Changing service from 1 phase to 3 phase

[Designer_82]

Working on a mixed-use building (retail/residential) currently supplied by single phase service.

The single phase needs to be upgraded to 3 phase to handle some larger loads going in one of the retail spaces.

There's an existing 1 phase meter center. I'm unsure if there would be any problems with the new supply voltage/compatibility to this meter center since building will now get 3 phase.
Maybe the heating units in the apts. will output less heat?
Maybe issues with any other existing 240V loads changing to 208V?

Appreciate your feedback.

 

[che12345]

Designer_82 (Mechanical)(OP)25 Dec 23 16:36
" #1.Working on a .... currently supplied by single phase service. The single phase needs to be upgraded to 3 phase to handle some larger loads going in one of the retail spaces. #2. There's an existing 1 phase meter center. I'm unsure if there would be any problems with the new supply voltage/compatibility to this meter center since building will now get 3 phase. #3. Maybe the heating units in the apts. will output less heat? #4. Maybe issues with any other existing 240V loads changing to 208V?"
I have the the following opinion for your consideration.
1. It is fine to up grade from single-phase to 3-phase due to increase of load. There can be two different conditions: a) the new larger loads are single phase, b) the larger loads having some three-phase motors, for example.
2. There is no great problem. The new three-phase kWh meter would be just slightly bigger in size. The new 3-phase would be fine for above a) or b) loads.
3. No. If the new 3-phase supply (Line to Neutral) the same as the existing single phase voltage, the output heat would be the same.
4. Attention: a) the new 3-phase (Line to Neutral) voltage shall be the same as the existing single-phase. All the existing appliances/loads would be fine. No actions need to be taken. The new three-phase (Line to Line) voltage would be 1.732 x the existing single-phase voltage; for connection to three-phase motors only. It does NOT affect the existing loads or any new larger (single-phase) loads. Note: ENSURE that the new three-phase (Line to Neutral) voltage IS be the same as the existing sing-phase voltage.
Che Kuan Yau (Singapore)

 

[davidbeach]

My guess is that the single phase is almost certainly 240/120V and the three-phase would be 208Y/120V. Definitely a difference between 240V and 120V.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[wcaseyharman]

If you have a lot of existing 240V loads you might be able to beg the utility to supply you with a 240V 4 wire service, also known as a high leg delta (one of the deltas are neutral tapped, so you get 120/240 and 240V 3 phase) You also may be able ask for a 120/208/240 service (two 120V transformers and a 120/240V transformer). Generally it seems utilities have stopped providing these odd service types and have preferred to standardize, but you might ask. These types do have some risk if the electricians are not familiar with them - you can blow up things if you wire to the wrong phase, and proper labeling is required.

It would probably be better in the long run to request a conventional 120/208 3 phase service and use dry type transformers to feed your existing 120/240V residential loads.

Just my $0.02

Casey

 

[LittleInch]

Designer,

Some information here would stop a lot of the speculation.....

Your existing single phase is what voltage exactly?
Your planned 3 phase is what voltage exactly and coming in as three wires or four?
How is your 240V supply (if it is 240V) being supplied by the utility?
Why can't they just add a separate 3 phase supply and you run a duplicate system?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cranky108]

Many 240 appliances will also operate at 208, but as someone above said a 240 delta with a grounded center tap on one of the 240 transformers might be preferred for you. But the utility may not want to do this. It is a standard type of service in many areas, but also many utilities don't want to do that.
Also many electrocutions don't like them, because of the 'wild leg'.
However, this would be less disruptive for the existing building occupants, that changing to a wye 208 arrangement.
Also less confusing as to if the existing 240 appliances will work on 208.
Warning, it may require a larger service transformer, and require a higher interrupting rating on existing panel boards. There are ways around changing out those panel boards.

 

[waross]

I have been out of the field for a few years now and I do not know current utility standards.
At one time, it was common to add a teaser transformer to an existing single phase transformer to supply three phase loads.
It was common to see a small teaser transformer (about 5 KVA, 10 KVA or 15 KVA paired with a larger 120/240 single phase transformer (possibly 50 KVA) to supply three phase loads.
You may replace the meter base with a meter base suitable for three phase metering (7 jaw vs 4 or 5 jaw).
You may replace service disconnect switch (if present) with a three phase switch and replace the panel with a three phase panel.
or
You may replace or add a three phase service disconnect switch and split out to the existing single phase panel and add a three phase panel for new three phase loads.
The down side:
The unbalanced primary phase currents will cause a primary neutral current and a primary neutral voltage drop.
On a firm primary supply, relatively close in this effect will be negligible.
On long rural distribution lines, the neutral voltage drop and resulting neutral displacement may cause phase angle errors and phase voltage unbalances.
This will cause circulating currents in any motors and the circulating currents will tend to reduce the effects the primary neutral current.
Solution:
Avoid 100% loading on close in installations.
Over-size the motors for remote rural installations.
Avoid closed delta secondary connections for distribution applications.
I have seen a lot of closed delta services and I never want to have to mitigate the effects of another one.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

It is common to feed apartment buildings 120/208. Each unit will get two legs and a neutral. The existing single phase meter bases and meters will not work. You will need "network" 5 lug bases and meters.

 

[waross]

Been there done that, Steven.
South of the border where the rules are different or non-existent;
A seafood plant was selling shore power to fishing boats tied along-side.
The millions of 120/240 single phase meters in use are not a Blondell solution.
The voltage coil measures the 240 Volts applied and relies on the assumption that both 120 Volt legs are equal.
In the case of badly unbalanced loads and a significant voltage drop on the neutral, the voltages on the nominal 120 Volt legs are no longer equal and metering accuracy suffers.

If a standard 4 jaw meter is used to meter two legs of 120/208 Volts, the meter will read low.
The reading may be corrected by a factor of 240/208.
The same assumption of equal equal nominal 120 Volt legs applies.
The corrected accuracy will be close to equal to the accuracy of the 4 jaw meter on 120/240 Volts.
Good enough for the tird world.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

waross,

Probably wouldn't pass with our meter department. To my knowledge, correction factors are only used when instrument transformers are involved.

Designer,

A few more comments regarding single phase 208 vs. 240:
Conventional thermostatically controlled resistance heating devices will take longer to reach set point. Pre-heating the oven will take longer and the water heater will take longer to recover. Motor controlled devices such as heat pumps and heat pump water heaters will need to be checked for suitability at 208.

 

[waross]

Probably wouldn't pass with our meter department.

Fortunately, I was the de-facto meter department.
There is a choice between accurately metering 120 Volt loads and accurately metering 208 Volt loads.
This was a special case, fishing boats alongside at the processing plant.
Their 208 Volt loads were 3 phase refrigeration fed from the onboard generator.
The catch has been unloaded and the refrigeration shut down.
When on shore power, there were no 208 Volt loads running.
In the tird world, you gotta do the best you can with what you got.

A few more comments regarding single phase 208 vs. 240:
Several tens of millions of apartment dwellers in North America have no idea that they have 208 Volts rather than 240 Volts.
That said, in the original design and build, the 208V vs 240 Volt difference should be considered when spec'ing infrastructure motor loads,
ie: elevators, central A/C, pumps etc.
Heating and the cycling of the thermostat is largely transparent to the apartment dwellers.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Designer_82]

I appreciate the excellent feedback! A couple things I took away...

1) Existing 120/240V Meter Center probably will not work correctly at 120/208V so a complete upgrade would be needed.
2) Existing 240V heating loads probably will not work as efficiently at 208V and will take longer to take effect.


Thanks!

 

[waross]

2) Existing 240V heating loads probably will not work as efficiently at 208V and will take longer to take effect.

The efficiency will be the same.
New users will not notice the difference in time to reach temperature.
If the voltage is changed from 240 Volts to 208 Volts, most users will not notice the difference.
With the lower voltage, there will be less overshoot on the first cycle when an oven element is first turned on.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!