480 step down

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Dbronx

Senior Member
Location
Colorado
I read the Mike Holt forum post ?Primary protection of xfmr? I want to step down 480 volts to 208 volts single phase. I need 23 per leg amps on secondary, the amp draw on the primary side is 10.4 amps table 450.3(B) states 125% for primary OP and 250% when both primary & secondary OP devices are used if transformer has primary thermal overload. I have not received the equipment, not sure if thermal protection is installed. I want to protect the transformer from overload by feeding with a 15 amp OP primary and a 20 amp for secondary OP work if no thermal protection is installed. Can any confirm I am on the right track Thanks?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Bob is, of course, correct... With that in mind I will foolishly take a stab at your question........:D
Remember Art 450, is for protection of the transformer.You also need to address Art 240 for conductor protection.
The currents you state indicate a transformer somewhere around a 5 kw, but, as stated, without knowing the actual size of the transformer, you can undersize by not considering inrush, or of course oversize.
Off the top of my head, based on the numbers you supply, the 15 amp primary protection is probably in the neighborhood, but a 20 amp OCP on a secondary rated at 23 amps is most likely going to be a problem.
Side note: 480 to 208 single-phase is a bit unusual in a transformer in my experience... you might recheck.
 

Dbronx

Senior Member
Location
Colorado
Bob is, of course, correct... With that in mind I will foolishly take a stab at your question........:D
Remember Art 450, is for protection of the transformer.You also need to address Art 240 for conductor protection.
The currents you state indicate a transformer somewhere around a 5 kw, but, as stated, without knowing the actual size of the transformer, you can undersize by not considering inrush, or of course oversize.
Off the top of my head, based on the numbers you supply, the 15 amp primary protection is probably in the neighborhood, but a 20 amp OCP on a secondary rated at 23 amps is most likely going to be a problem.
Side note: 480 to 208 single-phase is a bit unusual in a transformer in my experience... you might recheck.

The transformer is 5k
 

Dbronx

Senior Member
Location
Colorado
Bob is, of course, correct... With that in mind I will foolishly take a stab at your question........:D
Remember Art 450, is for protection of the transformer.You also need to address Art 240 for conductor protection.
The currents you state indicate a transformer somewhere around a 5 kw, but, as stated, without knowing the actual size of the transformer, you can undersize by not considering inrush, or of course oversize.
Off the top of my head, based on the numbers you supply, the 15 amp primary protection is probably in the neighborhood, but a 20 amp OCP on a secondary rated at 23 amps is most likely going to be a problem.
Side note: 480 to 208 single-phase is a bit unusual in a transformer in my experience... you might recheck.

The load is actually around 14 amps on one leg for a three 10X20 storage containers 6.9 amp AC unit, lights and one outlet. Trying to supply electrical at the lowest cost. A 480/277 volt 600 amp I line panel is 15 feet away, the 208 volt power is about 350 feet away. A 480 step down with single phase panel costs around 2k, no underground comms and utiilties to worry about
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
5 kva... 15 primary should be o.k.
I think you will find the transformer will be a 240/120 secondary.
20 amp secondary should be fine.
Dont forget to address Art 250.30
 

Dbronx

Senior Member
Location
Colorado
The load is actually around 14 amps on one leg for a three 10X20 storage containers 6.9 amp AC unit, lights and one outlet. Trying to supply electrical at the lowest cost. A 480/277 volt 600 amp I line panel is 15 feet away, the 208 volt power is about 350 feet away. A 480 step down with single phase panel costs around 2k, no underground comms and utiilties to worry about

After step down transformer the first means of disconnect will be a sub panel. I would install a equipment bond jumper from transformer cabnit to sub panel enclousure. A system bond jumper and Main bond jumper will be installed in sub panel. Also ground electrode to steel container wall, and then to ground rod.
There is one more question the transformer cabnit will have to be bonded to the steele container wall with same size conductor as equipment bond jumper?
 

kwired

Electron manager
Location
NE Nebraska
You really cannot select overcurrent devices and conductor sizes without knowing what size transformer will be used.

Yes you can - kind of.

You can protect the transformer at levels less than required in NEC. Now if you select conductors and protection and find out it is larger than allowed for the transformer there is a very good chance you don't have a large enough transformer for the need.

I have used oversized transformers for temporary power situations and protected them at lower levels than the transformer rating. I recall feeding a 15 KVA 480x120/240 with 10 AWG and 30 amp fuses and having no problem. We were only powering 2- 20 amp 120 volt receptacles with it and it was temporary for construction purposes - The 15 KVA was used simply because it was available and cost nothing. I even considered feeding it with 12 and a 20 amp fuse but decided not to take the chance that it may not hold when energizing. It was short run anyway.
 

Dbronx

Senior Member
Location
Colorado
Yes you can - kind of.

You can protect the transformer at levels less than required in NEC. Now if you select conductors and protection and find out it is larger than allowed for the transformer there is a very good chance you don't have a large enough transformer for the need.

I have used oversized transformers for temporary power situations and protected them at lower levels than the transformer rating. I recall feeding a 15 KVA 480x120/240 with 10 AWG and 30 amp fuses and having no problem. We were only powering 2- 20 amp 120 volt receptacles with it and it was temporary for construction purposes - The 15 KVA was used simply because it was available and cost nothing. I even considered feeding it with 12 and a 20 amp fuse but decided not to take the chance that it may not hold when energizing. It was short run anyway.

Thanks you have a good point heat is the enemy. The transformer is about 64% load, but not temporary instillation. The cycling of air con units cause harmonic problems, over sizing of transformer and dedicated neutrals may help
 

mbeatty

Senior Member
Location
Illinois
Thanks you have a good point heat is the enemy. The transformer is about 64% load, but not temporary instillation. The cycling of air con units cause harmonic problems, over sizing of transformer and dedicated neutrals may help

I agree. As kwired stated, using an oversize transformer with appropriate protection is a good way to go. :happyyes:
 
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