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Old 10-22-2009, 10:13 PM   #1
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transformer questions


Hi guys,

I need to install a single phase transformer to drop my 480 volts to 240 volts for a 120/240 volt panel. I'm looking at a 10 kVA transformer (I don't have a large load to be served). Based on my calculations it should supply approx 41 amps at 240 volts. I would prefer a breaker ahead of the transformer and one in the panel. What size breakers should I use. I was thinking a 20 amp breaker in my 480 panel to supply my transformer and a 40 amp breaker in my 120/240 panel. Would this be correct?

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Old 10-22-2009, 10:16 PM   #2
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A 2:1 current ratio sounds right but you may want different trip curves.

The short circuit current at my panel with the pole 'former just outside is 11 kA. The current available in your house downstream of your 'former will be less, and I guess this will cause your lights to dim when heavy loads are applied.
With a light load your voltage may get quite high.

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Last edited by Yoyizit; 10-22-2009 at 10:21 PM.
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Old 10-22-2009, 10:27 PM   #3
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Hi guys,

I need to install a single phase transformer to drop my 480 volts to 240 volts for a 120/240 volt panel. I'm looking at a 10 kVA transformer (I don't have a large load to be served). Based on my calculations it should supply approx 41 amps at 240 volts. I would prefer a breaker ahead of the transformer and one in the panel. What size breakers should I use. I was thinking a 20 amp breaker in my 480 panel to supply my transformer and a 40 amp breaker in my 120/240 panel. Would this be correct?

I will do it little diffrent and on primary you will need 30 amp breaker { that set for 175% OL } the 20 amp breaker may not hold espcally when you engerized the transfomer due inrush current it may trip the breaker and on secondary side you will need 40 amp breaker { this is the max value you can safely run the transfomer in stock format sans overload on it }

H1-H2 is most common primary connection however let me give you a head up some transfomer may have dual voltage connections it will get ya so it will have H1 - H2 - H3 - H4 if this transformer is 240/480 primary side so what you will have to hook up H1 as Line one tie the H2 and H3 together and H4 will be Line 2 { this is typical connection but WARNING please verify the nameplate for correct sequine }

The secondary side it will work in three ways straght 120 volts , 120/240 or straght 240volts so here what you will do here it will say X1-X0-X2 this is typical the X1 is line one on secondary side and X0 is netural and X2 is line two on secondary side.

make sure you have proper bonding on this set up.

Merci.
Marc
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Old 10-22-2009, 10:33 PM   #4
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Thanks guys. Marc - if you energize the transformer with no load connected it would trip a 20 amp breaker?
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Old 10-23-2009, 12:49 AM   #5
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Thanks guys. Marc - if you energize the transformer with no load connected it would trip a 20 amp breaker?
On primary side slim chance it may trip It did happned to me once a while when I engerzied larger 75KVA 3 480D 208Y120 volt transfomer I have 125 amp breaker for it on primary side and secondary side it is max is 200 amp but I know it called for 90 amps but I can safely add larger breaker as long I have OCPD on secondary side.

{ the 75 KVA transfomer is not very big to me due I work alot of commercal installment area and they use far much bigger units }

Merci.
Marc
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Old 10-23-2009, 01:09 AM   #6
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Most of the 10 KVA single phase transformers I've installed had either a 25 or 30 on the 480 side, and a 50 on the 120/240 side.

The code requires the secondary of this transformer to protected at not more than 125% of its rating. 125% of 42 = 52. The next standard size up is allowed, so 60 would be the maximum allowed. 50 is more common.

The primary can be protected at 250%. 250% of 21 = 52. The primary protection is not allowed to be upsized from a standard rating, so a 50 would be the maximum. 25 or 30 is more common.

The reason for the huge breaker allowed on the primary is because the secondary breaker protects the transformer from overloads, while the primary breaker protects the rest of the system if the transformer shorts out internally.

Too small of a breaker on the primary side might see the inrush current as a fault, and trip. Even with no load on the secondary.

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Old 10-23-2009, 06:09 PM   #7
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So a 40 amp would be okay, it just can't be more than 125% of the rating of the trans?
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Old 10-23-2009, 07:52 PM   #8
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A 40 would be OK.

The maximum is 125%, there is no minimum.

Like Marc stated above, keep the 480 breaker pretty good sized. Transformers have a huge inrush for the first full cycle, too small of a breaker could trip.

When I get a 10 KVA single phase transformer/panel combo, the factory installed 480 breaker will very likely be a 30, and the 240 breaker will be a 50.

Rob
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Old 10-25-2009, 09:39 PM   #9
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Thanks guys, that helped alot!

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