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

[ericwg]

Hello All,

This is officially my first original post. Before I list my questions I want to say thanks to everyone that posts on this forum. I can't begin to tell you how much help it has given me over the years. Ok...onto the good stuff.

Here is the situation, I have a three phase 208Y/120 panel via which I need to be able to power a single phase 240V electric induction oven. The datasheet for the oven calls for 240V (2 wire + N + Grd), 18.5kW. Furthermore, the install manual calls for Branch Circuit Protection of 50A. Admittedly, I am a little confused about the 50A BCP when 18.5kW equates to ~77A, but let's just skip that for now. In case someone suggests just running the oven at 208V...that maybe it just won't get as hot or will take longer to heat up, a rep from the manufacturer (AGA) confirmed that this oven will NOT operate correctly or be warrantied if operated at 208V. Thus, I'm not going to try that.

My plan was to employ a 16/32V boost transformer to get as close to 240V as possible. I understand an Autotransformer is an option as well...that it's similar to a boost transformer? Since I'm not clear on exactly which would be best to use, I mention both in my questions. My plan was to install this transformer in the Electrical room about 8 feet from the source panel. The range is then located ~30 feet away from the transformer. As I started to try to work out all the installation details for my plans, I realized there are a handful of questions I cannot answer RE: this setup. This is where I am hoping y'all can help out.

Branch Circuit Breaker:
What size (2-pole) breaker do I need for this circuit? If a 50A BCP is listed for use at 240V, the current would be higher at 208V. This is where my earlier confusion over the 50A breaker with an 18.5kW rating comes back into play. Not sure if this logic pans out, but with a 14.3% difference between 208V and 240V, should I assume that 50A BCP would increase to 50 X 14.3% = 57.2A, thus, a 60A breaker? I do know that 450.4(A) calls for an OCPD to be rated or set at not more than 125% of the rated full-load input current of the auto-transformer. Does that mean the OCPD should really be 57.2A X 125% = 71.5A...so 70A or 80A?

Primary/Secondary Transformer Protection:
1) Is primary and/or secondary protection required for boost / autotransformers?
2) If primary protection is required, does the branch circuit breaker provide it? If the boost really is akin to an autotransformer, then my understanding from section 450.4 (also Exhibit 450.6) is that the transformer I need would most likely be provided with a 2-pole disconnect switch with OCPD's (I assume fuses). If it isn't, I assume I could just specify one that is rated at the same current as the BCP? Would that cover the primary protection?
3) From what I am seeing, secondary protection is not required, but I would appreciate someone confirming this. If it is required, how is it typically provided?

Secondary Conductor OCP:
1) Is protection of the secondary conductors required? If so, how is it typically provided?

For clarification, there is a circuit diagram in the oven's install manual and though it shows multiple taps to L1 and L2, the N appears to pass through and not be used. There are also no control transformers shown in the circuit diagram. Lastly, in the "Connections" table there is a note referring to the "Maximum total electric load 240V (including oven fans, lights, etc.)". The fact that it mentioned 240V but not 120V, I can't help but wonder if this thing doesn't transform the 240V down to 120V for controls, lights, etc. BUT, for now, I am going to assume it does.

Providing a Neutral to the Load:
1) How do I provide a neutral to the load? In Exhibits 210.18 and 210.19 I don't see where I could tap either an autotransformer or boost transformer to get a neutral.
2) If I were to bring the neutral all the way from the panel to the oven, the line-neutral voltage would no longer be 120V, right? Please don't tell me I would need a second transformer for the neutral?

Sorry for the long post. Thanks for reading. I look forward to your responses.

-Eric

 

[MTW]

I would suggest that you confirm the equipment discrepancies first, don't skip that for now, it will affect everything else.
Post a link or photo to the equipment specs and diagram. If you need a balanced neutral is the first thing that needs to be determined. If needed a autotransformer will not provide that, you will require an isolation type. Then the true load, is it 77A or 50A ? Don't listen to the sales guy, get the data on paper from the manufacturer.

 

[ericwg]

Thanks for the quick reply. Yeah, I know I need to nail this down. Unfortunately, they are an English company and it has been really tough reaching someone knowledgeable. All I've been able to speak to so far is local distributors who can't really answer any engineering based questions. Until then, I will include snapshots of the datasheet and the section from the install manual I mentioned. Maybe you can make more out of it than I did.

 

[retirede]

The specs call for “2 wire plus neutral plus ground” which would mean you can’t use an autotransformer. This is consistent with the NEMA 14-50 plug it has.

Even though nothing in the schematic shows anything connected to the neutral.

 

[jim dungar]

A typical 'buck-boost' transformer is an autotransformer. There is no true primary or secondary. It is created by interconnecting the windings of an isolation transformer in parallel and series combinations.

 

[d0nut]

Who is providing this range? Is there a kitchen consultant or someone else selecting this that you can let know that you don't have 240V available and that they should select a 208V range?

The cutsheet shows a $7,750 MSRP. It may cost less to get a range at the correct voltage than what you will spend messing around to get this range to work.

Otherwise, to make it work correctly, you will need a transformer and a panel or enclosed breaker with shunt trip and maybe GFCI depending on the breaker size.

 

[gar]

210401-1048 EDT

ericwg:

Your circuit diagram shows no neutral connection. 14-50 plug has a neutral pin, but it may not connect to anything in the oven. You can test whether the neutral pin goes anywhere, start with an ohmmeter test.

An autotransformer and a buck-boost transformer are not necessarily the same. A buck-boost will have the boost winding wound with heavier wire than the primary, whereas an autotransformer may use the same wire size for the entire winding, for example a Variac. (Big problem with this website is changing my spelling of words. Variac gets changed to Varian.)

Suppose the neutral pin goes somewhere, then from your three phase source you could use two boost transformers to get the 240, and still use the wye system neutral as your neutral, and have neutral centered on 240. However, line to neutral voltage would be higher than 120.

You are correct that the power and current values do not correlate for 240 V or 208 V.

.

 

[kwired]

210401-1048 EDT

ericwg:

Your circuit diagram shows no neutral connection. 14-50 plug has a neutral pin, but it may not connect to anything in the oven. You can test whether the neutral pin goes anywhere, start with an ohmmeter test.

An autotransformer and a buck-boost transformer are not necessarily the same. A buck-boost will have the boost winding wound with heavier wire than the primary, whereas an autotransformer may use the same wire size for the entire winding, for example a Variac. (Big problem with this website is changing my spelling of words. Variac gets changed to Varian.)

Suppose the neutral pin goes somewhere, then from your three phase source you could use two boost transformers to get the 240, and still use the wye system neutral as your neutral, and have neutral centered on 240. However, line to neutral voltage would be higher than 120.

You are correct that the power and current values do not correlate for 240 V or 208 V.

.

A buck/boost basically is an autotransformer if used for buck/boost purposes. They can also be used as say a 120/240 x 12/24 separately derived supply. Not all "autotransformers" are necessarily buck/boost transformers though.

 

[suemarkp]

This is an electric range, they have special rules for branch circuit size due to diversity of the elements. They mention this on the middle picture attachment (load diversity calculated per NEC 220). Exact articles would be 220.18 and table 220.55. Whether you can use this depends on your occupancy -- it is for dwellings and instructional settings. If this is commercial cooking, you may to have to use 220.56, but I am unclear if 220.18 allows you to override that or if Note 4 of 220.55 applies to commercial occupancies.

Also, is it NRTL listed and is that a requirement for you? Being a UK design, it may only be CE marked which isn't sufficient.

 

[kwired]

The spec sheet says there is load demand diversity and is why you have 18.5 kW but only a 50 amp required supply circuit - they are figuring it will never be loaded beyond 50 amps, or at least not for long enough to be a problem.

The fact it appears to have no neutral load - a simple buck/boost transformer from 208-240 would likely work. Should someone ever replace this with another range that does utilize a neutral you may subject neutral loads in that new appliance to overvoltage if nobody knows about the transformer and simply unplugs the old and plugs in the replacement.

Neutral would still be same neutral and would still be 120 volts from one line, the common line of the autotransformer, to neutral. The boosted line would be about 152 volts to neutral though.

 

[winnie]

I would start with the manufacturer.

They state that they will not honor the warranty if the range is supplied with 208V. Find out if they will honor the warranty if supplied with 240V with unbalanced L-N voltages. Also find out if they have a recommended transformer arrangement if 120/208V 'single phase' is the power available.

Given that this was designed for service in the UK, it is very likely capable of operation on a 230V L-N supply and doesn't care at all about balanced L-N voltages.

Suemarkp's point is also very relevant. This unit was designed for a different part of the world and might not have the necessary certifications for the desired installation.

-Jon

 

[kwired]

I would start with the manufacturer.

They state that they will not honor the warranty if the range is supplied with 208V. Find out if they will honor the warranty if supplied with 240V with unbalanced L-N voltages. Also find out if they have a recommended transformer arrangement if 120/208V 'single phase' is the power available.

Given that this was designed for service in the UK, it is very likely capable of operation on a 230V L-N supply and doesn't care at all about balanced L-N voltages.

Suemarkp's point is also very relevant. This unit was designed for a different part of the world and might not have the necessary certifications for the desired installation.

-Jon

The fact it has a L1, L2 and N terminals seems to indicate it was designed to sell to North American market. European only market design would likely just have L and N.

 

[winnie]

The fact it has a L1, L2 and N terminals seems to indicate it was designed to sell to North American market. European only market design would likely just have L and N.

Yes, but given that the N terminal doesn't connect to anything suggests to me that it was a European design that was 'patched' for North America.

-Jon

 

[kwired]

Yes, but given that the N terminal doesn't connect to anything suggests to me that it was a European design that was 'patched' for North America.

-Jon

Yes, they probably build products for both markets with most of same components. Probably have a different power distribution block for NA markets though, many of them will connect to an existing receptacle with a neutral conductor. Find it hard to believe that it comes with a 4 wire 14-50 cord but likely it was added by a distributor/supplier.

 

[zbang]

Aga is a high-end brand in the UK, so they ought to have some competent technical help (unfortunately, their USA website is borked at the moment; the UK site is https://www.agaliving.com/contact-support).

Also, given that they're designed for the UK market at 230 volts, the strong likelihood is that the range is only labeled with L1/L2 to keep US electricians from connecting it to 120 volts. (And the 14-50 cord since that's the proper USA cord.)

 

[ericwg]

Wow. Thanks so much for all the great info folks. To address some of the questions raised...

I am a designer for CAL FIRE and this range is to be installed in the current fire station design I am working on. With the type of buildings I design I am vaguely aware of Article 220 and load demand diversity, however, I have not done a design yet where it was justified to implement. I know that there has been quite a bit of discussion in the past RE: fire stations and whether or not they are considered a dwelling, however, from all I have gleaned (and direction from Senior) that doesn't apply to us. Feel free to comment.

As far as the range, just to be clear, I didn't spec it. Appliances are spec'd by our Architect. However, after the issue with 208V vs 240V I did quite a bit of research and could not find another all electric induction range of this size (48" wide, 5 cook tops with plenty of spacing, glide out broiler, large multi-function oven, extra large convection oven) that was listed for use at 208V. If you know of one, please share the info with me. Wolf and Fulgar Milano both have 36" induction ranges but their cooktops aren't as spacious and they don't have as many ovens. These firefighters, God bless them, do some serious cooking and we want to provide them with what they need/want.

As far as someone replacing the range and wiring the new one improperly, etc., I am really not worried about that. A change like that would not happen without my team's involvement. I can even slap a warning sticker above the outlet for the oven.

With RE: to design, yes, as far as I understand, it was originally designed for the UK market. I tend to agree with winnie's observation, that they patched it up for use in the states, spec'd a common cord/plug which would couple to a common receptacle. Until I speak to a designer or engineer at AGA, I don't want to assume that the neutral isn't used, however, it sure looks that way. If that's the case, I would likely bring a spare conductor over for a neutral to be possibly used in the future, but I would not land it on either end. We would just have the contractor land L1, L2, and gnd. As for doing some testing on the neutral, the oven has not yet been purchased. As expensive as it is, I'm sure you understand I wouldn't want to green light purchasing this thing until I am supremely confident I can support the installation electrically.

I'm not sure if I have seen definitive answers yet RE: primary/secondary protection for Boost and Autotransformers. I did see someone say they don't have a true primary or secondary. As far as protection is concerned, though, I can't shake what I see in Exhibit 210.28. What are all those fuses for, which happen to be on both the 208V and 240V side? That's with an Autotransformer. Exhibit 210.19, on the other hand, shows a single phase buck/boost setup for 208-240V and there is no protection shown.

 

[winnie]

The issue with transformers in general is that they exchange voltage for current and can move power from one phase to another. This means that the OCPD that protects the transformer primary coils in general does not protect the secondary coils. That is why you have OCPD on both primary and secondary side.

For certain types of transformer, primary OCPD can also protect the secondary side sometimes.

Transformers can have issues with 'inrush current' thus the permission to have primary OCPD much larger than the transformer rating, but this depends upon having properly sized secondary protection. If you want the transformer primary to also protect the secondary then you can't oversize the primary.

This is the case for single phase autotransformer arrangements. A 208:240V single phase 15kVA autotransformer could be on a 90A OCPD, but if you did this then you would want to put a panel and 50A breaker on the secondary side. I believe you would be fine with a 60A breaker on the primary side and no other OCPD, but am not sure on that point.

-Jon

 

[petersonra]

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


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

 

[d0nut]

I would still push back on the architect to find something at 208V. Maybe two smaller ranges would work better? While you can make it work with a transformer, it still feels like forcing a square peg in a round hole.

 

[ericwg]

Hello Jon,

I have a pretty good understanding of the specs and requirements for LV distribution transformers, which I include in my designs all the time. Unfortunately, I just haven't had the need for a buck/boost or autotransformer until now. Given what you said about the autotransformer. I completely understand your suggestion for a 15kVA autotransformer if the transformer has to be sized for the full load amps of the load. The primary FLA would be 72A while the secondary would be 62.5A. I guess you sized the primary OCPD - 72 X 125% = 90A? The only downside with that 15kVA autotransformer is the size. 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?