Is this a psychotic plan? It might just be...

[carsongwalker]

Hello,

Im trying to hook up power to my new office trailer, from a subpanel of a nearby shed. The office has 120V ligthing and receptacles, and a 240V A/C unit. The subpanel that I intend to use for the source is a 60A 415Y/240V 3-phase panel.

It would probably be easiest to just get a ~25kva 240V to 240V-120V transformer and run the 240-120 split-phase panel on the office trailer as it is intended. Im trying to avoid this becasue I would like to reuse a cheap 5kva 240V to 120V single phase transformer I already have. The goal is to run the lighting and AC off the 5kva transformer while feeding the A/C unit directly with my already existing 240V at the subpanel. I am also adding an extra 240V receptacle at the office.

I attached a layout of how I think this can work, but have concerns regarding the grounding and whether half of what I am doing is even allowed.

To assist with the diagram, i will explain below...

1. Install 3x 30A breakers in the sub panel.
2. Run 3x lines of hot 10awg, 3x 10awg nuetrals, and a 10awg ground to the transformer in a single 3/4" conduit.
3. Pass circuits from P1 and P2 through the transformer to the office panel, no connections.
4. Connect P3 circuit to 5kva across H1/H3 and H2/H4 accordingly to get 120V on the secondary accross X4 and X1 (X2 and X3 connected). Tie ground to the transformer chassis.
5. Feed 120V into office panel along with the other 2x 240V circuits, single conduit.
6. Inside office panel, bond X1 (identified as the nuetral) from transformer to ground/nuetral bus. My understanding is this should be bonded despite being a sub panel since the 120 circuit is isolated. Drive a ground rod and connect to bus as well.
7. With a ground ran between the ground/nuetral bar to each 240V circuit, one can connect to the existing AC wiring, the other circuit can go to the newly installed 240V receptacle on the exterior of the bulding.
8. The 120V line will feed the already existing panel through a 30A breaker, the light and receptacles will be ran off 2x 30A breakers already inside the panel. They appear to share a nuetral (as the office came), and are accordingly on a shared double 30A breaker. Ground connected to the ground/nuetral.

So this might seem like a stupid, and possibly horrible, way to do this. Might be fun to pick apart and tell me I have no business doing this.

Thanks!

 

[Elect117]

You wired the transformer like a buck boost when I believe you wanted to rederive a separate system. You should be able to find a wiring diagram online to clear that up.

I do not think it is plausible to reuse the 5kva transformer.

All the circuits at the trailer should come from a single feeder. Right now you have multiple. See NEC 225.30.

EGCs from their source. Putting them in the panel being fed by the transformer is wrong. They come from the panel in building one. But that is besides the point since it is still a violation of 225.30.

 

[petersonra]

Your diagram shows three cicuits to the trailer. Not allowed.

 

[carsongwalker]

You wired the transformer like a buck boost when I believe you wanted to rederive a separate system. You should be able to find a wiring diagram online to clear that up.

I do not think it is plausible to reuse the 5kva transformer.

All the circuits at the trailer should come from a single feeder. Right now you have multiple. See NEC 225.30.

EGCs from their source. Putting them in the panel being fed by the transformer is wrong. They come from the panel in building one. But that is besides the point since it is still a violation of 225.30.

Ok, so in the trasnformers current configuration it is not a derived system, and the 120V secondary circuits should not have ground and nuetral connected, correct? The X1 nuetral would go direct to the nuetral only bus and feed the 120V equiopment from there. Ground would remain seperate. That right?

Assuming I keep neutral seperate in the panel, am I okay just pulling a single ground for all 3 circuits and attaching in the panel at ground bus (again, seperated from nuetral)?

I new doing it this way would not be NEC compliant, but does that necesarily make it unsafe? I understand its multiple feeders (at different voltages) at the office, but could I still get away with it?

 

[carsongwalker]

Your diagram shows three cicuits to the trailer. Not allowed.

Understood. Can you help me understand why that isnt allowed? These are pretty short runs, about 50ft total. If it was all one big building, this would have been okay?

 

[petersonra]

Understood. Can you help me understand why that isnt allowed? These are pretty short runs, about 50ft total. If it was all one big building, this would have been okay?

It's a rule. No idea why.

Trying to reuse the existing transformer is causing you to make bad choices.

Get a 2 pole breaker and feed two phases of the 415 to a 415 - 240/120 transformer. Run three wires and a ground to the trailer. Grounding electrode system required at the trailer.

 

[wwhitney]

I didn't look at the diagrams, just read the responses. But:

There's no reason you can't run a single properly sized 240V 2-wire L-N feeder to a 240V only panel in your trailer, and from that 240V only panel supply both the A/C and the 240V to 120V transformer. The secondary of the transformer is configured as an SDS and with an SBJ and a GEC to the same GES that the 240V panel uses. That secondary supplies a separate 120V only panel which supplies all the 120V loads.

Actually, you could do this with a single 2-bus panel if it is rated for 240V to ground. The 2-wire L1-N 240V feeder supplies the panel grounded conductor bar and one of the buses. The transformer supplies an ungrounded conductor L4 at 120V to ground, and L4 supplies the second bus; this could be done either the transformer as an SDS or as an autotransformer. All 120V loads would be supplied L4-N, and all 240V loads would supplied L1-N. Do not supply 120V load from L1-L4, I doubt they would be rated for both conductors ungrounded, and depending on the transformer configuration you choose, L1-L4 could end up at 360V.

The second option would definitely be even less standard and so probably the first option would be better.

Cheers, Wayne

 

[wwhitney]

It's a rule. No idea why.

Starting with the 2020 NEC, 225.30(B) does permit up to 6 feeders to a detached structure as long as all the feeders originate in a single piece of supply equipment.

Edit: 225.30(B) does require that each feeder terminates in a single disconnecting means. So with the 3 circuits to the building, there would need to be 3 disconnects in a row on the outside of the building (or inside nearest the point of entry), one for each circuit. And the circuits supplying the A/C and spare 240V receptacle would require 30A OCPD so that the supply to them is a feeder so that 225.30(B) can be used.

Cheers, Wayne

 

[wwhitney]

So a more normal version of the idea (still not necessarily normal) would be a single 30A 3P4W 415Y/240V feeder to a 3 pole disconnect on the exterior of the building, preferably with a 4-position neutral bar. Then from that disconnect L1-N supplies the A/C; L2-N supplies the spare 240V 30A receptacle; and L3-N supplies the transformer. That transformer would be configured as an SDS, and it supplies a 120V only panel. The building has a GES (2 ground rods) connected with a GEC to the EGC in the 3 pole disconnect, and to the grounded secondary conductor at the transformer or at the 120V only panel.

Cheers, Wayne

 

[carsongwalker]

It's a rule. No idea why.

Trying to reuse the existing transformer is causing you to make bad choices.

Get a 2 pole breaker and feed two phases of the 415 to a 415 - 240/120 transformer. Run three wires and a ground to the trailer. Grounding electrode system required at the trailer.

Do you think a 480 - 240/120 tapped at 432 primary would be close enough?

 

[petersonra]

Do you think a 480 - 240/120 tapped at 432 primary would be close enough?

Possibly. Especially if your source is more like 460 than 480.

That would be a design decision.

The P.E. that is putting his seal on the idea would have a say (if there is one), so would the AHJ.

 

[tom baker]

Understood. Can you help me understand why that isnt allowed? These are pretty short runs, about 50ft total. If it was all one big building, this would have been okay?

So there is a single disconnecting means. Multiple circuits of different characteristics are allowed if the disconnects are grouped
One big building same rules

 

[winnie]

My take:

Don't run 3 separate 30A circuits, even if permitted under the new rules. Run a single 3 phase 30A feeder to a suitable 3 phase disconnect. The disconnect could be a 3 phase panel with main breaker, which will provide you with a place to mount your 240V breakers.

It is totally fine to use a single 30A 240V L-N leg to run your transformer, 240V input, 120/240V output. I'm guessing that your existing transformer is designed for 480V or 240V in, and 240V or 120V or 120/240V out. If my guess is correct, then if wired as drawn your transformer is configured for 240V in and 240V out. I would not bother trying to run the transformer with 416V from your source.

You will need secondary protection on your transformer. I would provide this using a 120/240V. This gives you a panel to connect 120V loads.

Confirm that your HVAC loads are actually rated for 240V L-N. Common 240V supplies in the US are 120V L-N, 240V L-L. But as you are well aware most 240V equipment made for world wide consumption is happy with 240V L-N. So you are likely ok here but must double check. This is especially an issue if the equipment has TVS devices connected L-G. You don't want to save money on your transformer and then let the smoke out of your HVAC equipment.

Finally, you are an EE, not an electrician. Totally reasonably to tell your electrician 'this is the general idea of what I want done', but don't over-constrain the professional. I'm sure you understand this from the perspective of what happens when marketing tells you how to be an engineer

Jonathan

 

[Jpflex]

Hello,

Im trying to hook up power to my new office trailer, from a subpanel of a nearby shed. The office has 120V ligthing and receptacles, and a 240V A/C unit. The subpanel that I intend to use for the source is a 60A 415Y/240V 3-phase panel.

It would probably be easiest to just get a ~25kva 240V to 240V-120V transformer and run the 240-120 split-phase panel on the office trailer as it is intended. Im trying to avoid this becasue I would like to reuse a cheap 5kva 240V to 120V single phase transformer I already have. The goal is to run the lighting and AC off the 5kva transformer while feeding the A/C unit directly with my already existing 240V at the subpanel. I am also adding an extra 240V receptacle at the office.

I attached a layout of how I think this can work, but have concerns regarding the grounding and whether half of what I am doing is even allowed.

To assist with the diagram, i will explain below...

1. Install 3x 30A breakers in the sub panel.
2. Run 3x lines of hot 10awg, 3x 10awg nuetrals, and a 10awg ground to the transformer in a single 3/4" conduit.
3. Pass circuits from P1 and P2 through the transformer to the office panel, no connections.
4. Connect P3 circuit to 5kva across H1/H3 and H2/H4 accordingly to get 120V on the secondary accross X4 and X1 (X2 and X3 connected). Tie ground to the transformer chassis.
5. Feed 120V into office panel along with the other 2x 240V circuits, single conduit.
6. Inside office panel, bond X1 (identified as the nuetral) from transformer to ground/nuetral bus. My understanding is this should be bonded despite being a sub panel since the 120 circuit is isolated. Drive a ground rod and connect to bus as well.
7. With a ground ran between the ground/nuetral bar to each 240V circuit, one can connect to the existing AC wiring, the other circuit can go to the newly installed 240V receptacle on the exterior of the bulding.
8. The 120V line will feed the already existing panel through a 30A breaker, the light and receptacles will be ran off 2x 30A breakers already inside the panel. They appear to share a nuetral (as the office came), and are accordingly on a shared double 30A breaker. Ground connected to the ground/nuetral.

So this might seem like a stupid, and possibly horrible, way to do this. Might be fun to pick apart and tell me I have no business doing this.

Thanks!

I have always gone off of 2/3 phases of a 3 phase system to feed a 1 phase transformer for stepping down voltage. Although this my have balancing issues because of the third phase not used in the 3 phase source, I had to make do with the equipment my job supplied me with.

In your case I'm not sure if you are using all three phases from the source subpannel to feed a 3 phase transformer and come out with only 2 legs x1 x2 and a tapped neutral?

With a 5kVA transformer you would get only 20 amperes or less max which would be merely enough for one 20 ampere branch circuit. 5,000 VA / 240 = 20.8i or 5,000 /240 x 1.732

I'm not sure which equation to use because you are converters to a single phase system from a 3 phase system and a 3 phase system equation must factor in the square root of 3 or 1.732.

Lastly yes I believe you will either tie the neutral to EGC at the transformer or the trailer panel as if it were a service panel because you are starting over with the transformer.

Each point of disconnect gets a grounding system so possibly a couple ground rods if over 25 ohms at the trailer