have been shortcircuited and grounded b4 working on them, but sometimes mistakes happen.
Also the other way around, your generator will thank you.

Ola

Got it covered with a cummins OTEC400 ATS.....

 

[Fasthotrod]

Got it covered with a cummins OTEC400 ATS.....

I assume it's like this one?

https://www.electricgeneratorsdirect.com/Cummins-Power-Generation-A040C970/p14261.html

Do you know if your ATS is a three-pole or a four-pole switch? If you look at Figure 11 on page 22 of the manual, it shows the difference between the two.

https://www.electricgeneratorsdirect.com/manuals/218b5d302c201be3ff74846278414fc8.pdf

If it is a three-pole switch, then you need to be mindful of the NEC in regards to how the generator is connected to the system. If that ATS is a three-pole switch, you will see one pole per phase where the ATS contacts are located. There will be a neutral bar for the neutral connections on the upper right side of the switch. That means that this is NOT a Separately Derived Source per the NEC, and you cannot have a neutral/ground bond at the generator.

If this is a four-pole ATS, then you have a switched neutral. That means that the generator IS a Separately Derived System. Each source (utility and generator) will have it's own neutral/ground bond at each location.

Mark

 

[EasyE]

I ordered the 3 pole model. You can figure this is a permanently fixed unit so I did not run a ground wire back to the genset. I was planning on grounding the genset to a ground rod.

The neutral will not be bonded to the ground either. Am I missing anything there?

 

[wyliesdiesels]

I ordered the 3 pole model. You can figure this is a permanently fixed unit so I did not run a ground wire back to the genset. I was planning on grounding the genset to a ground rod.

The neutral will not be bonded to the ground either. Am I missing anything there?

That is incorrect. you are confusing grounding electrodes with equipment grounding conductors. They are NOT the same and do not serve the same purpose.

A grounding electrode will NOT "ground" the generator. This is because grounding electrodes are for shunting lightning. You dont need a grounding electrode for a generator. It wont do anything for it.

The grounding electrode will not enable a breaker to clear fault current. You need an equipment grounding conductor to do this. Since this is a 3-phase generator, you will need 5 wires.

 

[nadogail]

I have 120/208 Wye at my shop and am wiring in a generator with ATS. This may be a silly question but....

Is it important to match the phases to the incoming utility phases? I will not be running any motors while the generator is running. Only single phase loads.

If I have to match the phases what is the easiest way to do this?

Absolutely it is IMPORTANT, Robert Burns got it right when he wrote about the best laid plans of both mice and men.

The easiest way I have found to verify motor rotation is to "bump" the motor before connecting a load to the shaft. In your case you will need to do it twice once on Utility power and again on Generator power.

 

[EasyE]

Ok thanks for clearing that up. So I need to pull an additional wire for the ground and terminate it on a ground block in the ATS?
Should this wire be the same size as my hot legs or can I decrease the size?

 

[wyliesdiesels]

EGC is sized by the breaker size and table 250.122

https://www.academia.edu/11639110/N...onductors_for_Grounding_Raceway_and_Equipment

 

[EasyE]

EGC is sized by the breaker size and table 250.122

https://www.academia.edu/11639110/N...onductors_for_Grounding_Raceway_and_Equipment

90 amp breaker so looks like #8 copper will work for me. Thanks!

 

[wyliesdiesels]

90 amp breaker so looks like #8 copper will work for me. Thanks!

That is assuming you did not oversize the ungrounded conductors. What size are the ungrounded conductors?

 

[EasyE]

That is assuming you did not oversize the ungrounded conductors. What size are the ungrounded conductors?

I used 1/0 in case I upsized the generator later. Its also about a 100' run.

 

[wyliesdiesels]

I used 1/0 in case I upsized the generator later. Its also about a 100' run.

ok so then #8 will not work for the EGC.

you will need to increase the EGC proportionately the same amount that you increased the ungrounded conductors.

Assuming everything is copper

#3cu= 52633CM
#1/0cu= 105531CM

105531/52633 = 2.005

#8cu = 16509

16509* 2.005 = 33,101

nearest size is #4 cu so that is what is needed for the EGC

 

[EasyE]

ok so then #8 will not work for the EGC.

you will need to increase the EGC proportionately the same amount that you increased the ungrounded conductors.

Assuming everything is copper

#3cu= 52633CM
#1/0cu= 105531CM

105531/52633 = 2.005

#8cu = 16509

16509* 2.005 = 33,101

nearest size is #4 cu so that is what is needed for the EGC

You’re the man!! Thank you!