How to balance a high leg

[uwireme]

I was asked to balance the load at a store/gas station. service 200 amp 3 phase 4 wire 120/240 with a 208 volt high leg. At around noon
the loads were A 115amps B 124amps and C the high leg 51amps. I would like to take 25 amps from A and B and put the 50 amp load on C. Can I hook up a transformer to the 208 leg and transform it to 120 volts? All single phase 120/240 breakers cannot be moved due to
all 120 volt loads.

 

[Dennis Alwon]

How many DP loads are there. If the DP loads are on AB and not BC or AC that will cause the imbalance

 

[texie]

I was asked to balance the load at a store/gas station. service 200 amp 3 phase 4 wire 120/240 with a 208 volt high leg. At around noon
the loads were A 115amps B 124amps and C the high leg 51amps. I would like to take 25 amps from A and B and put the 50 amp load on C. Can I hook up a transformer to the 208 leg and transform it to 120 volts? All single phase 120/240 breakers cannot be moved due to
all 120 volt loads.

Most high leg services are built by the POCO are not intended to be balanced as they are usually used for only limited 3 phase loads and some POCOs even spell this out in the terms of service. In fact many, given the transformer sizes used, would be overloaded if fully balanced and loaded. If this was a 208/120 wye, then by all means a closer balance would be desirable. Also keep in mind that the only way to balance it would be to balance the 240 1 phase loads across all 3 phases (you probably don't have that many) but then you bring on new issues like the rating of the breakers.

 

[stickboy1375]

One of the disadvantages of using a high leg delta is not being able to balance it, since one phase to neutral voltage is higher than the others, no single phase loads can be used, with the lack of loading on the high leg will result in an unbalanced load.

 

[readydave8]

Can I hook up a transformer to the 208 leg and transform it to 120 volts?

can he?

 

[readydave8]

Most high leg services are built by the POCO are not intended to be balanced as they are usually used for only limited 3 phase loads and some POCOs even spell this out in the terms of service. In fact many, given the transformer sizes used, would be overloaded if fully balanced and loaded. If this was a 208/120 wye, then by all means a closer balance would be desirable. Also keep in mind that the only way to balance it would be to balance the 240 1 phase loads across all 3 phases (you probably don't have that many) but then you bring on new issues like the rating of the breakers.

I think he's wanting to put 100 amps on high leg large enough for 200 amp service, would the transformer likely be sized big enough for this?

 

[Smart $]

I think he's wanting to put 100 amps on high leg large enough for 200 amp service, would the transformer likely be sized big enough for this?

I think this is a very important question that needs answered, before any balancing occurs. POCO's often supply hi-leg delta systems (especially non-industrial) with an open delta secondary configuration (i.e./e.g. a two-POT bank), and they often size the two differently for this scenario... according to what they anticipate the 1? and 3? loads to be... and the service equipment and conductors don't always reflect this. I'd say someone has to call the POCO to verify before flopping any loads over to the hi-leg.

 

[uwireme]

Customer called the utilities because lights were dimming, utilities monitored the service for a week and thought balancing the panel would help. I was thinking of installing a 208 volt primary with a 120 volt secondary transformer and transfer some of the 120 volt loads from A & B to the new transformer. I figure that will increase C and decrease A & B and hopefully have close to the same ampacity on all three phases

 

[ptonsparky]

9 amps difference with that large of a total load is not an imbalance. IMO.

 

[hurk27]

You need to look to see what size the pots are, if you have a closed delta you will have 3 pots, two will be a different size then the center tap, the size will depend upon what the original design required, we have both here although being phased out as services are upgraded as they no longer install high leg delta's anymore, while most here will have a 50kva center tapped pot for the single phase loads with two 25 kva pots for the three phase loads (not many open deltas here) we also have 25 kva center tapped pots for smaller single phase loads with 50kva pots for the three phase loads, these are more found in industrial where more 3 phase is used and little single phase, now this ratio of sizing is common, we do have larger services but the same ratio of sizing is kept close to the same as above, I don't think I have ever seen the same size pots on a high leg delta, not around here at least.

So knowing the size of the pots will give you the correct ratio that the service will need to be balanced at.

another question all our high legs are landed on B phase not C ??? the NEC also requires this as well as the high leg being marked orange.

Edited to add: I have never understood why the two larger pots with a smaller center tap pot since for larger 3 phase loads the center tap pot will be always required all three pot to be at least the same, but I have seen a few, maybe they were what they had on the truck ?

 

[Smart $]

Customer called the utilities because lights were dimming, utilities monitored the service for a week and thought balancing the panel would help. I was thinking of installing a 208 volt primary with a 120 volt secondary transformer and transfer some of the 120 volt loads from A & B to the new transformer. I figure that will increase C and decrease A & B and hopefully have close to the same ampacity on all three phases

As mentioned earlier, are there any 240V 1? loads not connected to the high leg. Rearranging them so they are will help shift the load to the high leg.

Additionally, any breaker connected to the high leg must be fully rated for 240V. It cannot be a slash rated breaker (i.e. 120/240).

As Wayne mentioned, the high leg is required by the NEC to be the "B" leg.

 

[kwired]

If the transformers are all same size then it was likely designed with balanced loading in mind.

This is more typical for industrial loading with mostly motors or other loads that utilize all three phases. If you have a lot of 120 volt load as compared to three phase load or straight 240 volt single phase loads, this might not be the right system for the application, or at very least the center tapped transformer needs to be larger if getting a wye system is not going to be an option. Worst case you can install a separately derived system with wye secondary, but reserve that for the stubborn POCO situation that will not give you what you really need for service.

 

[uwireme]

As mentioned earlier, are there any 240V 1? loads not connected to the high leg. Rearranging them so they are will help shift the load to the high leg.

Additionally, any breaker connected to the high leg must be fully rated for 240V. It cannot be a slash rated breaker (i.e. 120/240).

As Wayne mentioned, the high leg is required by the NEC to be the "B" leg.

all 240v loads are connected to C

Customer will not pay me to change wire location...no benefit to him

 

[ptonsparky]

Other arrangements of hi leg position are, or used to be, allowed if they were part of an existing installation with . As long as it is consistent through out.

 

[Fulthrotl]

I was asked to balance the load at a store/gas station. service 200 amp 3 phase 4 wire 120/240 with a 208 volt high leg. At around noon
the loads were A 115amps B 124amps and C the high leg 51amps. I would like to take 25 amps from A and B and put the 50 amp load on C. Can I hook up a transformer to the 208 leg and transform it to 120 volts? All single phase 120/240 breakers cannot be moved due to
all 120 volt loads.

your numbers aren't that bad... i've seen high leg imbalance much worse.
you are under 80% on all three legs.

the only legal way you can do it, is to get a 240v delta / 120/208 wye transformer,
move your non 3 phase delta loads to another panel. that leaves your balanced
loads on the delta panel, and a 30 kva or whatever size xfmr you need feeding
a subpanel, and move the single phase loads over there, and balance them.

and that, as they say, is gonna cost more than eleventeen dollars, and the
cost/benefit ratio is a no sale, IMNSHO.

 

[Fulthrotl]

As Wayne mentioned, the high leg is required by the NEC to be the "B" leg.

ya gotta watch out for what Wayne says... he lives in indiana, and they
have their own time zone there, and everything.

he and i had a long chat on skype one night.... woke up everyone else
on the chat channel, and it was 'cause they do stuff different there....
i don't even want to TALK about utility neutrals any more. my head hurts.

he has to be as smart as he is, just to survive in a place that labors under
horrible handicaps... they can't even agree with people on what time
it is.....

and there is the weather.... don't get me started on that..... :happyno:

 

[Smart $]

... I would like to take 25 amps from A and B and put the 50 amp load on C. Can I hook up a transformer to the 208 leg and transform it to 120 volts? All single phase 120/240 breakers cannot be moved due to
all 120 volt loads.

all 240v loads are connected to C

Customer will not pay me to change wire location...no benefit to him

Using a transformer connected as proposed is... well, unheard of. But, I don't know of any prohibiting requirement. You will likely have to use a two-pole breaker, as it has been mentioned here before that no one manufacturers a full-rated single-pole.

Another issue... You are wanting to remove load from A-N and B-N and put it on C-N to balance the load to reduce or eliminate dimming of lighting. The question is whether the dimming is due to voltage drop (conductor loading) or voltage sag (transformer loading), or both. The proposed 208-120 arrangement will help with voltage drop (the current would no longer be on Lines A and B), but does very little to shift the transformer loading...

Consider first that current of the A-B connected load (which includes A-N and B-N connected loads) doesn't just flow on the A-B service winding. Assuming equal sized, full-delta pots, only 2/3 of it does. The other 1/3 flows on the A-C and C-B pots. Pot A-B has an impedance of 1z, while pots A-C and C-B have a combined series impedance of 2z. Your proposed 25A of A-N plus 25A of B-N load shift is 25A of A-B load shift. 16.7A flows on pot A-B, the other 8.3A through pots A-C and B-C.

The same loads shifted to your transformer present a 50A?120V?208V=28.8A load. Again assuming a full-delta service, 14.4A of that will flow on each pots A-C and B-C and each half of pot A-B. You effectively added an additional 73% (the 0.73 of 1.73) of the connected load to the service load (14.4A?240V?3=10.4kVA vs 25A?240=6kVA). If you consider winding current, you dropped 16.7A from pot A-B and added 14.4A back to it, while you dropped 8.3A each from pots A-C and B-C and added 14.4A back to them.

Can you see where you gain very little?

I would consider kwired's suggestion (if POCO remains stubborn) to go with a separately derived system transformer, such as 240D?208Y/120V.

 

[kwired]

Lets not get hung up on which bus is supposed to be the high leg, no matter which one it is you still can not directly connect 120 volt loads to it and unbalancing is still a problem. Which bus is supposed to be high leg is not the topic of thread, just a issue that goes along with having a high leg. You certainly don't want to start moving things around just because high leg is not in B position on something existing in most cases unless you are starting over - then it really is no longer existing.

 

[Smart $]

ya gotta watch out for what Wayne says... he lives in indiana, and they
have their own time zone there, and everything.

he and i had a long chat on skype one night.... woke up everyone else
on the chat channel, and it was 'cause they do stuff different there....
i don't even want to TALK about utility neutrals any more. my head hurts.

he has to be as smart as he is, just to survive in a place that labors under
horrible handicaps... they can't even agree with people on what time
it is.....

and there is the weather.... don't get me started on that..... :happyno:

You don't have to tell me. I lived in Indiana for a couple years and had all I could take. :happyyes:

(just kidding on the last part)

 

[uwireme]

Thanks for everyones help

I think I will look to see what 120 volt loads can be reduced.