480 volt unbalanced load

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Optimal1

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Sanford, Florida
I have a question that's was brought up to me... There is an electric engineer that is giving my customer a hard time about installing his equipment.
The customer has equipment and HVAC that runs on 120/240 volt single phase.
The power supply is from a 800 amp MDP 277/480 volt 3 phase panel.
We would normally install a 2 pole 100 amp 480v breaker to feed a 480v/120/240 volt single phase transformer 50kva.
The electrical engineer is saying that it can not be done that way. Because it will not be a balanced load. He says it will cause he 3 phase panel to now be unbalanced and has concerns about the neutral in his 480 volt panel?
Can you give me some insight on this please. I am not sure if I should just install a 3 pole 480 volt 3 phase breaker to feed a 480volt to 120/240 volt 3 phase 50 KVA and only use the A and C phase off the transformer to feed my 120/240 volt single phase panel?
My customer can not change any of his equipment.
 
If it were illegal to have unbalanced loads, we wouldn't have neutrals.

I have a hard time understanding the problem with the proposed installation as well. I fail to see the difference it would make to change to a three phase transformer.

Welcome to the forum.
 
Last edited:
Optimal1 said:
I have a question that's was brought up to me... There is an electric engineer that is giving my customer a hard time about installing his equipment.
The customer has equipment and HVAC that runs on 120/240 volt single phase.
The power supply is from a 800 amp MDP 277/480 volt 3 phase panel.
We would normally install a 2 pole 100 amp 480v breaker to feed a 480v/120/240 volt single phase transformer 50kva.
The electrical engineer is saying that it can not be done that way. Because it will not be a balanced load. He says it will cause he 3 phase panel to now be unbalanced and has concerns about the neutral in his 480 volt panel?
Can you give me some insight on this please. I am not sure if I should just install a 3 pole 480 volt 3 phase breaker to feed a 480volt to 120/240 volt 3 phase 50 KVA and only use the A and C phase off the transformer to feed my 120/240 volt single phase panel?
My customer can not change any of his equipment.
Click to expand...
You will still have an imbalance on the 480 volt supply if you go through the three phase transformer and only connect a single phase load.

The idea here is to try to connect any additional single phase load in a way to balance the supply.

One does need to watch total kVA - if you have a 50 kVA transformer (actually 45 is a standard size for a three phase) with a majority of the kVA for a load it needs to be balanced or a portion of your transformer may be overloaded. Same goes for the transformer supplying the service.

Now if you have multiple pieces of single phase equipment - you probably are wise in many instances to go with the three phase secondary and balance the load as much as possible.

Balancing the load may or may not be simple, all depends on kVA of each load, as well as considerations on how long certain loads run. Short duration loads can be something that you disregard to some extent over a lesser but more continuous load.
 
Does the engineer know as a matter of fact the 480/277 MDP is balanced to start with???

It could be unbalanced and adding the single phase load could make it closer to being in balance!!!
 
iwire said:
The engineer is nuts plain and simple.

What is the engineers solution?
Click to expand...

The engineer wants us to install a 100 amp 3 pole beaker with a 480 volt/120/240 volt 3 phase transformer. Delta/delta (if I understood that correctly ) and then to a 3 phase sub panel. He is saying to use this because it will have a balanced system. And by doing this it will not affect his 480 volt panel and the neutral won't have any issues then!!!! I Couldn't understand this reasoning. I was lost for words.
This would be great if my equipment was not all set up in a prefabbed shelter. With a 120/240 v single phase panel with a built in ATS/MTS /TVSS the cam locks are all pre wired as we'll. plus the back up generator has to be installed on a 120/240 single phase system since we have no access to a 3 phase generator.
 
Optimal1 said:
The engineer wants us to install a 100 amp 3 pole beaker with a 480 volt/120/240 volt 3 phase transformer. Delta/delta (if I understood that correctly ) and then to a 3 phase sub panel. He is saying to use this because it will have a balanced system. And by doing this it will not affect his 480 volt panel and the neutral won't have any issues then!!!! I Couldn't understand this reasoning. I was lost for words.
This would be great if my equipment was not all set up in a prefabbed shelter. With a 120/240 v single phase panel with a built in ATS/MTS /TVSS the cam locks are all pre wired as we'll. plus the back up generator has to be installed on a 120/240 single phase system since we have no access to a 3 phase generator.
Click to expand...
Wow! I don't know what to say. Sounds like some emails and meetings and a lot of time and energy being used up are in your future.

I am curious what kind of equipment you have. Is this for a cell site?
 
Optimal1 said:
...The electrical engineer is saying that it can not be done that way. Because it will not be a balanced load.
Click to expand...
The only way you can get a balanced three phase load when powering only a single phase load is to use a 3 phase to 1 phase converter. A transformer will not do it.


Can you give me some insight on this please. I am not sure if I should just install a 3 pole 480 volt 3 phase breaker to feed a 480volt to 120/240 volt 3 phase 50 KVA and only use the A and C phase off the transformer to feed my 120/240 volt single phase panel?
Click to expand...
You didn't mention actual load, but if it requires 50kVA 1 phase, a 50kVA 3 phase transformer will be too small. You'd need at least 133%-of-load 3 phase transformer.

He says it will cause he 3 phase panel to now be unbalanced and has concerns about the neutral in his 480 volt panel?
Click to expand...
The neutral will not be used.

Optimal1 said:
The engineer wants us to install a 100 amp 3 pole beaker with a 480 volt/120/240 volt 3 phase transformer. Delta/delta (if I understood that correctly ) and then to a 3 phase sub panel. He is saying to use this because it will have a balanced system. And by doing this it will not affect his 480 volt panel and the neutral won't have any issues then!!!!....
Click to expand...
Already answered above, but I'll reiterate....

The neutral will not be used, and using a transformer will not balance this load on the MDP (if it is already balanced).

That said, using a 3 phase transformer can help lessen the unbalance... but far from making it equally balanced across all three lines.
 
Smart $ said:
Does the engineer know as a matter of fact the 480/277 MDP is balanced to start with???

It could be unbalanced and adding the single phase load could make it closer to being in balance!!!
Click to expand...

I am actually going up to the job site on Wednesday. it is in north Florida and i am in central Florida.
Thank you for that help.
 
ActionDave said:
Wow! I don't know what to say. Sounds like some emails and meetings and a lot of time and energy being used up are in your future.

I am curious what kind of equipment you have. Is this for a cell site?
Click to expand...

Yes it happens to be a cell site. I was actually asked to talk with this engineer because I have done over 250 cell sites and have done over 25 high rise or existing site build outs for them with the 800 or larger MDP 3 phase 277/480 volt systems to the 480v/120/240 volt single phase 50 kva transformer and have never had any issue after the installation. I have also asked Mike Holt and Tim Henry on this and they thinks its odd that the engineer is giving me a hard time. Mike had thought if I post this i may get some good response to the problem and help me talk with this engineer.
 
kwired said:
You will still have an imbalance on the 480 volt supply if you go through the three phase transformer and only connect a single phase load.

The idea here is to try to connect any additional single phase load in a way to balance the supply.

One does need to watch total kVA - if you have a 50 kVA transformer (actually 45 is a standard size for a three phase) with a majority of the kVA for a load it needs to be balanced or a portion of your transformer may be overloaded. Same goes for the transformer supplying the service.

Now if you have multiple pieces of single phase equipment - you probably are wise in many instances to go with the three phase secondary and balance the load as much as possible.

Balancing the load may or may not be simple, all depends on kVA of each load, as well as considerations on how long certain loads run. Short duration loads can be something that you disregard to some extent over a lesser but more continuous load.
Click to expand...
I want to install a 2 pole 100 amp 480v breaker to feed a 480v/120/240 volt single phase transformer 50kva. this is what i have always done and never had any issues with this install.
 
Smart $ said:
The only way you can get a balanced three phase load when powering only a single phase load is to use a 3 phase to 1 phase converter. A transformer will not do it.



You didn't mention actual load, but if it requires 50kVA 1 phase, a 50kVA 3 phase transformer will be too small. You'd need at least 133%-of-load 3 phase transformer.


The neutral will not be used.


Already answered above, but I'll reiterate....

The neutral will not be used, and using a transformer will not balance this load on the MDP (if it is already balanced).

That said, using a 3 phase transformer can help lessen the unbalance... but far from making it equally balanced across all three lines.
Click to expand...
I want to install this the way I have for years. 100 amp 2 pole breaker in the 480 v panel to a 480v/ 120/240 volt single phase 50 kva transformer. the actually load is 78 amps max. customer always makes us install larger wire and gear for future. there are about (18) 2 pole breakers and (4) 20 amp breakers, mostly for outlets and lights that are never in use or on unless a tech come there once a week for an hour.
and this is what he is saying, it will cause his 480 volt panel to be unbalanced and have issues with the neutral. I explained to him that i am making my own neutral and sizing everything correct. and he then said i must do everything in 3 phase, i said there is no way that i can do that.
 
Optimal1 said:
I want to install this the way I have for years. 100 amp 2 pole breaker in the 480 v panel to a 480v/ 120/240 volt single phase 50 kva transformer. the actually load is 78 amps max. customer always makes us install larger wire and gear for future. there are about (18) 2 pole breakers and (4) 20 amp breakers, mostly for outlets and lights that are never in use or on unless a tech come there once a week for an hour.
and this is what he is saying, it will cause his 480 volt panel to be unbalanced and have issues with the neutral. I explained to him that i am making my own neutral and sizing everything correct. and he then said i must do everything in 3 phase, i said there is no way that i can do that.
Click to expand...
So how loaded and balanced is the existing 480 volt system? Again as others have said the neutral of the 480/277 system will not see any current you will just have higher current on the two phases you connect to - If existing load were unbalanced to begin with one should carefully select which two phases to connect this to in order to bring it closer to being balanced.

Is some of the equipment "pre-wired" and you would have to change it if you were to install the three phase transformer and attempt to balance the load better? Sorry not familiar with what typical loads are at a cell site.
 
Optimal1 said:
Yes it happens to be a cell site. I was actually asked to talk with this engineer because I have done over 250 cell sites and have done over 25 high rise or existing site build outs for them with the 800 or larger MDP 3 phase 277/480 volt systems to the 480v/120/240 volt single phase 50 kva transformer and have never had any issue after the installation. I have also asked Mike Holt and Tim Henry on this and they thinks its odd that the engineer is giving me a hard time. Mike had thought if I post this i may get some good response to the problem and help me talk with this engineer.
Click to expand...
I thought so. There are a lot of guys on this forum, including some very good engineers, who can help with the math and theory. I'll stay out of their way, however, be warned a totally well reasoned, factual argument may not get you as far as you hope.

I'll will join in with some empathy so that you don't feel like you are the first one to encounter something like this. The shop I work for does work on cell sites. Mercifully we now sub to another contractor who does battle with the big guns.
 
Optimal1 said:
I want to install this the way I have for years. 100 amp 2 pole breaker in the 480 v panel to a 480v/ 120/240 volt single phase 50 kva transformer. the actually load is 78 amps max. customer always makes us install larger wire and gear for future. there are about (18) 2 pole breakers and (4) 20 amp breakers, mostly for outlets and lights that are never in use or on unless a tech come there once a week for an hour.
and this is what he is saying, it will cause his 480 volt panel to be unbalanced and have issues with the neutral. I explained to him that i am making my own neutral and sizing everything correct. and he then said i must do everything in 3 phase, i said there is no way that i can do that.
Click to expand...
Well there is no way to tell how balanced a system is by the number and type of breakers. While a load calc' may show on paper how balanced a system is, real world balancing can only be determined by recording usage data.

The project may turn into such a headache pursuing the matter, it may just be easier to install a 45kVA 480-240/120 Delta-Delta 3? transformer and a 240V 3? panel, finish the job, and walk away. Let the engineer deal with the unbalance... :p
 
This is the part that confuses me:
Optimal1 said:
He says it will cause he 3 phase panel to now be unbalanced and has concerns about the neutral in his 480 volt panel?
Click to expand...
Unless the neutral was reduced in size on the basis of some loading calculation, and I never choose to do that myself, or unless there is a serious harmonics issue, and we have been given no indication that there is, then no amount of imbalance on the ungrounded conductors can possibly cause an overloading of the neutral.

 
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