Multiple transformers in series

[Opie11]

Any help would be appreciated in understanding how to calculate voltage drop, amps, and over current protection when having multiple transformers, up to 5 , connected in series.
In doing research I am not finding anything that clearly addresses this.

 

[Besoeker3]

Any help would be appreciated in understanding how to calculate voltage drop, amps, and over current protection when having multiple transformers, up to 5 , connected in series.
In doing research I am not finding anything that clearly addresses this.

Do you really mean five transformers in series ? Would you really do that?

 

[electrofelon]

Any help would be appreciated in understanding how to calculate voltage drop, amps, and over current protection when having multiple transformers, up to 5 , connected in series.
In doing research I am not finding anything that clearly addresses this.

1. Voltage drop, even for a single transformer is going to be a little complicated to figure out. You would need to know power factor of the load, loading, impedance, and XR ratio. Here is a calculator you can play with. https://voltage-disturbance.com/engineering-calculators/transformer-calculator/

2. Amps. Not really sure what you mean by amps. Amps is just KVA divided by volts (single phase), unless you want to bring losses into it.

3. OCPD. The transformers will need to be protected per 450.3. secondary conductors will need to be protected per 240.21c. note there are cases where the secondary can be predicted by the primar.

 

[Opie11]

Unfortunately, yes, there really are multiple transformers in series spec'd by an engineer.

Where my confusion is regarding voltage drop is when your going from say 480 to 120/240 then 240 to 480 and then back again to 120/240. Then yet another transformer to bring the voltage to 550. I
just dont know where to start.

For the amps, yes, figuring in the loss(es) to determine what my actual output would be.

We usually protect the initial transformer primary and secondary separately. I am confused if this protection, usually fused disconnects, would continue to protect subsequent transformers.

 

[xformer]

Unfortunately, yes, there really are multiple transformers in series spec'd by an engineer.

Where my confusion is regarding voltage drop is when your going from say 480 to 120/240 then 240 to 480 and then back again to 120/240. Then yet another transformer to bring the voltage to 550. I
just dont know where to start.

For the amps, yes, figuring in the loss(es) to determine what my actual output would be.

We usually protect the initial transformer primary and secondary separately. I am confused if this protection, usually fused disconnects, would continue to protect subsequent transformers.

Why not just provide primary protection for each Transformer? - 450.3(B)

 

[electrofelon]

Unfortunately there seems to be no allowance in the code to protect subsequent transformers downstream with one device. There is an allowance to do this for conductors, but not for the transformer itself per 450.3b.

You will have pretty low voltage stability with this setup. I have multiple 15 KVA 2400 to 120 / 240 transformers at my house and I see about 4 volts drop through each transformer

 

[HEYDOG]

Any help would be appreciated in understanding how to calculate voltage drop, amps, and over current protection when having multiple transformers, up to 5 , connected in series.
In doing research I am not finding anything that clearly addresses this.

What is the purpose of hooking the transformers in series….didn’t know you could do that. I am aware of in parallel!

 

[synchro]

Unfortunately, yes, there really are multiple transformers in series spec'd by an engineer.

Where my confusion is regarding voltage drop is when your going from say 480 to 120/240 then 240 to 480 and then back again to 120/240. Then yet another transformer to bring the voltage to 550.

I would refer to that arrangement as transformers in cascade, and not in series. "Series" implies that devices are connected within a single closed circuit.
In any case, the only reason I can think of that the engineer would want to do this is to intentionally provide a relatively high source impedance and/or a high level of isolation.

 

[xformer]

Are these 3-pahase transformers?

 

[petersonra]

What is the purpose of hooking the transformers in series….didn’t know you could do that. I am aware of in parallel!

It is not unheard of to have a MV transformer feeding a 480 V transformer feeding a 208 V transformer, feeding some lower voltage transformer, or needing to create a higher voltage from a lower voltage someplace. It is not real hard to get 5 transformers in series.

 

[Opie11]

First and foremost thank you all for your replies.
I have 450.3(b) as a document on my phone for quick reference. Use it often!

I thought about primary protection for each transformer but where they are close together, a lot of times, the secondary of one feeds right into the primary of another. Would it make sense to just protect each?

I am defining series in my situation as such: Either a single phase or 3 phase service that then feeds the first transformer. From there subsequent transformers are fed based on voltage need and distance. Yes, there is typically a load at each voltage but a number of times the load is just another transformer.

A common scenario for us is a 120/240 single phase service. We bump up to 480 go a little distance than bring it back to 120/240. Very common scenario I would think. The first transformer is protected separately on primary and secondary. The 2nd transformer has the secondary side protected.

We use both 3 phase and 1 phase transformers. Typically depends on what kind of service we get.

 

[Besoeker3]

Unfortunately, yes, there really are multiple transformers in series spec'd by an engineer.

But if they really are in series they they would all be the same voltage and in series.

 

[TwoBlocked]

Is the engineer specifying the KVAs? If so, just regard each as a separate transformer. If not, and you need to calculate the KVAs, start with the load on the last one and work your way back. In between each transformer, determine which needs the most protection and provide for it. Am I missing anything Folks?

 

[drktmplr12]

Any help would be appreciated in understanding how to calculate voltage drop, amps, and over current protection when having multiple transformers, up to 5 , connected in series.
In doing research I am not finding anything that clearly addresses this.

what is your objective?

comply with building code voltage drop requirements?

 

[Opie11]

Sometimes we get the KVa's spec'd sometimes we dont. Common sizes we use are 15 and 25KVa. However we do utilize both larger and smaller. Load is almost always spec'd at 40Amps whether true or not.

What's the objective? I have been asking myself that since I started the thread.
I just want to ensure a safe and proper installation where all wiring and OCPD's are properly sized for the equipment we are asked to install/ tie into.

 

[Opie11]

I have to add a couple of questions.

If 2 transformers are within 5 feet of each other and 1 feeds the other how would you treat them as separate? I am confused when a transformer secondary becomes a transformer primary in such a short distance.

Also, are transformers able to be used as a bidirectional device? Is the expectation there that they can be used this way?

 

[Besoeker3]

OK. What do you mean in series? If you have two resistors in series they add up to the sum of the two resistors. Surely that's not what you mean with your five transformers ? They are separate transformers so series doesn't come into the equation.

 

[Opie11]

Apparently I am using the word "series" incorrectly and possibly confusing myself and others. The 1st transformer is fed on the primary side. Cone out of the secondary side and feed the primary side of transformer #2. Come out of the secondary side and feed the primary side of transformer #3. This is basically the application we are dealing with. It is not always 5 transformers this way but always multiple transformers.

My post 11 has an example if it helps any.

 

[texie]

I think the OP is simply referring to step up/step down applications. I would be more concerned about the proper step up Xformer and the proper grounding and bonding. I've seen the RRs do some pretty unsafe wiring and then claim they are exempt from the NEC. Well OK, but that doesn't change physics.

 

[Besoeker3]

Apparently I am using the word "series" incorrectly and possibly confusing myself and others. The 1st transformer is fed on the primary side. Cone out of the secondary side and feed the primary side of transformer #2. Come out of the secondary side and feed the primary side of transformer #3. This is basically the application we are dealing with. It is not always 5 transformers this way but always multiple transformers.

My post 11 has an example if it helps any.

Appreciated, thank you!