Delta versus WYE for 240 1ph heaters? 2

[lrob]

What is the proper way to supply 240v heaters. I need to use a three phase transformer to supply multiple heaters in a production environment. My main concern is the safety factor. What is the difference/advantages between transformers with a Delta or a WYE secondary? Second part of question would be proper grounding for these?

 

[peebee]

Here's a couple options. Don't worry too much about which option you choose, either one will work OK.

Most Wye transformer secondaries would have a secondary voltage of 120/208. Running a 240-volt heater at 208 is OK, but beware that your output will be (208/240)^2 = 75% of rated -- that might or might not be a problem for you, you might or might not need to provide additional heaters or bigger heaters. Maybe you can get heaters rated at 208 rather than 240. This is a very common configuration, that alone might be a good reason to choose it.

Another option for you might be a 120/240-volt center-tapped delta. That would give you 3-phse 240, plus it would give you some 120-volt control voltage. Beware that you can only derive 120 on two out of the three legs; the third leg can be used only for 240-volt 2-pole or 3-pole loads, NOT 120-volt. This is kind of an unusual animal, you might want to avoid it for that reason, but otherwise it seems to lend itself well to your application.

Regardless of the secondary configuration, NEC 250.20(B) REQUIRES that it be a grounded system; ungrounded is NOT an option.

Regarding safety: well, the maximum voltage to ground on a 120/208-volt wye is 120; max line to line is 208. The maximum voltage to ground on a 120/240-volt center-tapped delta is 208; max line to line is 240. So, there's a slightly higher voltage to deal with on the 120/240, but it's not a tremendous difference. It's a moot point if you're used to 277/480-volt systems, which you probably are since I'm guessing that's your transformer primary voltage. Both systems will impose a voltage on both heater terminals, no way around that.

NOTE: you might want to double-check if your heaters have any maximum listed voltage to ground -- if there's a stated maximum of 120, then you CANNOT use the 120/240-volt center-tapped delta. You could, instead, use three single-phase 120/240-volt transformers, but I can't think of any good reason why you'd want to do that.

Regarding proper grounding method: Well, you need to bond the neutral of either system to ground in accordance with 250.30, the requirements there are pretty clear. The requirements are about the same as for any transformer or service entrance installation.

Hope this helps.

 

[lrob]

Thanks peebee, I plan on using a 480v pri./ 480/230 sec. transformer. Does all your info still carry over to the higher voltages? I have separate control transformer.

 

[peebee]

480/230 secondary?!? Are you sure that's right? 3-phase?

Maybe 480/277? (which would be no good for your 240-volt heaters)

Maybe 480/240 single phase?

Maybe 480/240 three-phase center-tapped delta?

 

[peebee]

Maybe 400/230? -- That's a European voltage. . . what country are you in?

 

[busbar]

peebee’s covered it well.

Using a 240V∆ secondary [X1/X2/X3] in an ungrounded mode is acceptable if there are only three secondary terminals and no midpoint or “lighting tap” [X4 or X6] by 99NEC250-20(b). [It should best be used solely for heaters.] If it’s a small system. there’s no real advantage to separately-derived-system grounding.

208Y/120V drytypes are probably a little more readily available, though. One advantage of a 3-wire ∆ primary—3-wire ∆ secondary is that primary overcurrent protection can also suffice for overcurrent protection of the secondary winding per 99NEC240-3(f).

Remember that loaded 150°C-rise transformers run very hot, and that a 115°C- or 80°C-rise may be more desirable.

 

[lrob]

Busbar & Peebee thank you for your feedback.

 

[peebee]

Any updates on that transformer voltage? Is it really 480/230? If so, can you provide some more detail on the exact transformer configuration?

 

[lrob]

Secondary will be 400/230.

 

[peebee]

Ahhh. . . . Well, that's a European voltage. What country are you in?

I'm in the US, and I'll do my best to tell you how to wire it if you were here designing to NEC. If you're somewhere else, take what I say with a grain of salt.

You'd want to solidly ground the neutral of your transformer. All three phases and ground should then be brought to a 4-wiredistribution panel -- here, I'd most likely select a panelboard rated for operation at 277/480-volts. It's not clear to me if 277/480-volt breakers are sold in the US which are rated to operate properly on a 120/400-volt system.

1-pole breakers would then be provided in the distribution panel, and each heater would be wired between a breaker and the neutral bus.

Your control transformer should be selected with either a 230 or 400 volt primary. It would then be fed with a 1-pole (230) or two-pole (400) breaker in the panel.