We are working on a project where there will be a portable generator brought to a facility in the event of a power failure. 480V 3-phase. We will have a fused disconnect located outside the building with cam lok connectors. Not knowing what portable generator will be used, I am trying to determine if a 3-pole or 4-pole transfer should be used. I am trying to find info on the portable generator (to see how the neutral is bonded at the genset), but they may have several and whichever one is available will be pulled up and plugged in. It is almost like there needs to be a sign someplace saying what type of generator can be used. How is this normally handled when you never know what unit will be plugged into it?
portable generator 3-pole or 4-pole
- Thread starter mjmike
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kingpb
Senior Member
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Is it powering a board that requires 277V loads? If so, than it will require a 480Y/277V machine. If it is powering a 480V 3 phase board, with no loads requiring a neutral, than a 480V 3 phase is all you need.
Whoever is responsible for getting the generator would be the person that needs to be knowledgeable about it.
i would say there needs to be a placard at the point of interconnection that states what the machine rating should be.
Whoever is responsible for getting the generator would be the person that needs to be knowledgeable about it.
i would say there needs to be a placard at the point of interconnection that states what the machine rating should be.
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- Placerville, CA, USA
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Since the important difference between an SDS and non-SDS connection is whether there is an earth-neutral bond at the generator (for non-SDS there cannot be, since that would be a second bonding point) I suggest the following:
If there is a ground-neutral bond internal to the supplied generator which you do not have the option of disconnecting, then you will want to switch the neutral, so that the ground-neutral bond used for the POCO service does not become a second bonding point. This will be an SDS installation.
That means that you will have to use a 4-pole transfer switch.
If there is not a ground-neutral bond at the supplied generator, you have the option of making one or else connecting the generator as a non-SDS.
In that case you do not want the neutral to be switched. The simplest way to do that with an installed 4-pole transfer switch is simply to connect a jumper across that pole of the switch.
So provide a convenient, well marked place to install that jumper, and document when it should or should not be installed.
You will also need to make sure that you provide for a 5 wire connection to the generator, not just 4. The EGC will never be switched though.
PS: If the generator is a 480V delta only, there will not be a neutral, while if it is a 480Y/277 there will be a neutral. It is my current understanding in the latter case that if your facility does not use the neutral for any loads, it still has to be brought to the the transfer switch and connected to any grounded conductor that may be present there.
In the first case, the grounding of the generator output has to be compatible with what your facility uses.
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got some more info. The generator neutral is bonded to ground at the portable generator. Therefore, the transfer switch needed would be 4-pole as it is separately derived. However, the portable generator is not currently using the neutral. It is set up to just use the phases and ground. The control panel after the transfer switch may not need the neutral. The distribution would be a fused service disconnect, then transfer switch, then control panel. If the neutral is not needed, I see landing the neutral at the service disconnect and bonding to ground there and not carrying it onto the transfer switch or control panel; generator can stay wired as it is. The transfer switch required is then questionable; 4-pole to plan for the future possibly. However, if there is a place for the neutral in the control panel, the neutral would be needed and the generator would have to be modified to run the neutral as well (5-wire as opposed to just 4-wire). The 4-pole switch would be needed.
iceworm
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MJ - I see a bunch of issues. I suspect you are already well aware:
- From your previous posts I suspect the premisis wiring is 480V grounded wye. If so you have to have the gen neutral. Not available does not make any sense. Is the neutral just not in the gen connection box? If so, that's odd - unless the gen is HRG or Delta, both of which are possible but surprising.
- You really shouldn't ground the neutral at the gen and at the outside disconnect - not good.
- What is the "control panel"? You really have to nail down if the 480V wye neutral is required.
- The temp gen connection is either going to be non-SDS, no gen N-G bond, 3pole transfer switch, 480V 5wire from gen to outside disconnect. Or is it SDS, gen has an N-G bond, 4pole transfer switch, still 480V 5wire from gen to outside disconnect.
Yes, I agree if the gen does not match the premisis wiring the gen will have to be modified. Also agree with the post that recommended the temp connection be placarded to show the required gen conection.
Let us know. This is an interesting job.
ice
To clarify, the service voltage is 480Y/277V with neutral to the service disconnect. From the service disconnect, you go to a 480V control panel not requiring a neutral because no 277V load. At the control panel, it feeds 480V loads and steps down the voltage. There are only connections for the phase conductors and ground at the control panel. There will now be a transfer switch to back up the control panel. Since there is no neutral to the control panel, there will be no neutral at the transfer switch to switch. So, the question becomes, 4-pole switch because the genset would be considered separately derived, or 3-pole switch. Maybe the thing to do, is have a 4-pole switch, bring the neutral over to it from the service disconnect and from the outside genset connection point, and stop there. The transfer switch neutral would not have a load side conductor.
iceworm
Curmudgeon still using printed IEEE Color Books
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- EE (Field - as little design as possible)
This "control panel" sounds like a panel with 100A - 1000A CB or maybe even an MCC. Is this "control panel" the only thing fed from the utility service? Are there any loads in the plant that require a 480V neutral?
If you want the gen to be non-sds, the gen neutral has to hit the utility neutral cause that is where the N-G bond will be.
You are the engineer-of-record. I suggest you draw up a one-line. Itis hard to design this stuff with out that basic document.
ice
If you want the gen to be non-sds, the gen neutral has to hit the utility neutral cause that is where the N-G bond will be.
You are the engineer-of-record. I suggest you draw up a one-line. Itis hard to design this stuff with out that basic document.
ice
Nothing else requires a neutral; as stated, the neutral stops at the service disconnect. Assume a 100A breaker in the control panel. Because the portable generator has a N-G bond, I believe this requires the use of a 4-pole transfer switch. I do not think you can have a 3-pole transfer switch and have the neutral bonded to ground at the generator.
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- Placerville, CA, USA
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- Retired PV System Designer
If you are not going to make a neutral connection on the load side of the transfer switch, I see no reason to have a switch pole for it. Simply do not connect the service neutral and the generator neutral to each other.
Landing them on two separate terminals of a terminal block will be just as good as connecting them to the unused pole of a 4-pole switch.
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Landing them on two separate terminals of a terminal block will be just as good as connecting them to the unused pole of a 4-pole switch.
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This helps. I was thinking because the generator was separately derived, I would be required to have a 4-pole transfer switch even if the 4th pole wasn't used. It sounds like it is not a code requirements, correct?
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- Retired PV System Designer
I cannot say for sure whether it is a code requirement in this case. I can just apply common sense, which is not always good enough. 
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abhishekbt
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Thanks for the detailed explanation here on this forum as I am trying to learn about the electrical calculation and understanding the logic for any other requirements. I think my stay at this forum is going to be very enjoyable and a good learning experience.
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