Details RE: Boost Transformer to Raise 1-phase 208V up to 1-phase 240V?

[ericwg]

I think the 50 amp branch circuit protection is a basic misunderstanding of the national electrical code on the part of these foreigners. They seem to be including the load calculation derating factor into the branch circuit protection

That sounds like the most plausible explanation I've heard thus far RE: the discrepancy between the data (18.5kW @ 240V) vs the OCPD they recommend.

I also don't see anywhere in this instruction manual that this range is UL listed.

From the user's guide:

 

[winnie]

I checked for a 208-240 single phase 15kVA model...TEMCo has one that is 16.75"W x 14"H x 15"D, weighing ~ 65lbs. This is significantly larger than the 2kVA boost xfmr I was looking at (SqD 2S46F) which is only 7.28"W x 9.55"H x 6.56"D, weighting 39lbs. I'm not even sure I can fit the 15kVA autotransformer in the same space.

What's your take on the boost vs autotransformer?

This may be a terminology misunderstanding on my part.

I use the term 'boost' transformer to mean a low voltage step down transformer connected in an autotransformer configuration.

The transformer that you mention is a 2kVA 240 to 32V isolation transformer, which you will connect as a 208V to 236V boost transformer. In the boost configuration your output capability is 14.75kVA. Many manufacturers would call this a 15kVA boost transformer, but the real capacity is 2kVA and the rest comes from the characteristics of the boost connection.

If this is the TEMCo transformer that you are thinking of: https://temcoindustrial.com/temco-a...secondary-volts-50-60-hz-single-phase-copper/ then it is essentially the same thing, probably in a bigger housing with the windings permanently connected, possibly with better detailed ratings (lower operating temperature, higher efficiency, etc.) But I've not looked at the details. The only significant difference is that the TEMCo transformer is a true 208V to 240V autotransformer, meaning internally it has a 208V to 32V coil.

For your application, if line-neutral voltage does not matter, then an autotransformer or autotransformer configuration makes the most sense. You get a smaller sized less expensive transformer with lower losses, and you don't need to deal with grounding electrodes for a separately derived system.

-Jon

 

[ericwg]

If this is the TEMCo transformer that you are thinking of: https://temcoindustrial.com/temco-a...secondary-volts-50-60-hz-single-phase-copper/ then it is essentially the same thing, probably in a bigger housing with the windings permanently connected, possibly with better detailed ratings (lower operating temperature, higher efficiency, etc.) But I've not looked at the details. The only significant difference is that the TEMCo transformer is a true 208V to 240V autotransformer, meaning internally it has a 208V to 32V coil.

Yup, you got it. That is the TEMCo Autotransformer I was looking at.

For your application, if line-neutral voltage does not matter, then an autotransformer or autotransformer configuration makes the most sense. You get a smaller sized less expensive transformer with lower losses, and you don't need to deal with grounding electrodes for a separately derived system.

-Jon

I want to clarify what you said here. The 15kVA TEMCo Autotransformer, for example, was nearly 2X the size and weight of the SquareD boost transformer I mentioned previously. You were saying, if the line-neutral voltage really doesn't matter, then an autotransformer or autotransformer configuration makes the most sense. Did you mean to say "boost transformer would make the most sense" since that's the smaller / cheaper unit? If so, I still fail to see how I would wire the neutral with that sucker. ??

 

[winnie]

What I am saying is that the 15kVA TEMCo Autotransformer and the SquareD boost transformer wired in autotransformer configuration are pretty much the same. I don't know the details that makes one weigh more than the other. The biggest difference that I saw other than weight and size is that the TEMCo transformer is a 208:32V unit, and so the output is 240V, and the SquareD unit is a 240:32V unit which gives a net 236V (since 240:32 is 208:28, and 208+28=236). I also question some of the details on the TEMCo site, since the image is of a 3 phase transformer and the spec mentions 3 phase.

With either unit you have a similar problem of unbalanced voltages relative to the neutral, which may or may not be problematic for your load and is questionably when applied to a standard NEMA 14-15 receptacle.

However if your load does not care about line to neutral voltage, then one of these autotransformer arrangements makes more sense than an isolation transformer.

If your load does care about line-neutral voltage, then you will need a true 208V to 120/240V single phase transformer, which will be considerably larger and more expensive, and require grounding and bonding as an SDS

-Jon

 

[LarryFine]

Without looking for it, I once suggested separately placing the two LV windings at opposite ends of the primary, splitting the boost between both lines, and preserving the centered neutral, voltage-wise.

 

[winnie]

If you have a 240V to 16/32V transformer, and connect it as LarryFine suggests to a 208V source, then you would get 236V L-L and 132V L-N, which would probably the closest you can get to 120/240 single phase with a normal autotransformer.

-Jon

 

[LarryFine]

If you have a 240V to 16/32V transformer, and connect it as LarryFine suggests to a 208V source, then you would get 236V L-L and 132V L-N, which would probably the closest you can get to 120/240 single phase with a normal autotransformer.

If a high L-N voltage is a greater concern than a low L-L voltage, use a 12/24v-secondary unit.

 

[kwired]

Post 3, the technical information page posted there says 50 amp is based on art 220 load diversity calculations. This likely would be T220.55.

For ranges 12-27 kw you use amount in column C which is 8 kw for a single range and add 5% for each kw or major fraction over 12 kw.

5% of 8 kW is .4 times 6.5 rounded up to 7 = 2.8. add that to 8 kw and total demand is 10.8 which is only 45 amps instead of the 50 they mentioned.

 

[kwired]

Yup, you got it. That is the TEMCo Autotransformer I was looking at.


I want to clarify what you said here. The 15kVA TEMCo Autotransformer, for example, was nearly 2X the size and weight of the SquareD boost transformer I mentioned previously. You were saying, if the line-neutral voltage really doesn't matter, then an autotransformer or autotransformer configuration makes the most sense. Did you mean to say "boost transformer would make the most sense" since that's the smaller / cheaper unit? If so, I still fail to see how I would wire the neutral with that sucker. ??

The VA rating of buck boost transformer only needs to be the difference in what is being bucked/boosted, the rest of the VA is coming directly from the supply line - kind of maybe could have worded this better but not sure how to say it.

So say you have a 50 amp load to carry, that 32 volt coil has to be able to carry 50 amps without being compromised. 50 x 32 = 1600 VA. That is where the need for a 2 kVA buck boost comes into play since there probably isn't a size between 1.5 and 2.0. Otherwise for isolation transformer 50 amps = 12 kVA. A 15 kVA transformer will be next size up and will be a lot bigger/heavier than a 2 kVA, but it will carry all the load instead of just the portion that is being bucked/boosted.

I bet the TEMCo you mentioned is a 15 kVA isolation transformer. Really the buck boost is also an isolation transformer but only when you input 120/240 to the high side and isolate and take 16/32 from the low side. Connect the low side in series with the high side and it becomes an autotransformer, exactly what you get out of it depends on a few possible ways to connect it along with consideration of what input volts actually is.

 

[ericwg]

Post 3, the technical information page posted there says 50 amp is based on art 220 load diversity calculations. This likely would be T220.55.

For ranges 12-27 kw you use amount in column C which is 8 kw for a single range and add 5% for each kw or major fraction over 12 kw.

5% of 8 kW is .4 times 6.5 rounded up to 7 = 2.8. add that to 8 kw and total demand is 10.8 which is only 45 amps instead of the 50 they mentioned.

I think you nailed it. I actually looked at that table last night and did the math too but didn't connect it to the 45A listed in the User's Guide. Like you said, 5% of 8kW = 0.4kW. 18.5kW - 12kW = 6.5kW. Since Note 1 says "or major fraction thereof", as you said, 6.5 gets rounded up to 7. Thus, 8 + (7 x 0.4) = 10.8kW, which correlates to 45A @ 240V. Not sure why they didn't specify a 45A breaker, but for whatever reason they moved up to 50A.

Now, unless I am looking at this wrong, the major flaw I see with them doing this is that they are assuming that EVERY installation of their range will be able to implement these load diversity calculations. In my second or third post I mentioned that this range is going into a fire station which I do not think qualifies. I have no problem speaking with the CSFM (my AHJ) about this. If they permit this, it will cut down on the OCPD, conductor, and transformer sizes, however, it still leaves me with the concern over the neutral.

 

[ericwg]

What I am saying is that the 15kVA TEMCo Autotransformer and the SquareD boost transformer wired in autotransformer configuration are pretty much the same. I don't know the details that makes one weigh more than the other. The biggest difference that I saw other than weight and size is that the TEMCo transformer is a 208:32V unit, and so the output is 240V, and the SquareD unit is a 240:32V unit which gives a net 236V (since 240:32 is 208:28, and 208+28=236). I also question some of the details on the TEMCo site, since the image is of a 3 phase transformer and the spec mentions 3 phase.

With either unit you have a similar problem of unbalanced voltages relative to the neutral, which may or may not be problematic for your load and is questionably when applied to a standard NEMA 14-15 receptacle.

However if your load does not care about line to neutral voltage, then one of these autotransformer arrangements makes more sense than an isolation transformer.

If your load does care about line-neutral voltage, then you will need a true 208V to 120/240V single phase transformer, which will be considerably larger and more expensive, and require grounding and bonding as an SDS

-Jon

Since you recommended this, I started looking around to see what options are out there. Assuming my AHJ permits the load diversity calc, at 10.8kW, I would only need a 15kVA transformer. Via Platt's website, I found the Acme linked below. It's 16.94"H x 14.12"W x 11.59"D and weighs only 77.6lbs. Granted, not as small as that boost transformer, but not too bad. Also, it's only $1,669. As for the grounding and bonding, honestly, that won't be a problem. This transformer would go in the Electrical room where there is access to a grounding electrode system. The feeder for the panel in this room, which I mentioned previously, comes from another building, so I already have a GE installed for it.

Acme Encapsulated 120/208 - 120/240 Single Phase Dry Dist Xfmr

 

[kwired]

I think you nailed it. I actually looked at that table last night and did the math too but didn't connect it to the 45A listed in the User's Guide. Like you said, 5% of 8kW = 0.4kW. 18.5kW - 12kW = 6.5kW. Since Note 1 says "or major fraction thereof", as you said, 6.5 gets rounded up to 7. Thus, 8 + (7 x 0.4) = 10.8kW, which correlates to 45A @ 240V. Not sure why they didn't specify a 45A breaker, but for whatever reason they moved up to 50A.

Now, unless I am looking at this wrong, the major flaw I see with them doing this is that they are assuming that EVERY installation of their range will be able to implement these load diversity calculations. In my second or third post I mentioned that this range is going into a fire station which I do not think qualifies. I have no problem speaking with the CSFM (my AHJ) about this. If they permit this, it will cut down on the OCPD, conductor, and transformer sizes, however, it still leaves me with the concern over the neutral.

The chances of turning every heating element on "high" and leaving it on for any significant amount of time is pretty slim. Don't know a lot about induction ranges, but definitely not happening with more conventional resistance heating elements. Once a pan is heated up to a certain amount you turn the setting down, then it cycles. The oven - draws high demand when preheating, but once you reach desired temp it only kicks on when needed to maintain set temp.

Look how low the values in column 3 are compared to how many ranges @ 12 kW those values can be used for. They know these aren't drawing max rating for very long at a time when/if they ever do draw max rating. 2 ranges has a demand of only 11 kW 10 ranges only a demand of 25.

I have a customer that I couldn't get a 40 amp breaker last summer for a double oven (covid related supply shortages) , temporarily put it on a 30, haven't been back to install the 40 yet though I do have other work to do there soon. So far they never had it trip, never really used both ovens at same time since then (covid kept from having a couple family gatherings where both may have been used), but bet there is a good chance it never trips a 30 amp breaker anyway.

 

[kwired]

What I am saying is that the 15kVA TEMCo Autotransformer and the SquareD boost transformer wired in autotransformer configuration are pretty much the same. I don't know the details that makes one weigh more than the other. The biggest difference that I saw other than weight and size is that the TEMCo transformer is a 208:32V unit, and so the output is 240V, and the SquareD unit is a 240:32V unit which gives a net 236V (since 240:32 is 208:28, and 208+28=236). I also question some of the details on the TEMCo site, since the image is of a 3 phase transformer and the spec mentions 3 phase.

With either unit you have a similar problem of unbalanced voltages relative to the neutral, which may or may not be problematic for your load and is questionably when applied to a standard NEMA 14-15 receptacle.

However if your load does not care about line to neutral voltage, then one of these autotransformer arrangements makes more sense than an isolation transformer.

If your load does care about line-neutral voltage, then you will need a true 208V to 120/240V single phase transformer, which will be considerably larger and more expensive, and require grounding and bonding as an SDS

-Jon

I see that also. I think they have title of the product wrong, the information in specifications seems fairly expected for a three phase unit though. Don't know how this unit got selected, for single phase load like we have per this thread shouldn't need to be anymore than a 2 kVA unit.

 

[winnie]

I see that also. I think they have title of the product wrong, the information in specifications seems fairly expected for a three phase unit though. Don't know how this unit got selected, for single phase load like we have per this thread shouldn't need to be anymore than a 2 kVA unit.

If you go to the TEMCo site looking for single phase autotransformers, it links to that page.

-Jon

 

[kwired]

If you go to the TEMCo site looking for single phase autotransformers, it links to that page.

-Jon

It probably coming up with a search hit because of incorrect info on main page content, still think it is actually a three phase 15 kVA unit.

 

[LarryFine]

Once a pan is heated up to a certain amount you turn the setting down, then it cycles.

Are you suggesting that a thermostatically-controlled element reaches desired temperature faster on a higher setting?

An oven will heat faster if there's a preheat setting that energizes both elements, but otherwise, the heat rise rate is fixed.

Now, when I'm baking, I usually preheat it to a higher setting, then set back to the desired temp after putting in the food.

 

[kwired]

Are you suggesting that a thermostatically-controlled element reaches desired temperature faster on a higher setting?

An oven will heat faster if there's a preheat setting that energizes both elements, but otherwise, the heat rise rate is fixed.

Now, when I'm baking, I usually preheat it to a higher setting, then set back to the desired temp after putting in the food.

Simple thermostatically controlled no. PID type control maybe, but not going to normally see that on residential or even most commercial "ranges".

Most surface element controllers typically called "infinite control switches" are more of a variable percentage timer type of controller, often with no switching occurring when in "high" position. So in high position it will get hotter and hotter until you either burn your food or turn it down, so even then it won't stay on high for all that long most the time. Maybe if you put on a 30 gallon pot of water covering multiple elements and turn them all on high it might take a while before it ever brings it to a boil though.

 

[ericwg]

Just to clarify, that Acme transformer I posted the link to earlier, it's not 77 lbs...it's 77 kg! Or, 171 lbs. Well, it is encapsulated.

 

[retirede]

Are you suggesting that a thermostatically-controlled element reaches desired temperature faster on a higher setting? :

No, but unless one is very familiar with the characteristics of a particular stove, the only practical way to initially heat a pan is to max out the control. Otherwise you risk choosing a setting that never gets to the desired temperature.

 

[kwired]

Just to clarify, that Acme transformer I posted the link to earlier, it's not 77 lbs...it's 77 kg! Or, 171 lbs. Well, it is encapsulated.

Most the smaller buck boost transformers will be encapsulated also.