Transformer Install

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Rex Hungerford

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I hate asking such broad questions but I have not touched a transformer in 15 years. Now I am carrying a product line and need to ensure I am specifying the installation correctly.

I have a 4.5kva 240 volt to 120 volt isolation transformer.
The transformer is in a wall mount enclosure and has integral 25A Primary fuses and a 50A secondary circuit breaker. These transformer has a built in temperature limit switch set to 130*. These transformers will generally be installed in a residence.

450.3(B) Over current per table 450.3(B) Primary and Secondary protection 9A or more
Primary 250% - Secondary 125%

Does this mean I can size the circuit breaker from the main panel feeding the transformer as follows. 4500/240=18.75x2.5=46.875A. So a 45 Amp circuit Breaker and a #8 feeder. Is this correct?

The transformer already has a secondary 50A circuit breaker. Does this mean I can use #8 wire from the transformer to a panelboard and the panel can be main lug only.

450.14 Disconnecting Means Locked and Field Marked

This looks to say I can put a lock on the Circuit breaker in the panel feeding the transformer and affix a label to the transformer noting where the transformer is fed from? No other disconnect required. Assume the panel serving the transformer will be in another part of the home. Correct.

What does "Field Marked" require for materials. Can it be hand written? Can it be a sticky back label I print on with my computer? Does it have to be phenolic? Does it have to be applied to the face of the transformer or can it be inside the hinged cover.

What am I forgetting?

Thanks
 
I forgot to add, one of my customers want to put the unit in a mechanical closet. Are there code requirements for ventilation to control the temperature in the room. Where would I look to find the guidelines.
 
Note: You mentioned "residential" use which would normally be NM wiring which is limited to the 60° rating for ampacity so if you plan on using the units 50 amp breaker as protection for you panel a #6 Romex would be required.

How are you addressing grounding ??
 
Rex Hungerford said:
I hate asking such broad questions but I have not touched a transformer in 15 years. Now I am carrying a product line and need to ensure I am specifying the installation correctly.

I have a 4.5kva 240 volt to 120 volt isolation transformer.
The transformer is in a wall mount enclosure and has integral 25A Primary fuses and a 50A secondary circuit breaker. These transformer has a built in temperature limit switch set to 130*. That is a curious addition. These transformers will generally be installed in a residence. Can't install in a closet.

450.3(B) Over current per table 450.3(B) Primary and Secondary protection 9A or more
Primary 250% - Secondary 125%

Does this mean I can size the circuit breaker from the main panel feeding the transformer as follows. 4500/240=18.75x2.5=46.875A. So a 45 Amp circuit Breaker and a #8 feeder. Is this correct? That would work, but since it already includes a 25 A fuse on the primary side, it could be a 30 A CB with #10 wire.

The transformer already has a secondary 50A circuit breaker. Does this mean I can use #8 wire from the transformer to a panelboard and the panel can be main lug only. Yes, with the NM caveat another poster mentioned.

450.14 Disconnecting Means Locked and Field Marked

This looks to say I can put a lock on the Circuit breaker in the panel feeding the transformer and affix a label to the transformer noting where the transformer is fed from? No other disconnect required. Assume the panel serving the transformer will be in another part of the home. Correct. That would be fine.

What does "Field Marked" require for materials. Can it be hand written? Can it be a sticky back label I print on with my computer? Does it have to be phenolic? Does it have to be applied to the face of the transformer or can it be inside the hinged cover. There is no spec on the way it is marked to be found in the code.

What am I forgetting?

Thanks
Click to expand...
See comments in red above.

Off hand I can't think of anything you are missing other than the grounding electrode connection another poster mentioned.
 
Understand on NM-B wire size to 60*.

Why not in a mechanical closet. This is a walk in space with other equipment related to the building. It's not a coat closet. How does one differentiate a closet from a room?
 
Rex Hungerford said:
Understand on NM-B wire size to 60*.

Why not in a mechanical closet. This is a walk in space with other equipment related to the building. It's not a coat closet. How does one differentiate a closet from a room?
Click to expand...
240.24 Location in or on Premises.
(D) Not in Vicinity of Easily Ignitible Material. Overcurrent
devices shall not be located in the vicinity of easily ignitible
material, such as in clothes closets.
Click to expand...
I would not call a mechanical room a closet, but a clothes closet would be off limits.
 
Talking NM-B. If I run a feeder to a subpanel and use #2 copper NM-B , at 60* its 95 Amps. Can I use the next standard size up CB and use a 100 amp.
 
Rex Hungerford said:
Talking NM-B. If I run a feeder to a subpanel and use #2 copper NM-B , at 60* its 95 Amps. Can I use the next standard size up CB and use a 100 amp.
Click to expand...
Why would you want to? Secondary of transformer is limited to a 50 amp breaker isn't it?
 
qcroanoke said:
Why would you want to? Secondary of transformer is limited to a 50 amp breaker isn't it?
Click to expand...
I also have 10kva transformers that have 100A fused outputs.
And I also have a project with a feeders from a main distribution to a subpanel. I want to use the existing 100A breaker. 240.4(B)(1)(2). I think I'm good. I have 2/3 NM-B
 
Last edited:
Tulsa Electrician said:
If 250 102 c applies. 6-2g may not work.
Click to expand...
I see what your asking. Here is the schematic. The derived neutral is solidly bonded to the ground in the transformer enclosure. That should be my SBJ SSBJ. The load feeder wire from the transformer to my panel will have a Hot, Neutral and ground. The ground should only be an equipment grounding conductor per 250.122.

My panel is a custom panel Ul labeled 120 volt only.
 

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6-2 g NM cu has a # 10 bare.
#8 is minium size for a SSBJ in this case.
The drawing did indacte #8
I would either pipe or MC or use a SEU ( ser) cable.
I know some may say use a 6-3g NM however the black or red would be used as SSBJ and the bare #10 cut out. If we call it an EGC than we can not use it and it must be green in Color untill you hit #4.
If you use Al you must size accordingly.
I do believe the SSBJ can be bare.
I will need to read to be sure.
I have always used insulated. Maybe some one else will know for sure.
 
It seems so odd to me I can legally run a feeder from my main panel (separately derived system) where the neutral and ground are bonded together, eminated from a branch CB, then wire to a sub panel where the ground floats and the ground is an equipment ground.

Yet I have a transformer (separately derived system) where thr neutral and ground are bonded together, with a built in overload CB to protect the feeder, and a feeder wire to the panel where the ground floats, yet this ground is a SBJ.

Does the fact I have a CB disconnect in the enclosere make this different than other transformer your use to powering. Years ago on commercial union pronects I never saw a transformer with a CB in the enclosure. A very different install.

If were to toss a regular old dry type on a floor, bond in the enclosure then exit to a fused disconnect switch, then run from the fused switch to a MLO panel, would my wire from the fused disconnect to the panel still be a SBJ? Or just an equipment ground.

If I have to, I can run #4. Still less than pipe and wire when I calculate the labor.
 
Most general use transformers do not have any OCPD included.

Some intended for a specific use might have that as well as other accessories included. 4.5 kVA is also not a industry standard size for single phase is for three phase though (basically three 1.5 kVA single phase units equivalent).

If there is an OCPD and you are using it for general use feeder/branch circuits pretty sure the OCPD needs to be rated as a branch circuit device otherwise it is nothing more than a supplemental device for the unit.
 
If the first means of disconect (50amp) is in the same enclosure. The rules are different.
You do not need a SSBJ.
It's an egg at that point and is sized per 250.122.
 
Mikes book does not clearly address the question of a grounding electrode, but these iimages are typical. They do not reflect what I have with a CB disconnect in the transformer enclosure with all my bonding in the enclosure
 
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