480V transformer

Transformer suppliers probably have stock of 240D to 480Y 277.

Regular electrical supply houses may have lesser chance of stocking them as they wouldn't be as high in demand as 480 delta to 208/120 wye.
That is the transformer I have looked into already wouldn’t the wye secondary require the 4th wire (neutral) ? It’s not required on any of the loads. There all 3 wire loads. Would delta delta step up be better ? Maybe cheaper although at $1600 for refurbished I’m not complaining to much. It’s doubful I would ever need the 277v for lighting loads.
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
A Delta Delta transformer will bring you right back to your corner grounded situation. A Delta Wye unit has the HO midpoint on the secondary, which you would bond to establish a more "conventional" 480/277 SDS. This also allows the use of regular slash rated breakers if you were going to install a panel downstream of the transformer.

Technically you'd not need to bring a neutral out of the transformer if you had no Line to Neutral (277) loads. I'd leave it to more experienced electricians to chime in if they'd pull one for use in the future. I am just an engineer, and tend not to care about budgets, so I'd pull one if I was supplying a panel.

Good luck with the job. Hope it works out well for you.
 
Last edited:

kwired

Electron manager
Location
NE Nebraska
That is the transformer I have looked into already wouldn’t the wye secondary require the 4th wire (neutral) ? It’s not required on any of the loads. There all 3 wire loads. Would delta delta step up be better ? Maybe cheaper although at $1600 for refurbished I’m not complaining to much. It’s doubful I would ever need the 277v for lighting loads.
You are not required to utilize the neutral, which basically means if there is no 277 volt loads to supply you don't have to install a neutral conductor. You still must ground the system and 250.26 tells us:

For ac premises wiring systems, the conductor to be grounded shall be as specified in the following:

  1. Single-phase, 2-wire — one conductor
  2. Single-phase, 3-wire — the neutral conductor
  3. Multiphase systems having one wire common to all phases — the neutral^ conductor
  4. Multiphase systems where one phase is grounded — one phase conductor
  5. Multiphase systems in which one phase is used as in (2) — the neutral conductor
Number 3 fits what you have and therefore you must ground the neutral conductor.

with those backfed transformers you had no neutral on the high voltage side and therefore there only option to ground is to ground a phase conductor.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
That is the transformer I have looked into already wouldn’t the wye secondary require the 4th wire (neutral) ? It’s not required on any of the loads. There all 3 wire loads. Would delta delta step up be better ? Maybe cheaper although at $1600 for refurbished I’m not complaining to much. It’s doubful I would ever need the 277v for lighting loads.
You don't need to bring the neutral to the equipment you are running you do need to bond it at the transformer or first disconect.
 
Ok this will shine some light on this job. Turns out this building has three services as it used to be multiple tenants. The one machine bead blaster was as discribed corner grounded delta, 240 to 480 reverse fed. Adjusted tap for incoming voltage. Phase is grounded to a ground lug not XO and is joined together with a separate ground wire to the building steel. They ran a 8/3 MC cable from trans to disconnect fused at 40amp all three phases including grounded phase which is marked with yellow tape. That phase which is grounded into a lug at bottom of transformer where all grounds tie together. Several problems with this install which I may or may not have to repair. Good news for me I can use a different service in the next building that is three phase 208/120 so I can now use there 75KVA trans that’s sitting on a shelf. Will need to be reverse fed but will be fine. Will run the 208V to a disconnect from a 100amp breaker from disconnect to trans from trans to load center load center to line of site crane and furnace. Done
 

kwired

Electron manager
Location
NE Nebraska
Ok this will shine some light on this job. Turns out this building has three services as it used to be multiple tenants. The one machine bead blaster was as discribed corner grounded delta, 240 to 480 reverse fed. Adjusted tap for incoming voltage. Phase is grounded to a ground lug not XO and is joined together with a separate ground wire to the building steel. They ran a 8/3 MC cable from trans to disconnect fused at 40amp all three phases including grounded phase which is marked with yellow tape. That phase which is grounded into a lug at bottom of transformer where all grounds tie together. Several problems with this install which I may or may not have to repair. Good news for me I can use a different service in the next building that is three phase 208/120 so I can now use there 75KVA trans that’s sitting on a shelf. Will need to be reverse fed but will be fine. Will run the 208V to a disconnect from a 100amp breaker from disconnect to trans from trans to load center load center to line of site crane and furnace. Done
Trying to make sense of what you said.

Incoming service is 240 corner ground delta?
That is supplying a reverse fed transformer that is normally 480 delta x 208Y120?

If that is correct XO is a primary terminal when reverse fed, it should be left "floating" when used as reverse feed. The 480 volt delta side is the secondary when reverse fed, and there is no neutral for this setup, there is no choice except to corner ground it, can be any of the three corners that gets grounded, but only one can be grounded or else you will have fault current between the two that are grounded. Incoming voltage tap on back fed ends up being on the secondary side. You may be able to get secondary voltage to a point you like, but the fact still remains that you are inputting 240 nominal into a coil designed to operate at 208 nominal. You may get away with it easier if incoming voltage is at low end of what is tolerable for 240 volts as that is also close to the high end of what is tolerable for a nominal 208 volts. If tolerance level is 10% then upper end for 208 does overlap lower end of 240.
 
Trying to make sense of what you said.

Incoming service is 240 corner ground delta?
That is supplying a reverse fed transformer that is normally 480 delta x 208Y120?

If that is correct XO is a primary terminal when reverse fed, it should be left "floating" when used as reverse feed. The 480 volt delta side is the secondary when reverse fed, and there is no neutral for this setup, there is no choice except to corner ground it, can be any of the three corners that gets grounded, but only one can be grounded or else you will have fault current between the two that are grounded. Incoming voltage tap on back fed ends up being on the secondary side. You may be able to get secondary voltage to a point you like, but the fact still remains that you are inputting 240 nominal into a coil designed to operate at 208 nominal. You may get away with it easier if incoming voltage is at low end of what is tolerable for 240 volts as that is also close to the high end of what is tolerable for a nominal 208 volts. If tolerance level is 10% then upper end for 208 does overlap lower end of 240.
Sorry for the confusion I’ve been reading up on this for two days and all I get is more confused. The one job is already done but was done wrong.
Main concern is the my job of providing 480V power to two machines both 3 phase one 50amp the other 30amp the 50 draws about 22amps the 30 draws 12.5 amps at 480V.
My transformer supply voltage is 208 some 130’ away. Ideally I’d like to install a 3 pole 100amp breaker in the 208V panel run that the 130ft to a disconnect feed transformer come out of transformer secondary twice with 480V to two fusible disconnects installed within 10ft of the transformer to feed each machine. Confusion has risen when trying to use there own transformer sitting on the shelf which is 480V pri and 208 sec I get that it would need to be corner grounded and unfused grounded phase initial influx etc but other factors with the EGC and the overall stress on a unknown used transformer etc. So option two is to supply a proper step up transformer and basically install as stated above. All the references and different chapters concerning this install all the advice given sometimes contradictory has left me confused as to which is the best way to proceed. If I can avoid a panel board with two breakers for the loads and simply use the disconnects that would offset the cost of a new transformer and would be my recommendation to the client. Now I bet anyone who reads this is confused as well sorry. Bottom line is to let them know of the code violations on previous install and provide the client two options for hooking up there two 480V machines. One with there transformer and one with a step up transformer. Pros and cons of each.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Absolutely you can use 2 disconnect switches on the secondary of a transformer. If each feed originates at the transformer lugs, just follow the appropriate tap rule for load on the '30' or '50A' conductors. If you plan to leave the transformer with (1) set of conductors and then tap to each device, you conductors will need to be sized for combined load.

Corner grounding a 480V delta does not but any stress on the transformer. Reverse feeding a transformer does not add any additional stress to the transformer, however its inrush current is increased, so the circuit feeding it might be impacted (FWIW, except for some specialty transformers, I have not seen any manufacturer with reverse feed prohibition on units larger than 15kVA, but you still need to ask).
 

kwired

Electron manager
Location
NE Nebraska
Sorry for the confusion I’ve been reading up on this for two days and all I get is more confused. The one job is already done but was done wrong.
Main concern is the my job of providing 480V power to two machines both 3 phase one 50amp the other 30amp the 50 draws about 22amps the 30 draws 12.5 amps at 480V.
My transformer supply voltage is 208 some 130’ away. Ideally I’d like to install a 3 pole 100amp breaker in the 208V panel run that the 130ft to a disconnect feed transformer come out of transformer secondary twice with 480V to two fusible disconnects installed within 10ft of the transformer to feed each machine. Confusion has risen when trying to use there own transformer sitting on the shelf which is 480V pri and 208 sec I get that it would need to be corner grounded and unfused grounded phase initial influx etc but other factors with the EGC and the overall stress on a unknown used transformer etc. So option two is to supply a proper step up transformer and basically install as stated above. All the references and different chapters concerning this install all the advice given sometimes contradictory has left me confused as to which is the best way to proceed. If I can avoid a panel board with two breakers for the loads and simply use the disconnects that would offset the cost of a new transformer and would be my recommendation to the client. Now I bet anyone who reads this is confused as well sorry. Bottom line is to let them know of the code violations on previous install and provide the client two options for hooking up there two 480V machines. One with there transformer and one with a step up transformer. Pros and cons of each.
Some considerations.

As Don mentioned inrush when energizing a backfed transformer is likely higher than would be for similar capacity unit that is designed for 208 primary. This may mean the 100 amp breaker you wish to use may hold for the conventional type unit but may not on the backfed unit, but then the 130 foot supply conductors might have enough resistance to help out here as well.

VD will be bigger issue for 130 feet of conductor @ 208 volts then it is for same VA load @ 480 volts, plus smaller conductor can be used even before factoring in VD so there is reason to consider placing transformer near the 208 volt supply. Even if you run two separate circuits 130 feet you are looking at a set of 12 AWG and a set of 10 AWG most likely vs a set of 3AWG or maybe even 2AWG for VD, and larger raceway as well. Probably will need a disconnect near the loads supplied, but can likely just be a non fused safety switch or for no more current than involved here even motor rated switch (some are called manual motor switches). Would only need to be 30 amp switches, your 30 and 50 amp "circuit" is possibly recommended circuit breaker to allow motor starting? Time delay fuses can be less, and are still fine on 12 AWG and 10 AWG (depends on details) if supplying motor loads of the ratings you mentioned even though the overcurrent devices are over 20 and 30 amps. (your loads sound just about right for a 10 HP and a 20 HP motor or maybe smaller motors and other combination load types included.

Another option is a single 480 volt feeder and feeder taps near the load end to two disconnects. need to run pricing on all these options because each one has higher cost in some areas and lower cost in others, but nothing is necessarily equally proportional in price in each of these options. Single 480 volt feeder will need your primary breaker, a secondary disconnect, then your two disconnects near the load plus possibly a splice box or something to make the taps in, but only single raceway run or no need to have to adjust ampacity for multiple current carrying conductors. This off top of my head may be able to be as little as 8 AWG feeder conductor and if supplying motor loads only still could possibly be on 60 amp time delay fuses. 8 AWG might be borderline too small, all depends on more details.

Using less copper and smaller raceway may offset enough cost to justify spending some more an additional 600 volt disconnect.

Add: you also need a GEC on the secondary of the separately derived system. If you have building steel as part of your GES and it is everywhere in the facility this may not make much difference, if your SDS is close to the service equipment and you don't have other electrodes near the other end of this run it may mean spending more on a GEC, though I believe if you are running a EGC with the 208 volt supply it would likley be able to also be used for GEC in this instance, but would be a larger EGC than for a 480 volt feeder also.
 
Last edited:
I was going to run a 2/3 MC cable or conduit and pull #2s not sure which yet. with my 208 supply to a disconnect to the transformer out to 2 fusible disconnects not more then 10’ away from the transformer. The one load is a 5Ton crane and will need that ground to the 4th rail. The second load is a furnace with two 3hp motors a control panel with its own small 480/208/120 step down built in for idiot lights (120v)and a few pressure switches that are 208 I believe it’s 2 KVA
If the transformer is located at the 208V side I would then have to supply a OCPD on the secondary anything after 25’ for industrial building I thought. Here’s where some of my confusion comes in do I need to carry a neutral from the 208V supply to then use the X0 terminal if a step up transformer is used ? This would now be my grounded conductor for a 4 wire wye system. If there transformer is used reverse fed wye/delta it will require corner grounding plus a separate EGC and do not use the XO at all. My loads do not require a neutral strictly 3hase with a ground.
 

MD Automation

Member
Location
Maryland
Occupation
Engineer
If the transformer is located at the 208V side I would then have to supply a OCPD on the secondary anything after 25’ for industrial building I thought. Here’s where some of my confusion comes in do I need to carry a neutral from the 208V supply to then use the X0 terminal if a step up transformer is used ? This would now be my grounded conductor for a 4 wire wye system. If there transformer is used reverse fed wye/delta it will require corner grounding plus a separate EGC and do not use the XO at all. My loads do not require a neutral strictly 3hase with a ground.
Just to clarify...
You don't need to bring a neutral from the 208 panel to the transformer.

If you are using a step up transformer, there is no XO terminal at all. There is an HO terminal on the 480 Wye Secondary. This you need to bond and can be your neutral on the 480 side - but if you don't need one you don't need to come out of the transformer with it. But you do need to bond and ground that for your SDS.

As you indicated, if you are using their step down transformer and back feeding it, you don't connect anything to the XO terminal. No neutral, no bond, leave it floating. It's a common mistake to want to land something on that lug.

Lastly, if the furnace and crane load are running at the same time, from your numbers that will be drawing 80 amps on your 208 primary. Which you indicate you are protecting with a 100A breaker. There are good reasons to locate the transformer near the 208 supply - like copper costs and VD. Like KWired said in his previous post, make sure to price out that option.
 
Just to clarify...
You don't need to bring a neutral from the 208 panel to the transformer.

If you are using a step up transformer, there is no XO terminal at all. There is an HO terminal on the 480 Wye Secondary. This you need to bond and can be your neutral on the 480 side - but if you don't need one you don't need to come out of the transformer with it. But you do need to bond and ground that for your SDS.

As you indicated, if you are using their step down transformer and back feeding it, you don't connect anything to the XO terminal. No neutral, no bond, leave it floating. It's a common mistake to want to land something on that lug.

Lastly, if the furnace and crane load are running at the same time, from your numbers that will be drawing 80 amps on your 208 primary. Which you indicate you are protecting with a 100A breaker. There are good reasons to locate the transformer near the 208 supply - like copper costs and VD. Like KWired said in his previous post, make sure to price out that option.
Thank you and much appreciated. The crane load is 12.5 amps and the furnace is 22.5 with an additional motor running a cooling fan which will run sometimes at about 3.5 amps. The furnace comes with a three phase 50a breaker inside of this control panel. Yet another reason I’ve been hesitant about a corner fed delta.
Wouldn’t I need OCP on the secondary at that 130’ distance ? There’s also a guestion of available space but it’s probably possible to install trans at the 208 supply. Never crossed my mind reading over 25’ rule
 
Thank you and much appreciated. The crane load is 12.5 amps and the furnace is 22.5 with an additional motor running a cooling fan which will run sometimes at about 3.5 amps. The furnace comes with a three phase 50a breaker inside of this control panel. Yet another reason I’ve been hesitant about a corner fed delta.
Wouldn’t I need OCP on the secondary at that 130’ distance ? There’s also a guestion of available space but it’s probably possible to install trans at the 208 supply. Never crossed my mind reading over 25’ rule
Meant to say corner ground delta not corner fed
 

kwired

Electron manager
Location
NE Nebraska
Thank you and much appreciated. The crane load is 12.5 amps and the furnace is 22.5 with an additional motor running a cooling fan which will run sometimes at about 3.5 amps. The furnace comes with a three phase 50a breaker inside of this control panel. Yet another reason I’ve been hesitant about a corner fed delta.
Wouldn’t I need OCP on the secondary at that 130’ distance ? There’s also a guestion of available space but it’s probably possible to install trans at the 208 supply. Never crossed my mind reading over 25’ rule
Outdoors maybe not. Read 240.21(C) carefully and determine what may or may not apply to your installation.

Indoors you generally will need overcurrent protection at least within 25 feet max from transformer secondary though.
 
So now it’s recommended to install a step up trans at the 208 primary side and carry the 480V the 130’ to both machines. I would most certainly need to protect each load so two 600V disconnects fused at the proper amperage per load from fusible to service disconnects line of sight and whip into each machine. EGC from source to machine disconnects and GC also would be carried in the 12/3 MC and the 6/3 MC
 

kwired

Electron manager
Location
NE Nebraska
So now it’s recommended to install a step up trans at the 208 primary side and carry the 480V the 130’ to both machines. I would most certainly need to protect each load so two 600V disconnects fused at the proper amperage per load from fusible to service disconnects line of sight and whip into each machine. EGC from source to machine disconnects and GC also would be carried in the 12/3 MC and the 6/3 MC
Not saying it is recommended, am saying compare options and make a decision. Less copper over the length of the circuit means more $$ left over for additional items it may end up requiring. Also if you end up needing a disconnect at a particular location either way the 30 amp possibly is less cost than a 60 amp even though it is a different voltage rating, so you need to look at the entire install not just one component at a time.

130 feet is short enough run it may not matter all that much which way you do it, make that run longer and price of conductors can become a bigger factor especially if you need to increase size for voltage drop.
 
The fact that if the transformer is installed on the 480V load side the two machines will be within the 25’ range and not require overcurrent protection I can then simply feed each machine with a service disconnect. Eliminate the need for a panel board. Could use a 100amp service disconnect at same location to feed transformer. Protection comes from 100amp 208V primary breaker. The furnace load has a control panel with a 3pole 50amp breaker (another reason I don’t want to corner ground and reverse feed another transformer). and the crane simply has the rail boots with the ground rail which I will need to carry back to the source ground of my SDS. The problem I see the other way around is 4 disconnects will be needed. 2 fusible for OCP on the secondary 480V due to the 130’ run then service disconnects for the two loads. The cost is lower even carrying # 2s the 130’. Everything I read tells me OCP is needed on the secondary over 25’ some exceptions with outdoor transformers but as it pertains to this building it’s 25’ which makes the bead blaster installation already completed with a reverse fed corner grounded trans approx 75’ 480V secondary protected by a 100Amp 3phase 208V primary breaker not within code.
 

kwired

Electron manager
Location
NE Nebraska
The fact that if the transformer is installed on the 480V load side the two machines will be within the 25’ range and not require overcurrent protection I can then simply feed each machine with a service disconnect. Eliminate the need for a panel board. Could use a 100amp service disconnect at same location to feed transformer. Protection comes from 100amp 208V primary breaker. The furnace load has a control panel with a 3pole 50amp breaker (another reason I don’t want to corner ground and reverse feed another transformer). and the crane simply has the rail boots with the ground rail which I will need to carry back to the source ground of my SDS. The problem I see the other way around is 4 disconnects will be needed. 2 fusible for OCP on the secondary 480V due to the 130’ run then service disconnects for the two loads. The cost is lower even carrying # 2s the 130’. Everything I read tells me OCP is needed on the secondary over 25’ some exceptions with outdoor transformers but as it pertains to this building it’s 25’ which makes the bead blaster installation already completed with a reverse fed corner grounded trans approx 75’ 480V secondary protected by a 100Amp 3phase 208V primary breaker not within code.
You need secondary protection regardless of secondary length, but can have so called "tap conductors" up to 25 feet in length as the general rule.

One other option is primary only protection, but this would have to be three wire delta to three wire delta or two wire to two wire (single phase) to do so. In those cases you have proportional current on primary and secondary and the primary overcurrent protection can protect both primary and secondary conductors. Can't do this with secondary that has neutral because unbalance load can overload things yet not draw overcurrent on the primary.
 
Top