Boosting 208V to 240V

[Little Bill]

Trying to help a friend out with a problem. He needs 240V with a 30A load. The service is 208V. I know a B&B transformer can do this, and I have hooked them up. However, the load also needs a neutral. Would you just run the neutral straight though, bypassing the transformer, or would it have to connect through the transformer?

 

[LarryFine]

Yes, the neutral must feed through. You didn't say 1ph or 3ph.

 

[wwhitney]

I'm going to assume that this is a single phase 3-wire load, as I'm not aware of loads that require 240Y/138V 4-wire.

You can't get 120/240V 3-wire from 120/208V 3-wire with just a normal boost arrangement. If you boost the ungrounded conductors from 208V to 240V in a balanced fashion, then the voltage of the boosted ungrounded conductors relative to the 208Y/120V neutral will be 138V.

So you need to provide more information about the load and its exact voltage requirements. If it expects L1-L2 to be 240V, and L1-N to be 120V, and N to be at earth voltage, and doesn't have anything connected L2-N, then you could possibly boost only L2.

Cheers, Wayne

 

[fastline]

Ya, would help to know what the load actually is first.

 

[LarryFine]

I'm going to assume that this is a single phase 3-wire load, as I'm not aware of loads that require 240Y/138V 4-wire.

Good point. Stop that.

You can't get 120/240V 3-wire from 120/208V 3-wire with just a normal boost arrangement. If you boost the ungrounded conductors from 208V to 240V in a balanced fashion, then the voltage of the boosted ungrounded conductors relative to the 208Y/120V neutral will be 138V.

I did once propose the idea of wiring the two secondaries at opposite ends of the primary.

 

[wwhitney]

I did once propose the idea of wiring the two secondaries at opposite ends of the primary.

Right, that's the balanced boost arrangement I referred to. The boosted L-N voltage is 138V.

If the load requires 120V L1-N, 120V L2-N, and 240V L1-L2, and it doesn't care about the N-G voltage, then you could use an autotransformer with a center tap to derive a new N' for the load. That N' would be 60V to N.

Cheers, Wayne

 

[LarryFine]

If the load requires 120V L1-N, 120V L2-N, and 240V L1-L2, and it doesn't care about the N-G voltage, then you could use an autotransformer with a center tap to derive a new N' for the load. That N' would be 60V to N.

Wouldn't that produce an ungroundable system that requires grounding?

If it doesn't require grounding, it seems like a good solution.

What about auto-transformer rules such as a common conductor?

 

[wwhitney]

Wouldn't that produce an ungroundable system that requires grounding?

It's not an SDS, so no additional grounding is required. N' is just another ungrounded conductor. If it faults to N-G, a breaker should trip. You just need to confirm that the load is compatible with an N-G voltage of 60V.

What about auto-transformer rules such as a common conductor?

2017 NEC 210.9 in part:

"210.9 Circuits Derived from Autotransformers. Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts."

Seems like Exception No. 1 applies, as it doesn't restrict to 2-wire circuits.

Cheers, Wayne

 

[kwired]

Your typical single phase buck/boost output has one line that is same potential as one of the input conductors, the other output has the boost winding in series with it. If you are only needing 120 volt from one of the ungrounded conductors make sure it is connected to the "common" line of the autotransformer as it is still 120 volts to the source neutral. If you need 120 from both lines to neutral it gets more complicated or the simple thing is separately derived system with 120/240 secondary.

 

[kwired]

It's not an SDS, so no additional grounding is required. N' is just another ungrounded conductor. If it faults to N-G, a breaker should trip. You just need to confirm that the load is compatible with an N-G voltage of 60V.



2017 NEC 210.9 in part:

"210.9 Circuits Derived from Autotransformers. Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts."

Seems like Exception No. 1 applies, as it doesn't restrict to 2-wire circuits.

Cheers, Wayne

yes. one line will still be 120 to the grounded conductor, the other one will be 120 plus the 32 volts added by the boost coil so it should be 152 to the grounded conductor and not the previously mentioned 138. 138 is what you get if you made a separately derived wye system that is 240 phase to phase.

 

[wwhitney]

yes. one line will still be 120 to the grounded conductor, the other one will be 120 plus the 32 volts added by the boost coil so it should be 152 to the grounded conductor and not the previously mentioned 138.

You can boost L2 32V relative to L1 and get what you described, and that would work for a load that matches the description in the last paragraph of my first post. The voltage to ground of the boosted L2' is a bit less than 152V, as the 32V boost is not in phase with the L2-N voltage.

But what Larry and I are referring to would be to boost both L1 and L2 relative to their midpoint. E.g. a transformer arrangement resulting in a single coil that is 240 units long, call the ends -120 and 120. There would be 3 additional connections, at -104, 0 and 104. Connect L1 and L2 to the -/+104 connections, and take L1', L2', and N' at -120, 120, and 0 respectively. N' is ungrounded at 60V to ground (N-N' is in phase with L3-N).

Cheers, Wayne

 

[kwired]

You can boost L2 32V relative to L1

Which is what probably 99% of what is done when someone needs to boost 208 to 240 volts and uses simple selection tables or on line selectors, then it refers you to a connection diagram that puts the 32 volt boost winding in series with L2.

I suppose it could be possible if the low volt winding of your autotransformer is 16/32 to put each 16 volt segment in series with each input lead and get ~120 volts to neutral on each output lead?

 

[hillbilly1]

I'm going to assume that this is a single phase 3-wire load, as I'm not aware of loads that require 240Y/138V 4-wire.

You can't get 120/240V 3-wire from 120/208V 3-wire with just a normal boost arrangement. If you boost the ungrounded conductors from 208V to 240V in a balanced fashion, then the voltage of the boosted ungrounded conductors relative to the 208Y/120V neutral will be 138V.

So you need to provide more information about the load and its exact voltage requirements. If it expects L1-L2 to be 240V, and L1-N to be 120V, and N to be at earth voltage, and doesn't have anything connected L2-N, then you could possibly boost only L2.

Cheers, Wayne

You can get 120/240 three wire, only boost the one leg, but you will only have 120 on one leg to neutral, but leg to leg would be 240.

You can boost L2 32V relative to L1 and get what you described, and that would work for a load that matches the description in the last paragraph of my first post. The voltage to ground of the boosted L2' is a bit less than 152V, as the 32V boost is not in phase with the L2-N voltage.

But what Larry and I are referring to would be to boost both L1 and L2 relative to their midpoint. E.g. a transformer arrangement resulting in a single coil that is 240 units long, call the ends -120 and 120. There would be 3 additional connections, at -104, 0 and 104. Connect L1 and L2 to the -/+104 connections, and take L1', L2', and N' at -120, 120, and 0 respectively. N' is ungrounded at 60V to ground (N-N' is in phase with L3-N).

Cheers, Wayne

but why make it complicated, when probably only one leg is used for the 120 volt load?

 

[LarryFine]

yes. one line will still be 120 to the grounded conductor, the other one will be 120 plus the 32 volts added by the boost coil so it should be 152 to the grounded conductor and not the previously mentioned 138. 138 is what you get if you made a separately derived wye system that is 240 phase to phase.

Not the way I'm talking about. I'm saying to wire each secondary to opposite ends of the primary.

Each 104v half gets one half of the boost, maintaining the secondary's center tap centered-ness.

 

[wwhitney]

You can get 120/240 three wire,

I would not call it 120/240V three wire, that term has a well defined meaning where each ungrounded is 120V to N and the L-L is 240V.

but why make it complicated, when probably only one leg is used for the 120 volt load?

Sure, if you can confirm and identify that only one ungrounded conductor is used for 120V loads.

Cheers, Wayne

 

[LarryFine]

I suppose it could be possible if the low volt winding of your autotransformer is 16/32 to put each 16 volt segment in series with each input lead and get ~120 volts to neutral on each output lead?

Bingo.

 

[wwhitney]

Which is what probably 99% of what is done when someone needs to boost 208 to 240 volts

Sure for a 2-wire load. But we're discussing an unknown 3-wire load.

I suppose it could be possible if the low volt winding of your autotransformer is 16/32 to put each 16 volt segment in series with each input lead and get ~120 volts to neutral on each output lead?

Relative to the 208Y/120V neutral, if you boost the 208V to 240V in a balanced fashion like this, you get 134V to neutral on each ungrounded conductor.

Cheers, Wayne

 

[LarryFine]

but why make it complicated, when probably only one leg is used for the 120 volt load?

Two reasons:

1. Having to ascertain that the safe line to boost is always maintained.

2. It's fun to do the unusual.

 

[LarryFine]

I would not call it 120/240V three wire, that term has a well defined meaning where each ungrounded is 120V to N and the L-L is 240V.

It would apply if your reference is N'.

 

[LarryFine]

Relative to the 208Y/120V neutral, if you boost the 208V to 240V in a balanced fashion like this, you get 134V to neutral on each ungrounded conductor.

Then use N' instead of N. Perhaps clearly label the outlet, or build it into or onto the load/equipment.