generator/transfer switch questions

[ericsherman37]

So, being an apprentice, I am still in the process of grasping some of the more basic concepts of electrical installations. That being said, I'll soon be helping with a project involving the installation of a transfer switch and generator for a small city Water Treatment plant. I helped haul down some parts and materials today, and my journeyman gave me a brief walkthrough of what we were going to be doing. It raised a few questions in my mind that I would like to receive input on, if y'all would be so kind

Following is a picture of what we will tentatively be doing. There is no currently no transfer switch, just an MCC which contains pretty much everything the plant runs from. As is, the service (red wires) comes from underground right into the PoCo CT Metering cabinet, and then up over to the 600A service breaker. We will install the transfer switch immediately adjacent to the MCC.

As the caption states, the service breaker's load side is tied directly to the MCC bussing (busing?) with some jumper bars. My journeyman said we are going to remove these and run wires straight from the service breaker's load side over to the transfer switch (blue). The transfer switch load wires will come right back (purple) and land on some bus lugs which we will install at the location where we removed the bus jumper bars. A big switch loop, kind of.

Here's the picture I made, as I envision how this would work. A couple of questions follow:

1) With the blue set of wires coming from the service breaker load side, would a neutral need to be run with those phase wires? Intuition tells me so, but the transfer switch is a 3-pole and doesn't switch the neutral... it wouldn't be necessary for functionality, but something is telling me that it should be there. Or can it just stay where it originally is, landed on the MCC neutral bus?

2) I'm kind of confused about the generator EGC. It won't be a separately derived system, so the EGC isn't bonded to the generator neutral at its breaker enclosure. So if there was a fault to ground in, say, the generator conduit while the generator was running, am I correct in observing that the fault current would have to go allll the way back to the MCC ground bus, through the main bonding jumper and alllllllll the way back to the source via the generator neutral?

3) For that matter, how does the transfer switch go about getting bonded? Is a bonding jumper over to the MCC ground bus sufficient, as per the illustration? If so, would it be sized as an EGC based on the generator OCPD? Or the service OCPD? Or whichever is largest? Or would it be sized based on Table 250.66? Why?

4) Keep in mind that this illustration is based on a very quick glance at the setup and scenario. I haven't gotten to poke around at all so there may be some inaccuracies in the picture that don't reflect the actual setup. I am of course not running this job, but I haven't really had a chance thus far in my career to be involved in a backup system installation from the onset, so my curiosity is piqued and I'm just trying to understand the system. Therefore, the final question is: are there any glaring errors that I have overlooked? Anything to note? What did I miss or screw up on (aside from the previous questions).

Sorry for the really long post, but I greatly appreciate the time that anyone is willing to take to help me understand these concepts. Thanks!

P.S. PoCo approved and is fine with having the wiring run through their CT cabinet.

 

[ibew441dc]

Here's the picture I made, as I envision how this would work. A couple of questions follow:

1) With the blue set of wires coming from the service breaker load side, would a neutral need to be run with those phase wires? Intuition tells me so, but the transfer switch is a 3-pole and doesn't switch the neutral... it wouldn't be necessary for functionality, but something is telling me that it should be there. Or can it just stay where it originally is, landed on the MCC neutral bus?

View attachment 3286 I'm thinking it'll end up something like this.:smile:

2) I'm kind of confused about the generator EGC. It won't be a separately derived system, so the EGC isn't bonded to the generator neutral at its breaker enclosure. So if there was a fault to ground in, say, the generator conduit while the generator was running, am I correct in observing that the fault current would have to go allll the way back to the MCC ground bus, through the main bonding jumper and alllllllll the way back to the source via the generator neutral?

correct:smile:

3) For that matter, how does the transfer switch go about getting bonded? Is a bonding jumper over to the MCC ground bus sufficient, as per the illustration? If so, would it be sized as an EGC based on the generator OCPD? Or the service OCPD? Or whichever is largest? Or would it be sized based on Table 250.66? Why?

The normal power, generator power, and load conductors should all have an appropriately sized EGC ran with them, and each should be bonded to the transfer switch enclosure.

4) Keep in mind that this illustration is based on a very quick glance at the setup and scenario. I haven't gotten to poke around at all so there may be some inaccuracies in the picture that don't reflect the actual setup. I am of course not running this job, but I haven't really had a chance thus far in my career to be involved in a backup system installation from the onset, so my curiosity is piqued and I'm just trying to understand the system. Therefore, the final question is: are there any glaring errors that I have overlooked? Anything to note? What did I miss or screw up on (aside from the previous questions).

I think in no time, it is obvious that one such as yourself will conquer the basics of this set up. It takes some guts to actually want to know how something is properly put together and you got em.....I can very much relate(you probably do a lot of teeth biting ...wanting to get the whole picture, it'll come as long as you stay on the same track)

Disclaimer If I missed anything of major importance....I'm sure somebody will pick it up.:smile:

 

[ericsherman37]

Hey that makes a lot more sense that way Although I don't appreciate the violation of the sanctity of my artwork.

So the normal power and the transfer switch load power both need their own EGC even though they're in the same cabinet?

 

[ibew441dc]

Hey that makes a lot more sense that way Although I don't appreciate the violation of the sanctity of my artwork.

So the normal power and the transfer switch load power both need their own EGC even though they're in the same cabinet?

If those nipples are PVC then I would say yes.....if they are rigid, emt, close, chase, ect. then no additional conductor is required.

It probably took me longer to dirty up your art work than it took you to make it pretty:grin: It is a paint in the butt

 

[ibew441dc]

So the normal power and the transfer switch load power both need their own EGC even though they're in the same cabinet?

The following are important areas to be aware of ....there are more but these stand out to me right now.

300.3(B) Conductors of the Same Circuit.
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

250.90 General.
Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed.

250.102(D)Size ? Equipment Bonding Jumper on Load Side of Service.
The equipment bonding jumper on the load side of the service overcurrent devices shall be sized, as a minimum, in accordance with the sizes listed in Table 250.122, but shall not be required to be larger than the largest ungrounded circuit conductors supplying the equipment and shall not be smaller than 14 AWG.
A single common continuous equipment bonding jumper shall be permitted to connect two or more raceways or cables where the bonding jumper is sized in accordance with Table 250.122 for the largest overcurrent device supplying circuits therein.

 

[ericsherman37]

Yeah.. 250.90 "Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed."

I've been staring at this (revised) picture trying to envision the fault current path. I pick a random point and say "This phase just grounded out - what happens?" and figure out if and how it will get back to its source and trip its breaker.

It's kind of a challenge. But then again, when I first started I couldn't understand why it didn't electrically matter which terminal screw you attached a switch leg to on a single pole switch. And it took me who knows how long to figure out 3-ways. Now, with practice, it's a piece of cake, and I'm sure that, given time, I'll get this kind of situation down too. But for now, I treat it like I treat ladder diagrams: stare blankly at it until I go "duh!" Results vary.

Thanks for the input, keep 'em coming!

 

[ibew441dc]

Yeah.. 250.90 "Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed."

I've been staring at this (revised) picture trying to envision the fault current path. I pick a random point and say "This phase just grounded out - what happens?" and figure out if and how it will get back to its source and trip its breaker.

It's kind of a challenge. But then again, when I first started I couldn't understand why it didn't electrically matter which terminal screw you attached a switch leg to on a single pole switch. And it took me who knows how long to figure out 3-ways. Now, with practice, it's a piece of cake, and I'm sure that, given time, I'll get this kind of situation down too. But for now, I treat it like I treat ladder diagrams: stare blankly at it until I go "duh!" Results vary.

Thanks for the input, keep 'em coming!

This may be a little more clear....In your situation imagine another phase, this is only single phase

 

[steve66]

Sorry, but I will admit I didn't read your entire post. However, I see one thing that might be an issue:

I don't think the service conductors are allowed in the same CT enclosure or wireway with either the feeders from the service disconnect, or the feeder from the generator.

Steve

 

[hurk27]

Sorry, but I will admit I didn't read your entire post. However, I see one thing that might be an issue:

I don't think the service conductors are allowed in the same CT enclosure or wireway with either the feeders from the service disconnect, or the feeder from the generator.

Steve

Edited because I thought the OP diagram was a combo CT/service disconnect type

 

[ericsherman37]

Sorry, but I will admit I didn't read your entire post. However, I see one thing that might be an issue:

I don't think the service conductors are allowed in the same CT enclosure or wireway with either the feeders from the service disconnect, or the feeder from the generator.

Steve

Check the P.S. at the very end of the OP. Power company checked it out and they're fine with it - there aren't very many options in this building They just want us to make sure the non-service conductors are adequately secured and tucked off to the side as best as we can.

The place was built in 1992 - I don't know why they didn't install or at least make provisions for a backup system at that point. I mean it's the town's water supply, you know? Maybe they just wanted to leave it so I could learn how to do it some day

 

[Pierre C Belarge]

I am surprised the POCO permits the "line-load" or "other conductors" in the same location.
The POCO and the local inspectiors do not permit this type of install.

 

[ericsherman37]

I am surprised the POCO permits the "line-load" or "other conductors" in the same location.
The POCO and the local inspectiors do not permit this type of install.

There's a few other existing buildings in the area that I can think of that have a big ol' CT cabinet located inside and occupying the only viable access point for getting any new wiring to service equipment.

PoCo generally grumbles at these, but given the expense of the alternative to the municipality (at least in this case), apparently they said okay. The electrical inspector said that if PoCo is fine with it, so is he.

It's a small community out here and everyone tries to help each other out when possible. The City of Yachats (the place in question) has a population of roughly 600, and was mandated by the state DEQ to build a brand new sewer plant. So after bond levies, grant applications, tax increases, increased traffic fines, city budget cuts, and lay-offs, they managed to scrounge up the money to do it. Our shop wired it. But now they're just scraping by for the timebeing. So PoCo is lenient with them.

When I briefly looked at the place, I wondered the same thing, and suggested something along the lines of "Well why can't we put a gutter or something up and extend the wiring out of the MCC without going through the CT section?" But apparently they had already gotten approval and my opinion was moot to them