replacing delta high leg services with straight 3-phase

[malachi constant]

We are replacing a 120/240V delta high leg service with a straight 208V/3P/4W service. The utility transformers, service conductors, main switchboard, and a few panels are scheduled to be replaced. A few other switchboards and panels are scheduled to remain. What concerns do I need to have regarding existing loads and equipment?

My questions:
* Some of our 3-phase motor nameplates say 208-230, and some only say 230. Do we need to be concerned about the "230V only" equipment? We have pumps, an elevator, RTUs, compressors, lifts. (For instance one compressor says it is rated at 230/460 at 60hz, and 208/415 at 50hz. What are the chances there are issues here?)
* We plan on intercepting the feeders to existing-to-remain 3-phase switchboards and extending to the new service with three equal phase wires plus neutral and ground. Is it possible (likely?) the switchboard is currently fed with different size conductors? Are there potential issues here?
* We are replacing a few of the three-phase breaker panels with new. I assume the 120V/1P loads get refed from the same, and 208V/1-pole loads get refed from 2-pole breakers. What about 2-pole loads that straddled the 120V and 208V phases - just put those on new 208V 2-pole breakers?

The utility originally said they would let us keep the delta high-leg, but last week changed their mind and said we had to replace it with a straight-up 208/3. If there are enough concerns with existing panels/equipment I will challenge their decision. (This is all being driven by needing to replace an aged service entrance switchboard.)

Thanks!

 

[Jraef]

We are replacing a 120/240V delta high leg service with a straight 208V/3P/4W service. The utility transformers, service conductors, main switchboard, and a few panels are scheduled to be replaced. A few other switchboards and panels are scheduled to remain. What concerns do I need to have regarding existing loads and equipment?

My questions:
* Some of our 3-phase motor nameplates say 208-230, and some only say 230. Do we need to be concerned about the "230V only" equipment? We have pumps, an elevator, RTUs, compressors, lifts. (For instance one compressor says it is rated at 230/460 at 60hz, and 208/415 at 50hz. What are the chances there are issues here?)
* We plan on intercepting the feeders to existing-to-remain 3-phase switchboards and extending to the new service with three equal phase wires plus neutral and ground. Is it possible (likely?) the switchboard is currently fed with different size conductors? Are there potential issues here?
* We are replacing a few of the three-phase breaker panels with new. I assume the 120V/1P loads get refed from the same, and 208V/1-pole loads get refed from 2-pole breakers. What about 2-pole loads that straddled the 120V and 208V phases - just put those on new 208V 2-pole breakers?

The utility originally said they would let us keep the delta high-leg, but last week changed their mind and said we had to replace it with a straight-up 208/3. If there are enough concerns with existing panels/equipment I will challenge their decision. (This is all being driven by needing to replace an aged service entrance switchboard.)

Thanks!

MOST, but not ALL, 230V listed motors 10HP and under are fine with 208V, and a lot of that has to do with loading and your actual voltage under load. Most motors are over sized and/or have a 1.15SF, so you often have some "wiggle room", but if your 208V ends up as 192V at the motor terminals, that may have exceeded even that. You really have to check them individually, there is no generalize statement that applies other than that.

Heating loads will not really matter unless reaction time is critical to a process, and most switch mode power supplies for electronics now have a fairly wide voltage tolerance. Older linear power supplies might be problematic, but those are getting more and more rare now.

 

[ActionDave]

I'm just a guy who works on stuff so take what I say as far as you like...
I have seen very few problems using 230V motors on a 208V system.

I have seen more problems using 208V motors on existing high leg systems.

No amount of planning or paperwork is enough to head off every problem. This is true for new construction and double true times five for remodels.

 

[malachi constant]

Ooh, another question. There is a generator on site, probably 30+ years old. Couldn't find any nameplate on it, and got distracted before I could locate the ATS. Is there a chance the ATS will need adjusted to look for a lower drop-out voltage? (i.e. 120V/120V/120V instead of 120V/208V/120V)

 

[ptonsparky]

We are replacing a 120/240V delta high leg service with a straight 208V/3P/4W service. The utility transformers, service conductors, main switchboard, and a few panels are scheduled to be replaced. A few other switchboards and panels are scheduled to remain. What concerns do I need to have regarding existing loads and equipment?

My questions:
* Some of our 3-phase motor nameplates say 208-230, and some only say 230. Do we need to be concerned about the "230V only" equipment? We have pumps, an elevator, RTUs, compressors, lifts. (For instance one compressor says it is rated at 230/460 at 60hz, and 208/415 at 50hz. What are the chances there are issues here?)
* We plan on intercepting the feeders to existing-to-remain 3-phase switchboards and extending to the new service with three equal phase wires plus neutral and ground. Is it possible (likely?) the switchboard is currently fed with different size conductors? Are there potential issues here?
* We are replacing a few of the three-phase breaker panels with new. I assume the 120V/1P loads get refed from the same, and 208V/1-pole loads get refed from 2-pole breakers. What about 2-pole loads that straddled the 120V and 208V phases - just put those on new 208V 2-pole breakers?

The utility originally said they would let us keep the delta high-leg, but last week changed their mind and said we had to replace it with a straight-up 208/3. If there are enough concerns with existing panels/equipment I will challenge their decision. (This is all being driven by needing to replace an aged service entrance switchboard.)

Thanks!

Generally the switch to 208 does not cause a great deal of grief but some of the items you list may have a problem. I would be most concerned with the RTU, compressors and the Elevator. 208V/1pole loads probably should never been hooked up that way. Those items that need the boost to 230 can be connected via Buck/Boost transformers for relatively low costs.

Years ago it was common to feed a service disconnect with smaller wires on the high leg because that phase generally had a much smaller load. Usually via fused disconnects, not CBs.

FWIW the available fault current will change and never entered into the picture back when, so you may have consider that.

 

[jglavin427]

* We are replacing a few of the three-phase breaker panels with new. I assume the 120V/1P loads get refed from the same, and 208V/1-pole loads get refed from 2-pole breakers. What about 2-pole loads that straddled the 120V and 208V phases - just put those on new 208V 2-pole breakers?

I didn't think putting single-pole breakers on the high leg was allowed, but if there are any and you reconnect to a 2-pole, you'll have to make sure when connecting what was the neutral back into a phase that you mark those wires to identify them as ungrounded.

As for the existing 2-pole breakers, voltage between any 2 phases on the delta was the same 240V, so shouldn't matter which ones they are connected to.

 

[GoldDigger]

I didn't think putting single-pole breakers on the high leg was allowed,

As long as the voltage is what is needed and the current is not going to overload the 120/240 winding, there is no prohibition against wiring a high leg to neutral load with a single pole breaker. But that breaker has to be full voltage rated rather than slash rated since the voltage to ground is too high.

 

[JRW 70]

Different Perspective

Different Perspective

I don't know what legal/regulatory rights you have, but
at my utility what you have is what you get to keep (if
the system has been in continuous service) We have
480Y277, 208Y120, center grounded delta and 480 and
240V corner grounded in the system and have backup
transformers in all existing configs. out there in the stock
yard.

See if you could get someone in management or engineering
that would understand the massive work over this change
will cause. I had a pole that the transformer was hanging on
the phase conductors (just up the street from me that supplied
a pump for the fire hydrants) I work central plant, but calling
the right person over at the district office of the transmission
and distribution department got them there the next day. The
city where I stay while working called the number they had
for weeks with no results. So if you have regs. that allow a
continuation of your working service please check that out

And depending on the pickup/dropout on your generator
that in and of itself may cause problems. We had to replace
the entire control system at a pump site because the generator
was/is in it's 30's as well and had plug in boards that controlled
starting/load pickup/retransfer and cool down. That is all gone
electriconic with a cost of around fifteen thousand on a 230kW
Onan. The engine control was incompatible with the transfer
switch, etc.

In summary, if your system works now try to see if you can
get in contact with someone above the customer rep. level
before enbarking on what sounds like a change I would try
to avoid.

JR

 

[kwired]

How significant is the motor load?

If you switch from 240 to 208 the motors are going to draw more current - some actually may require larger conductors for 208 then for 240. Any motor already operating near full load very well will end up drawing enough to trip overload protection if it is not re-adjusted, and that is assuming the motor can take the increase in current in the first place.

If this is a facility with a lot of 120 volt and limited three phase load then maybe not a bad idea. If a facility with mostly three phase motor load then you are likely better off staying with the 240 volt system if possible.