120V/240V systems

[adamscb]

All,

I had a question regarding the 120v/240v split phase system.

When calculating the maximum current on the secondary side of a 480v primary, 120v/240v secondary transformer, which secondary voltage do you use in your calculations? i.e. we have a lot of 50kVA transformers with these voltage ratings, so is the max secondary current 208 or 416?

 

[sparkyrick]

Calculations always use the higher voltage.

 

[GoldDigger]

All,

I had a question regarding the 120v/240v split phase system.

When calculating the maximum current on the secondary side of a 480v primary, 120v/240v secondary transformer, which secondary voltage do you use in your calculations? i.e. we have a lot of 50kVA transformers with these voltage ratings, so is the max secondary current 208 or 416?

The maximum current is the current that the wires of the secondary winding can take without overheating the transformer. The maximum power will be routed through the transformer when you draw that current line to line from the full 240V of the secondary. But you are free to instead multiply the maximum current times 120V instead and then multiply by two since you have two half windings and two sets of 120V loads.

The result comes out just the same either way.

It gets a little more complicated when you get to three phase, and I will not try to go there.

 

[Iron_Ben]

All,

I had a question regarding the 120v/240v split phase system.

When calculating the maximum current on the secondary side of a 480v primary, 120v/240v secondary transformer, which secondary voltage do you use in your calculations? i.e. we have a lot of 50kVA transformers with these voltage ratings, so is the max secondary current 208 or 416?

It's both. The 240 volt transformer winding is center tapped - split in half, you might say. Each half has 120 volts across it and can carry 208 amps. So if all your loads were 240 volts, you could supply 208 amps at 240 volts, roughly 50 kva. If all loads were 120 volts,and *perfectly balanced*, you would have 208 amps at 120 volts on both halves of the winding, again 50 kva.

Of course in the real world you will usually have a mix of 120 and 240 volt loads in your panel. If your 120 volt loads aren't balanced, or reasonably close, you will have a problem. You could end up with say, 28 kva on one leg (winding half) and 20 kva on the other leg (winding half). 48 kva on your 50 kva transformer looks okay on the surface. But since each half of the winding is rated for 25 kva (208 amps at 120 volts), one of them is overloaded. 28/25 = 112%.

 

[topgone]

It's both. The 240 volt transformer winding is center tapped - split in half, you might say. Each half has 120 volts across it and can carry 208 amps. So if all your loads were 240 volts, you could supply 208 amps at 240 volts, roughly 50 kva. If all loads were 120 volts,and *perfectly balanced*, you would have 208 amps at 120 volts on both halves of the winding, again 50 kva.

Of course in the real world you will usually have a mix of 120 and 240 volt loads in your panel. If your 120 volt loads aren't balanced, or reasonably close, you will have a problem. You could end up with say, 28 kva on one leg (winding half) and 20 kva on the other leg (winding half). 48 kva on your 50 kva transformer looks okay on the surface. But since each half of the winding is rated for 25 kva (208 amps at 120 volts), one of them is overloaded. 28/25 = 112%.

I guess the OP is asking what secondary current will he be using (protection setting or CB selection). Transformers with center taps are actually designed with its secondary winding wires sized on the full voltage amps. In his case, that 50 kVA, 120/240V secondary has windings rated for 50,000/240 = 208 amperes.

 

[adamscb]

So let's say I have 400A fuses on the secondary of this 50kVA, 480:120/240 transformer. Are these fuses too large based on NEC transformer protection criteria?

 

[augie47]

So let's say I have 400A fuses on the secondary of this 50kVA, 480:120/240 transformer. Are these fuses too large based on NEC transformer protection criteria?

Actually it would depend on the primary protection. As far as the transformer is concerned it needs to be protected per Article 450.3 which can be accomplished by properly sized overcurrent protection on the primary or secondary. In your situation with a 50kva transformer the 480 primary would be 104 amps so using any primary OCP 150 amps or less would protect the transformer and there would be no limit on secondary protection (as far as transformer protection).
Likewise with a 300 amp secondary OCP device, you could protect the primary by a device no greater than 250 amps and the transformer would be protected.
FWIW. Locally, on a 50kva single-phase transformer I normally find a 225 amp secondary panel. That may be based more on panel economics.

 

[adamscb]

The primary protection is 100A, and like I said the fuses on the secondary are 400A. I'm suggesting using 250A fuses instead.

 

[gadfly56]

The primary protection is 100A, and like I said the fuses on the secondary are 400A. I'm suggesting using 250A fuses instead.

What would be the point? As far as coordination, the primary side will always blow first unless you do some very tricky coordination setup.

 

[augie47]

Keep in mind, if you are feeding a panelboard, 408.36 requires a panel have overcurrent protection not greater than the rating of the panelboard *

*(There is an exception covering two wire-two wire and delta delta transformers)

 

[topgone]

What would be the point? As far as coordination, the primary side will always blow first unless you do some very tricky coordination setup.

IMO, it's useless coordinating the primary breaker with the secondary main breaker, they serve the same load! Losing either side leaves the load in the dark!
@adamscb,
If you already have a primary protection that's very restrictive (100A vs 104 FLA), what gadfly said is true. The primary protection will blow ahead of your proposed 250A protective fuse at the secondary! Having an overcurrent protective device rated 125% of primary FLA is even enough to protect your transformer without having a fuse/breaker at the secondary (Table 450.3(B)).

 

[kwired]

Regardless of what code may allow, if you put more then 25 kVA but less then 50 kVA of 120 volt load on just one side of the secondary you will overload that half of the secondary. If you have 400 amps OCPD on secondary it will not trip. If you have 200 amp OCPD it will. If you have 225 or 250, you can have some overloading but are much more limited then what the 400 amp device would allow.

Primary still sees full kVA drawn whether it is in one or both halves of the secondary.