Converting 120/208 to 120/240

[jsciarlo]

Hello all,
I have a 120/208 panel in a mechanical room. The panel services only 120 circuits. I now have a need in this area for a 240 circuit. Being that I already have a 25 KVA tranformer in stock , I was wanting to change the feed to the panel from 120/208 to 120/240 from this tranformer. I can't see any problem with this other than having to be clearly marked, but having never done this type of conversion with a panel before, I was wanting to know if anyone had experience doing this, or any safety concerns that might arise with me doing this. Thank you for your time, JSCIARLO

 

[augie47]

Can be done, but, depending on your load, it might be easier and less expensive to purchase a buck & boost transformer for that one load
(buck 208 to 240)

 

[jsciarlo]

yes i agree, but some time before I started someone intalled a tranformer right above the MCC that now feeds the line side of the panel. So since it is there i figured i would try to use it. The panel now is a 100 Amp

 

[augie47]

Most 25 kva 240/120 transformers are single phase. Is the one you plan on using single phase ?
You state your panel is 208/120. That often means a 3 phase panel. Is the panel 3 phase ?

 

[Jack Linville]

208

208

In my experience 208 is three phase wye connected secondary. Everything else relating to 220-240 would be single phase or three phase or open delta type configurations. I don't recall ever seeing a system that was 208 and not 3 phase.

Then again, I have a lot more experience with controls than power distribution.

 

[LarryFine]

J, welcome to the forum!

The simple answer, presuming the transformer and loads are single-phase, is a 208-to-240v transformer, and the secondary center tap needs to be bonded as an SDS.

If everything is three-phase, you won't have a neutral, so you'll end up with 240 only, unless one secondary has a center tap, and you'll end up with a high-leg secondary.

 

[jsciarlo]

Yes the panel is 3 phase, and yes the transformer is single phase. The plan was to mark the B phase dead in the panel, and put covers on all the b phase breaker possitions. I know it doesn't sound likem the most ideal way. Just trying to use what i have!

 

[augie47]

You need to look at the branch circuits and see if any are multiwire (shared neutral). If they are, and were installed based on a 3 phase
supply, you neutrals will be subject to overload if you change to a single phase arrangement.
Otherwise, a bit unortahdox but permissible.

 

[steve66]

So you just happen to have a 208 volt to 240/120 volt transformer there? Or is the transformer voltage something else???

Steve

 

[jsciarlo]

The tranformer that is there is a 460 primary to 120/240 secondary

 

[steve66]

The tranformer that is there is a 460 primary to 120/240 secondary

Oh, so the main service is 480. Doesn't sound like it would be a problem, although its more common to stick with equipment that works with the available voltage.

A lot of equipment will work with either 208 or 240 volts. Are you sure you equipment won't do that?

Steve

 

[jsciarlo]

Oh, so the main service is 480. Doesn't sound like it would be a problem, although its more common to stick with equipment that works with the available voltage.

A lot of equipment will work with either 208 or 240 volts. Are you sure you equipment won't do that?

Steve

The equipment is a household oven. I don't believe that it could be safely wired to 208 without a increase in the amperes, which I dont think the oven internal wiring could handle.

 

[Jraef]

I don't think it would pass inspection though. A 3 phase panel being fed 1 phase power and the B phase bus "blocked off and marked dead"? I think it's a violation of the "intended use" clauses in various places of the code (don't have it on this PC to cite a reference). You know what you are doing now, but will Joe Blow understand what you did 20 years from now?

 

[jsciarlo]

I don't think it would pass inspection though. A 3 phase panel being fed 1 phase power and the B phase bus "blocked off and marked dead"? I think it's a violation of the "intended use" clauses in various places of the code (don't have it on this PC to cite a reference). You know what you are doing now, but will Joe Blow understand what you did 20 years from now?

AHHH. The "intended use" never thought about that aspect of it. I guess I am buying a new panel. I would really hate to confuse "Joe Blow" haha.
Thanks for all the reponses..

 

[LarryFine]

The equipment is a household oven. I don't believe that it could be safely wired to 208 without a increase in the amperes, which I dont think the oven internal wiring could handle.

The oven, which is a fixed resistance, will use less current on 208v than on 240v, not more. To use more current on less voltage, a load must be redesigned with less resistance for the lower voltage, so that more current can flow, resulting in the same overall power level.

Don't confuse the theoretical math, which says lower voltage means greater current (current varies inversely with voltage) for a given power, with the real-world math that says lower voltage means lower current (current varies proportionately with voltage) for a given resistance.

You wouldn't be modifying the range for the lower voltage, so the same heating elements will run at approximately 3/4 the power they would on 240v. The elements will take longer to heat, but once hot, the thermostatic controls will compensate with longer heat-cycle times.

 

[Jraef]

AHHH. The "intended use" never thought about that aspect of it. I guess I am buying a new panel. I would really hate to confuse "Joe Blow" haha.
Thanks for all the reponses..

Just for future reference:

Par. 110-3.b Installation and use states, “Listed or labeled equipment shall be installed and used in accordance with any instruction included in the listing or labeling.”.

I don't know of any panelboard that is going to be listed for both 1 phase and 3 phase feed.

 

[hurk27]

The equipment is a household oven. I don't believe that it could be safely wired to 208 without a increase in the amperes, which I dont think the oven internal wiring could handle.

Household ovens run on 208 all the time, many apartment buildings are fed with 208/120 services and have regular ranges form a big box store.
Just the preheat time will be just a little longer, and the current draw will be less as Larry pointed out.

The only loads that are affected severely are motor loads, high torque applications such as air compressors will have a problem starting up and will over heat if a 200 volt motor is not used, but many motors will have no problem.

 

[BJ Conner]

Confused

Confused

Yes the panel is 3 phase, and yes the transformer is single phase. The plan was to mark the B phase dead in the panel, and put covers on all the b phase breaker possitions. I know it doesn't sound likem the most ideal way. Just trying to use what i have!

I am confused. Refering to anther thread on MCC cubicals . IF we take an MCC cubical and rework it in conformance with the vendors manuals, instructions and components were viloating a UL listing and that is not acceptable.

Now we can take a 3 phase 120/208 panel and modify it to become a single phaes 120/240 panel?
UL listing aside, and just utilizing a little common sense. Which one of these situations is going to cause trouble in 5 or 10 years when someone who wasn't in on the "Conversion" comes on to the scene?
The least we could hope for is just supprize and a exclamation of "WTF were these guys doing". The worst is getting someone hurt or starting a small fire.

 

[jsciarlo]

I am confused. Refering to anther thread on MCC cubicals . IF we take an MCC cubical and rework it in conformance with the vendors manuals, instructions and components were viloating a UL listing and that is not acceptable.

Now we can take a 3 phase 120/208 panel and modify it to become a single phaes 120/240 panel?
UL listing aside, and just utilizing a little common sense. Which one of these situations is going to cause trouble in 5 or 10 years when someone who wasn't in on the "Conversion" comes on to the scene?
The least we could hope for is just supprize and a exclamation of "WTF were these guys doing". The worst is getting someone hurt or starting a small fire.

I do agree that this is not an ideal way to this. With all the reponses I now think we are just going to add a sub panel that is feed from the tranformer. This way the sub-panel is closer to the breakroom(where the oven is at). It also seams much safer this way, and will accomplish space for future use

 

[Jraef]

I do agree that this is not an ideal way to this. With all the reponses I now think we are just going to add a sub panel that is feed from the tranformer. This way the sub-panel is closer to the breakroom(where the oven is at). It also seams much safer this way, and will accomplish space for future use

But BJ Connor brought up another good point. Technically, you can't change out that panel unless you use one that is listed by the manufacturer for use in that MCC. May not be a problem unless the MCC is really old, maybe not even then. Just something to consider.