Disconnects

[rustyryan34]

I am running power for 2 trailers, each has 100 amp panel, I am bring one 480 feed to a tranformer for each trailer. I know I need a disconnect on each transformer for the secondary, But can i use one disconnect for both transformers for the primary coming in? Can I bring the power into the disconnet then run it to the junction box and tap off of it to both transformers. They are only 20 feet apart

 

[augie47]

You need to keep Art 450 & Art 240 in mind. If you have 125% secondary OCP on each transformer then your primary OCP can be 250% per Art 450, however, per Art. 240, your conductor would have to be sized per your OCP all the way to each transformer. You could only reduce your tap if you add overcurrent protection.
If you are going from 480 to 240/120 you will need secondary protection anyway as your primary OCP cannot serve as secondary in this arrangement.

 

[rustyryan34]

My disconnects are fused so i can put in whatever fuse size i need i was just wondering if i need a disconnect on each transformer on the primary side? It would be alot cheaper if i could just use one disconnect for this they are within 20 feet, can i do that?

 

[augie47]

I'll try again. If you have secondary protection on each trasformer sized at 125% then you could serve both transfomers frm the same primary overcurretn device provided it is rated at not over 250% of one transformer primary curreent and your wiring to each transformer is rated at the 250%.
Assuming your tansformers are 25 kw, with a primary current of 52 amps, you cold connect both transformers to a circuit rated up to 250 amps using 250 kcmil condcutors. Whether that is the most economical way for you to go (vs an individual 70 +/- amp branch circut) is another matter.

 

[RUWired]

Assuming your tansformers are 25 kw, with a primary current of 52 amps, you cold connect both transformers to a circuit rated up to 250 amps using 250 kcmil condcutors. Whether that is the most economical way for you to go (vs an individual 70 +/- amp branch circut) is another matter.

Augie, are you sure about that. If you used a 250 amp breaker, primary protection would be over 250% for any of the two. Seems like 250% would be the max for any of the two. In the 25kw single phase example, 52 x 250%= 130 amps max.OCPD =125 amps w/ either # 1awg or #2awg.

 

[don_resqcapt19]

Assuming that this is an outside installation, you can eliminate the disconnects and overcurrent protection for the secondary at the transformer and use the disconnect at the trailers for this purpose. 240.21(C)(4)
Each transformer must have protection on the primary side per the rules in 450.3. If you can size one OCPD to be less than 250% of the primary current on the smallest transformer and still have the OCPD hold on the inrush, you could use a single device, that is assuming that the secondary protection does not exceed 125% of the secondary current.

 

[augie47]

Augie, are you sure about that. If you used a 250 amp breaker, primary protection would be over 250% for any of the two. Seems like 250% would be the max for any of the two. In the 25kw single phase example, 52 x 250%= 130 amps max.OCPD =125 amps w/ either # 1awg or #2awg.

You are correct, my friend. My brain went down the wrong track and I was looking at the 240 volt current.