Sizing a WYE Delta transformer

[Mike Lang]

I'm looking at a job where I need to supply 480 volts for a pump 10 Horsepower and the building has a 120 / 208 3 phase WYE. Can you guys double check my calculations so I know im on the right page.

The new pump operates at 480 volts 3 phase and FLA is 16.1.

To figure out the size of the WYE Delta transformer I did 480 x 16.1 x 1.732 = 13385 VA or 13.3kVA The next available transformer is a 15 kVA.

My primary side OC protection would be 60 Amps. 15000 VA divided by 360(208 x 1.732) = 41.666 Since it's over 9 amps I multiply by 125% = 52.08 Amps next higest size = 60 amp breaker on primary.



I bet someone asks why I don't just order a 120 / 208 3 phase pump at 10 horsepower.

 

[LarryFine]

I bet someone asks why I don't just order a 120 / 208 3 phase pump at 10 horsepower.

Um, Mike, why don't you just order a 10 hp 208v 3ph pump?

Hey, you were right! :grin:


A couple of points:

1. You might need more than a 2kva headroom for starting current. I bet some others will tell us if I'm correct.

2. If the transformer you end up using has a wye primary, do NOT connect the neutral terminal to anything.

 

[benaround]

Mike,

I'll answer first and look second, I'm thinking 13385/360=37.1 x 1.25=46.47 or 50a ocpd.

Reason: 15kva full load amps is 41.66 on the 208v side and a 60a ocpd is 150% of that,

seems kind of high ?

 

[Mike Lang]

Believe me I'm trying to get a 120 / 208 pump. The problem is apparently they don't make a 4" 10 HP well pump in a 120 / 208. The next size in 120 / 208 a 6" 10 HP so I'm trying to get the customer to drill a bigger well and use the 6" pump.

I suppose if I do connect the neutral terminal I'd create some fireworks?

 

[benaround]

You can't put a reducing fitting inbetween ?

 

[Mike Lang]

You can't put a reducing fitting inbetween ?

To couple the 4" to the 6" ? the problem is the customer is drilling a 6" well for the 4" pump, so to install the 6" pump that operates at the 120/ 208 we need a 8" well. I'm guessing it will be a cheaper alternative than installing this transformer.

 

[LarryFine]

The problem is apparently they don't make a 4" 10 HP well pump in a 120 / 208. The next size in 120 / 208 a 6" 10 HP so I'm trying to get the customer to drill a bigger well and use the 6" pump.

That seems weird. HP is HP.

I suppose if I do connect the neutral terminal I'd create some fireworks?

Possibly. I learned that lesson here, and my best friend the EE confirmed it.

 

[jim dungar]

It involves all sorts of other adjustments but the result is, I will often use the next available transformer larger than the formula of 1KVA = 1HP as long as there is about a 20% margin.

So 10HP would mean 10KVA
Next standard size is 15KVA
Check 15KVA is > 12KVA (10 + 20%).