1 mva transformer

Status
Not open for further replies.
B

badgerfan08

Member
Have a question regarding overcurrent protection for a transformer. This is a code question from our class.

1 mVA transformer, 480 v-delta primary and 208/120 wye secondary. I came up with 1202.85 amps for the primary and 2775.8 A for the secondary. If this is correct how do you size the wire based on 310.16 for the secondary. Run 6-1000kc/mil's, that seems crazy to me but that's why I'm asking. How would I size the breaker/fuse for the secondary?
 
How do you figure the secondary at 2500 A. If I'm doing the formula correctly 1mva/208/1.732, that comes to 2775.8.

And if from the load center the secondary (208) goes 250' to a panelboard would you put a main breaker on the panelboard if the load center is already protecting it or would you use a main lug panel?
 
badgerfan08 said:
1 mVA transformer, 480 v-delta primary and 208/120 wye secondary. I came up with 1202.85 amps for the primary and 2775.8 A for the secondary. If this is correct how do you size the wire based on 310.16 for the secondary. Run 6-1000kc/mil's, that seems crazy to me but that's why I'm asking. How would I size the breaker/fuse for the secondary?
Click to expand...

badger

1202 amps is the primary current. What is your primary breaker size?

chris
2500 amp would not allow the FLA of the transformer. Why not go larger?
check 240.21.C
 
Last edited:
badgerfan08 said:
How do you figure the secondary at 2500 A. If I'm doing the formula correctly 1mva/208/1.732, that comes to 2775.8.
Click to expand...
208?360=2778A. You want go go the next size up??? 240.4(C)

And if from the load center the secondary (208) goes 250' to a panelboard would you put a main breaker on the panelboard if the load center is already protecting it or would you use a main lug panel?
Click to expand...
You would be hard pressed not to have secondary OCP on this install. 450.6(B)
 
If I'm looking at the article correctly (450.3A), and I have a supervised location and use fuses I can go 250 % and a size up if the setting does not correspond with a standard rating. So my primary protection would be 3000 Amp fuses. Correct?
 
Or in any location if I use a breaker on the primary it could be 400% if the impedance is from 6-10%. So I could use an 4000 amp breaker on the primary and a 6000 amp breaker on the secondary. Correct?
 
badgerfan08 said:
If I'm looking at the article correctly (450.3A), and I have a supervised location and use fuses I can go 250 % and a size up if the setting does not correspond with a standard rating. So my primary protection would be 3000 Amp fuses. Correct?
Click to expand...

Table 4503A is for transformers over 600 volt. Look at table 450.3B for your settings.
 
oops i see. 250% on primary and 125% on secondary. so primary could be up to 3000 amp breaker and secondary could be also 3000 amps. Correct?
 
badgerfan08 said:
If I'm looking at the article correctly (450.3A), and I have a supervised location and use fuses I can go 250 % and a size up if the setting does not correspond with a standard rating. So my primary protection would be 3000 Amp fuses. Correct?
Click to expand...
You need to be checking 450.3B and not 3A. Look at the heading of the tables. Look at the primary and secondary protection.
Normally the transformer secondary is covered under 240.21.C. However since
you run exceeds all of the tap rules, you can not use this section.
Where should the OC protection be located? You should also check the voltage drop.
 
badgerfan08 said:
oops i see. 250% on primary and 125% on secondary. so primary could be up to 3000 amp breaker and secondary could be also 3000 amps. Correct?
Click to expand...

Thats right if you need that much primary ocp.
 
badgerfan08 said:
oops i see. 250% on primary and 125% on secondary. so primary could be up to 3000 amp breaker and secondary could be also 3000 amps. Correct?
Click to expand...

The secondary protection would be on a load center on the inside of the building. Whats the easiest way to check voltage drop. 250'
 
bob said:
chris
2500 amp would not allow the FLA of the transformer. Why not go larger?
check 240.21.C
Click to expand...
Doesn't this say what I said. What am I missing?

240.21 Location in Circuit.
(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
Click to expand...
chris kennedy said:
You want go go the next size up??? 240.4(C)
Click to expand...
 
badgerfan08 said:
what do you mean if i need that much primary ocp? thats the max right?
Click to expand...

The minimum required is 1500 amps, 3000 amps would be the max. the higher the primary the more conductors required as well.
 
badgerfan08 said:
so you would go witht the 1500 amp?
Click to expand...

I would but i am not an engineer. It would depend on the type of load you have on the secondary. If there are large motors or electric furnaces that take high starting current, then go with higher primary ocp.
 
so on my secondary 208 volts, 3000 amp breaker 250' from load center to panel board, how would i calculate voltage drop and wire size properly?
 
badgerfan08 said:
so on my secondary 208 volts, 3000 amp breaker 250' from load center to panel board, how would i calculate voltage drop and wire size properly?
Click to expand...

Here you would have to know the actual load. (I)
 
Status
Not open for further replies.
Top