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

[Open Neutral]

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.

I predate the first transatlantic telephone cable. It's astonishing to me that I pay 1¢ per minute to call the UK now.
I've had cases where calling made the difference. So first find the rates you'll pay, & what needed get sane rates.

 

[drcampbell]

Unless you're providing a circuit that supplies 120v on both L1-N and L2-N, I wouldn't install a NEMA 14-50 receptacle. Sooner or later, someone will plug in an all-American appliance and the boosted 152v L-N will destroy something. Or worse, cause apparently-idiopathic failures every so often, maybe several times per year. The whole point of the NEMA family of plugs & receptacles is assuring that the P&R are electrically compatible if they're mechanically compatible.

If you can confirm that the N is not used, (it would be nice if the schematic and text agreed) a NEMA 6-50 might be appropriate. (if the actual current draw isn't excessive)

I would definitely ask a manufacturer's applications engineer for a recommendation in writing. And maybe have a native Brit look it over to assure that it doesn't say something you think it doesn't say.

 

[ericwg]

Unless you're providing a circuit that supplies 120v on both L1-N and L2-N, I wouldn't install a NEMA 14-50 receptacle. Sooner or later, someone will plug in an all-American appliance and the boosted 152v L-N will destroy something. Or worse, cause apparently-idiopathic failures every so often, maybe several times per year. The whole point of the NEMA family of plugs & receptacles is assuring that the P&R are electrically compatible if they're mechanically compatible.

If you can confirm that the N is not used, (it would be nice if the schematic and text agreed) a NEMA 6-50 might be appropriate. (if the actual current draw isn't excessive)

I would definitely ask a manufacturer's applications engineer for a recommendation in writing. And maybe have a native Brit look it over to assure that it doesn't say something you think it doesn't say.

You make a good point RE: the 14-50R. Instead of showing that receptacle on my plans, I can just draw a 4SQ behind the range, have the Contractor flex over to it, and perform the make up in the box. Depending on what I find out RE: the use of the neutral, I may do as I said previously and slap a warning sticker on the wall or the 4SQ cover.

I actually just gave AGA's HQ in the UK a call. What I did not know is that today (4/5) is a UK Bank Holiday. Oh well, I am typically up this late during the week so I will try to give them a call again tomorrow. I sent them an email last week as well. So, between the calls and email, hopefully I can reach someone knowledgeable soon. I'm also hoping to get a call back from Middleby International later today. I spoke with someone on Friday who was unable to answer my questions but said he had someone on his team he was confident could. Unfortunately, he was off on Friday but is supposed to be back at work today. <fingers crossed>

If this range really cannot operate at 208VAC and they confirm that the schematic in the user's guide is not accurate and the neutral is needed, I am honestly leaning towards installing a general purpose dry type distribution transformer like one of the following:

Eaton S29N11E15N - Platt
Acme T279747S - Platt

They are definitely more expensive than the boost / autotransformers I was considering, but they will provide everything I need. I have room in my Elec room for either. I also have room on an adjacent wall for a fused disconnect to cover the secondary protection. Although, with the added cost of the transformer, disconnect, and wiring, the overall cost of this range would be >$10k. That's a bit much. I will definitely be pushing back on my Architect to try and find a suitable replacement.

 

[kwired]

You make a good point RE: the 14-50R. Instead of showing that receptacle on my plans, I can just draw a 4SQ behind the range, have the Contractor flex over to it, and perform the make up in the box. Depending on what I find out RE: the use of the neutral, I may do as I said previously and slap a warning sticker on the wall or the 4SQ cover.

I actually just gave AGA's HQ in the UK a call. What I did not know is that today (4/5) is a UK Bank Holiday. Oh well, I am typically up this late during the week so I will try to give them a call again tomorrow. I sent them an email last week as well. So, between the calls and email, hopefully I can reach someone knowledgeable soon. I'm also hoping to get a call back from Middleby International later today. I spoke with someone on Friday who was unable to answer my questions but said he had someone on his team he was confident could. Unfortunately, he was off on Friday but is supposed to be back at work today. <fingers crossed>

If this range really cannot operate at 208VAC and they confirm that the schematic in the user's guide is not accurate and the neutral is needed, I am honestly leaning towards installing a general purpose dry type distribution transformer like one of the following:

Eaton S29N11E15N - Platt
Acme T279747S - Platt

They are definitely more expensive than the boost / autotransformers I was considering, but they will provide everything I need. I have room in my Elec room for either. I also have room on an adjacent wall for a fused disconnect to cover the secondary protection. Although, with the added cost of the transformer, disconnect, and wiring, the overall cost of this range would be >$10k. That's a bit much. I will definitely be pushing back on my Architect to try and find a suitable replacement.

Unless it is already ordered maybe see if there isn't other voltage options available?

 

[synchro]

Transformers with isolated primary and secondary windings will also draw more current than a smaller autotransformer when it's idling without a load. So there will be some added ongoing cost from that to consider if it makes any difference to the client.

 

[petersonra]

My personal opinion - if they can afford to buy such a pricey item, best bet is to just get an isolation xfmr and wire it with a normal plug. If the N is not used on the range end, who cares then and no need for a sign.

 

[ericwg]

Unless it is already ordered maybe see if there isn't other voltage options available?

The unit has not been ordered yet. It will not be for another ~8-10 months. However, I know for sure it is not available in other voltages...yet.

 

[ericwg]

Ok gang, I just got off the phone with the national service director for La Cornue, the snootier sister to the AGA line. But, this guy knew the AGA Mercury line really well too. He was able to confirm the following:

1. The oven portion of the range will operate at 208VAC, albeit with a slower preheat time and an overall lower output. However, the induction cooktop will NOT operate at 208VAC. They recognize this is an issue for the US market and have plans to update the design for use over 208-240VAC in the future. He pointed out that most New York hotels and restaurants actually operate at 208VAC so they are designing themselves out of that market with the current design. Bottom line, the current model needs 240VAC, however, there should be a future model that operates from 208-240VAC.

2. The circuit diagram for the Mercury range is accurate. The neutral is NOT used. In fact, he mentioned that the neutral is missing from the NEMA 14-50 plug they provide with the oven. On the other hand, the user's guide for the La Cornue, CornuFe induction ranges, which is essentially a higher end art deco version of the Mercury, does show the neutral being used for two switched oven lights. Point being, their circuit diagrams are accurate. Then the neutral is used, they show it.

3. As discuss earlier in this thread, the 45A load / 50A breaker was defined based off load diversity demand calculations via article 220. The true load is closer to 18.5kW, i.e. ~77A @ 240VAC.

Soooo, with this new information, it would seem that I am back to where I started, i.e. a boost or autotransformer should work just fine. Assuming I am able to employ the load diversity demand calc'd values, with a 50A breaker (w/ 2#6,#10G), please comment on:

Boost vs. Autotransformer? Referencing the aforementioned Exhibits 210.18 vs 210.19, it appears that the boost xfmr setup doesn't require any primary/secondary protection whereas the autotransformer setup requires fuses for each hot on both the 208 and 240 side?

 

[jim dungar]

Watch your terminology.

Boost means to raise the voltage (buck means to lower it), every transformer can perform this task.

An isolation style transformer is probably the most common construction. There is a primary winding and an isolated secondary windings. It is common to call these two winding units.

An autotransformer is a configuration of windings such that there is a direct electrical connection tion between the high and low voltage windings. For large voltage differences, like 480V to 208V, these transformers are often built at the factory using a single tapped winding.

What our industry calls a 'Buck-Boost' transformer, used for small voltage changes like 240V - 208V, is usually a two winding transformer with its primary and secondary windings interconnected into an autotransformer configuration.

 

[synchro]

An autotransformer is a configuration of windings such that there is a direct electrical connection tion between the high and low voltage windings. For large voltage differences, like 480V to 208V, these transformers are often built at the factory using a single tapped winding.

What our industry calls a 'Buck-Boost' transformer, used for small voltage changes like 240V - 208V, is usually a two winding transformer with its primary and secondary windings interconnected into an autotransformer configuration.

Agree. A single tapped winding makes sense if the number of turns on either side of the tap are reasonably close to each other, and therefore the currents in them are also close to each other. In Jim's example, the side of the winding with 208V across it would have (480-208)/208 ≅ 1.31 times the current going through the other side. However, with a boost transformer going from 208V to 240V the 32V winding would have 208/32 = 6.5 times the current flowing through the 208V winding. And so a single tapped winding of a uniform gauge would be grossly oversized on the 208V side while also having 6.5/(1 + 6.5) ≅ 0.866 (~ 87%) of the total turns. And so a transformer with two separate windings, each appropriate for its own current level, is the only practical configuration in this case.