Buck Boost Transformer Neutral

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solarEI

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I am trying to understand the buck boost transformers and have been searching everywhere for literature that talks about the neutral. I have been unsuccessful so i'm hoping someone might be able to help me out.

My situation is a 120/240V single phase service feeding a 208V single phase panel (panel feeds both 120V and 208V loads). The contractor suggested using a buck transformer to step down from 240V to 208V. My concern is the neutral conductor and a 120/208V single phase voltage at the panel.

From what i can gather the buck boost transformer would buck the voltage on the phase conductors only (with no connections for the neutral). I know not to derive a ground/neutral on the transformer secondary (for this single phase application), but does this mean the neutral just bypasses the buck transformer and feeds through? I'm not sure if i need to be cautious of potentially elevated neutral voltages or currents? It doesn't appear there are any codes that prohibit this but i'm not convinced this is acceptable or safe or if it's an inappropriate application of the buck boost transformer.

Does anyone know if this is a standard practice or if there are any other issues that could arise from this? Thanks in advance!!
 
Buck-boost transformers are autotransformers, meaning a common connection to the primary and secondary. In the case of a system involving a neutral (grounded conductor), the neutral should be the common to both sides. In other words, the neutral should run stright through from input to output.

120/240 single phase is a different animal than 120/208. Typically, 120/208 is from a three phase system, and the only way to have 208 phase to phase giving you 120 phase to neutral is to have a phase angle of 120 degrees between them.

Bucking the 120/240 single phase is going to result in 104/208 single phase. You would need two buck-boost xfmrs to accomplish this.

As usual, my thoughts are exactly and only that, and are based on my elementary understanding of electrical theory.
 
Originally this panel was connected to an existing 208V three phase system but the owner wanted it fed from a 240V single phase system (to comply with the building department requests). Since the equipment and appliances have already been purchased for 208V (and some loads are existing) we were trying to find a cost effective way to keep the panel at 208V single phase and have it fed from the 240V single phase system.

The 104/208V system makes sense. When you say you need 2 transformers is one to buck from 240V to 208V for the phase conductors and the other to therefore boost the neutral from 104V to 120V? Thanks!!
 
What kind of loads, exactly, are we talking about?

I am assuming no 3 phase loads. I am assuming some 208 volt single phase line-to-line loads. And a bunch of 120 volt single phase loads?

How much of each?
 
You are correct, there are no 3 phase loads. The breakdown is as follows

(5) 208V branch circuits (AHU, CU, Water Heater and some fuel dispensing pumps) totaling about 19KVA

(8) 120V branch circuits (lighting, recepts, exhaust fan and a few more fuel dispensing related circuits) totaling about 12.2KVA

The total connected load on the panel is about 31.2KVA and feeds a little maintenance building.

Thanks,
 
Equipment Rating

Equipment Rating

Have you checked to see if the equipment is rated 208-240? You may not need to reduce from 240v to 208. I think I have seen some equipment that is rated 200-240. May be worth checking into. Otherwise I think you will have to use buck-xfmr for the specific equipment that is 208V only.
 
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1) A buck transformer can create 208V single phase from 240V single phase. It _cannot_ create the necessary phase angle difference to give you 120/208V. If you want 120/208V, then you have to start with or synthesize three phase power. You could use some sort of rotary converter to develop the third leg...but if you already have three phase, then why bother.

2) You could use the 120/240 to feed all of your 120V loads directly, and then use a buck transformer to feed a panel that only has the 208V loads. One leg of this second panel would be 120V line to neutral, the other leg would be 88V line to neutral.

3) Or you could use two buck transformers to feed a second panel, and have 104V line to neutral on both legs.
1) note that 208/120 wye is a three phase system that has three 'hot' conductors and

4) _Officially_ 120/208V consisting of 2 hot wires and 1 neutral, is _single phase_ power. This has a 120 degree phase difference, and the physics of the situation is the physics of three phase supplies, but the official designation according to the IEEE is single phase.

-Jon
 
grant brings up a good point, and winnie is right on target.

brianjohn, I'm on dial-up tonight so didn't look at your link, but I am not seeing how 120/240 singlephase can be converted to 120/208 singlephase. I'll check that link tomorrow.

jrannis, I don't see that xfmr as doing us any good. We only have single phase 120/240 input... I can't see getting singlephase 120/208 out of that xfmr.
 
I'm curious why you need 208, single phase? That's a bit odd. Normally people have 208 and would rather bump it up to 240. Do you have some special equipment that will be operating off that panel that needs 208 only and can't tolerate 240?
 
I can think of no buck-boost transformer connection that will provide any single-phase 3-wire or a three-phase 4-wire output. Buck-boost transformers should only be used with Line-Line loads (well okay there are some L-N 2-wire only connections possible).
 
wow, great info!! It sounds like you basically can't use a neutral for the loads of a single phase buck boost transformer, (except a load at 240V since that would put the neutral at 120V)? Does this sound correct or am i missing something else? Thanks in advance!
 
grant said:
Have you checked to see if the equipment is rated 208-240? You may not need to reduce from 240v to 208. I think I have seen some equipment that is rated 200-240. May be worth checking into. Otherwise I think you will have to use buck-xfmr for the specific equipment that is 208V only.
Click to expand...

Also mdshunks comments above:

To reiterate: Why are you trying to drop the voltage? From what you've described, the 240v system should be fine for the loads you're listing.
 
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Post #3 explains the situation. The AC units and the water heater will be ok at 240V but the fuel dispensing pumps are existing and have been buried in the ground for 20+ years. I think i'm going to leave the panel at 120/240V but i'm not sure if i can verify if the fuel dispensing circuits will be ok at 240V, they were originally circuited at 208V (but the old panel has already been removed). If i can't verify i can always try to use a buck boost transformer just for those 2 circuits but i think that might only work if they don't require a neutral.
 
I can think of no buck-boost transformer connection that will provide any single-phase 3-wire or a three-phase 4-wire output. Buck-boost transformers should only be used with Line-Line loads (well okay there are some L-N 2-wire only connections possible).
Click to expand...

Jim you are right, I mis-spoke (typed) you can of course achieve 208 VAC but there would be no 208/120 usable from a buck boost.
 
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