Transformer sizing

Fred B

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Customer has 208/120Y coming into facility at 800A. He states he is getting a bunch of equipment he says "requires" 240V single phase. Can that even be derived from the 208Y? Or do I go to POCO and ask for a secondary service be provided?
 
If the 240 volt loads are all line to line loads, I would look at 208 to 240, 4 wire high leg transformer.
As far as the sizing of the transformer, you will need to know what the loads are.
 
You are going to need to get more detail on what exactly is required.

In particular do the loads _require_ 120/240V 'single phase (like a residential service)? Or are they 240V L-L loads which don't care about L-N or L-G voltage?

How many kVA of loading will be added?

How divided up can the loads be?

Jon
 
winnie said:
You are going to need to get more detail on what exactly is required.
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Kind of what I thought. Tried find out, but I think I need to find who at the company really knows what is going on, not just the guy who writes the checks. What I do know there are about 24 individual motor loads that I haven't been given details of other that hinted they are 30A 240V single phase. There also control and heating element loads associated with the equipment. Also he indicated they will not be all operating concurrently, but won't commit to how many will. Would think all these factors also come into play to calculate total loads.
But again don't really know what the owner means by just stating 240V single phase. All I've gotten after asking a couple of time was that answer. Is there a different way to ask the question?
 
IMHO, ask for a data sheet or wiring diagram for the equipment.

Ok, you know that the new load is potentially a very large portion of the existing service, but you don't know how much because you don't know how many will operate concurrently. (800 * 208 * 1.732 = 288kVA service, 24 * 30 * 240 = 173 kVA of total new circuits, which is more than the total new load but you don't know how much more.)

Still you probably want to balance the loading across all 3 phases.

The key thing you need to know is if the loads are all true 240V L-L loads, or if there is any 120V L-N loads. Control boards are often 120V, but it is very common for industrial machinery to have an internal transformer to provide that 120V. If you have 120V L-N loads, your transformer choices are tightly constrained. If your loads are pure 240V with no neutral reference needed, then you have lots of choices.

In particular, if you have no L-N loads and no L-G references, you can use buck-boost transformers which are much cheaper.

-Jon
 
LarryFine said:
I would start by seeing whether the loads are compatible with 208v.
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My thoughts exactly. The equipment is not typical "off the shelf" and should be able to be designed and mfg. to the specs of the "in house" system, I would think.

So I'm thinking I need the following answered.

Name plate info from the motors.
Induction heating label info.
Any control circuitry requirements.
Duration of the run times of each portion of operation.
How many units will be running concurrently. and each or any operation overlap.
Additionally info related to L-L and L-N loads.

Anything else I'm forgetting?

Then can start thinking about the transformer needed to fit the equipment.
 
I would figure out the price for installing a 3 phase 208 to 240 volt transformer to include the price of a three phase panel, circuit breakers and wire / conduit runs and price for installing a buck boost transformer at each piece of equipment.
 
garbo said:
I would figure out the price for installing a 3 phase 208 to 240 volt transformer to include the price of a three phase panel, circuit breakers and wire / conduit runs and price for installing a buck boost transformer at each piece of equipment.
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One other option is to have a single buck boost bank serve a dedicated 138/240 panel board for the 240 loads (if they are all L-L)
 
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