Eliminating 208 to 115 VAC transformer by using phase to neutral from supply circuit 4

[PaulKraemer]

Hi,

I am trying to design an Industrial Control Panel that will replace an older Control Panel that is currently being used in a machine control application. I have posted a several questions related to this project, and the responses I have received have been incredibly helpful. If everyone here hasn't grown too tired of me by now, I have one more...

While the new components I'll be using in the new panel in most cases are a bit smaller than the old components in the old panel, I have found that the old panel has virtually no branch fusing. The only over-current protection for many of the components inside the panel is from the main circuit breaker that supplies the entire panel. In the new panel, I am planning to include branch fuses for each individual component. This has resulted in my current (preliminary) panel layout being more cramped than I would like it to be.

One opportunity I thought of that would free up some space would be if I could eliminate a transformer in the old panel that takes the 208 VAC supply voltage down to 115 VAC for a few components that require the lower voltage. I will have components in the new panel that will require 115 VAC, but I am wondering if the facility can provide me with a four wire 208 VAC supply circuit (3 phase plus neutral), if I can get the 115 VAC I need between one of the phase lines and the neutral.

Before I consider doing this (and asking our electrical contractor if he can give me the neutral, which is not in use on the old panel), I figured I'd ask here if there are any drawbacks I should consider if I think about going this route.

Any advice will be greatly appreciated.

Thanks in advance,
Paul

 

[TugboatEng]

The control transformer allows a single panel to be used for common industrial voltages (208-240 or 440-480). It also allows the panel to work without a neutral. It's a one size fits all solution. Your specific cases can work without the control transformer provided your fuses have an interrupt rating suitable for a mains based system. In the event of a failure the fault current will be higher which could cause more damage.

 

[FacEngrPE]

Some AHJ's are requiring new panels to have an Short-Circuit Current Rating (SCCR). This is something that appeared in the NEC 2017, so check the code version that applies in your location. If this requirement applies it is easier to meet in the control circuits if they are decoupled from the mains by a control transformer.

 

[jghrist]

120/208 4-wire supplies are common in North America. 120V to ground should be usable for your 115V requirement.

 

[TugboatEng]

208Y/120 is a common power source but it's unlikely that a 4 or 5 conductor cable will have been pulled to the existing equipment.

 

[jraef]

Yes, the problem will be that the end user / installing contractor will be REQUIRED to run the Neutral conductor out to the panel. In many 3 phase applications, they don't do that; the wire costs more and having more than 3 conductors in a raceway may mean bigger conduit, etc., so higher installation costs. So from my EXPERIENCE with this exact issue, you get in trouble with the end user later when the installer does not look carefully at your drawings and did not pull the 4th conductor, because he has never had to before...
I know it makes perfect sense to be ABLE to do that, but the real world often does not make perfect sense.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[PaulKraemer]

Thank you TugboatEng, FacEngrPE, jghrist, and jraef for your responses,

While 208Y/120 is available in the building where this will be installed, I am sure that a 4 of 5 conductor cable has not been pulled to the existing control panel. This means that if I don't include a control transformer in my new panel, I will need our electrical contractors to pull new wires and possibly replace the existing conduit with something larger. I was not aware of the fact that using a control transformer makes it easier to satisfy SCCR requirements. This is useful information. While I know I won't be subjected to UL inspections or to come up with a SCCR for this particular project, I am trying to comply with current UL guidelines to whatever extent I can, with the hope that I might be able to use this panel as the prototype for other panels that might be installed in other locations with more stringent requirements.

For now, I will do my best to arrange my components efficiently in a way that will allow ample space for a control transformer, without having to go to a larger enclosure size than the original panel I will be replacing (which would be difficult due to space constraints). If I can't do it, now I feel like I understand the pros and cons of eliminating the control transformer.

I really appreciate all your help.

Thanks again,
Paul

 

[TugboatEng]

If space is a concern, transformers with enclosures are available that could be mounted on the outside of your panel.