Overcurrent Protection for transformer article 450 & 240

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notmyspace

Member
Location
Maryland
Could someone interpret this for me? I am slightly confused here.

I have a site that has an existing 45KVA transformer 600V 3 phase 208Y/120V rated at 29A primary and 83A on the secondary

There is an existing 60A disconnect with (4) #6 and (1) #8gnd connected to the transformer. On the secondary side, we have a 100A breaker panel connected to the transformer with a (4) #1 and (1) #6 gnd. The total load on the secondary side is 63.83A that is calculated with 125%.
Distance transformer to the breaker panel is around 10ft.

Based on the article 450.3(b) and 240, do I cover the overprotection to the transformer and conductor?

I saw article 450, there are two methods

Method 1: Primary protection only ? set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

So, given those requirement, I should have less than 250% primary current protection and slightly less than 125% (around 120.5%) right?

thanks
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Could someone interpret this for me? I am slightly confused here.

I have a site that has an existing 45KVA transformer 600V 3 phase 208Y/120V rated at 29A primary and 83A on the secondary

Your rated currents don't match a 45kVA transformer. It matches a 30kVA transformer.

There is an existing 60A disconnect with (4) #6 and (1) #8gnd connected to the transformer. On the secondary side, we have a 100A breaker panel connected to the transformer with a (4) #1 and (1) #6 gnd. The total load on the secondary side is 63.83A that is calculated with 125%.
Distance transformer to the breaker panel is around 10ft.

Based on the article 450.3(b) and 240, do I cover the overprotection to the transformer and conductor?

I saw article 450, there are two methods

Method 1: Primary protection only – set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

So, given those requirement, I should have less than 250% primary current protection and slightly less than 125% (around 120.5%) right?

thanks

Your transformer is properly protected using the primary & secondary method of 450.3(B). (Per 450.3(B), you could have as high as a 70A OCPD on the primary and a 110A OCPD on the secondary.)

Your conductors are properly protected per Art 240. (I think you mean 3#6 on the primary side.)
 
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notmyspace

Member
Location
Maryland
Your rated currents don't match a 45kVA transformer. It matches a 30kVA transformer.



Your transformer is properly protected using the primary & secondary method of 450.3(B). (Per 450.3(B), you could have as high as a 70A OCPD on the primary and a 110A OCPD on the secondary.)

Your conductors are properly protected per Art 240.

Yes, sorry it was a typo. It was meant to be 30KVA. I was looking at another print instead this. So, based on that, my 60A disconnect on the primary and 100A breaker panel on the secondary is suffice per NEC code?

Thanks again
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Here's a rule to remember...

A wye-configured secondary always requires secondary protection.

A wye-configured secondary does NOT always require TRANSFORMER secondary protection (that is to say that Primary Only transformer protection from 450.3(B) can be used for a delta-wye transformer.) A wye-configured secondary will always require secondary conductor protection.
 

notmyspace

Member
Location
Maryland
Could someone interpret this for me? I am slightly confused here.

I have a site that has an existing 45KVA transformer 600V 3 phase 208Y/120V rated at 29A primary and 83A on the secondary

There is an existing 60A disconnect with (4) #6 and (1) #8gnd connected to the transformer. On the secondary side, we have a 100A breaker panel connected to the transformer with a (4) #1 and (1) #6 gnd. The total load on the secondary side is 63.83A that is calculated with 125%.
Distance transformer to the breaker panel is around 10ft.

Based on the article 450.3(b) and 240, do I cover the overprotection to the transformer and conductor?

I saw article 450, there are two methods

Method 1: Primary protection only ? set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

So, given those requirement, I should have less than 250% primary current protection and slightly less than 125% (around 120.5%) right?

thanks

Your rated currents don't match a 45kVA transformer. It matches a 30kVA transformer.



Your transformer is properly protected using the primary & secondary method of 450.3(B). (Per 450.3(B), you could have as high as a 70A OCPD on the primary and a 110A OCPD on the secondary.)

Your conductors are properly protected per Art 240. (I think you mean 3#6 on the primary side.)

i believed is 4 wires - 3 phases and a neutral
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
i believed is 4 wires - 3 phases and a neutral

You have indicated that the transformer is 600-208/120V. To me this would indicate a Delta-Wye configuration. There will be no place to connect a neutral conductor on the primary (delta) side.

Yes, sorry it was a typo. It was meant to be 30KVA. I was looking at another print instead this. So, based on that, my 60A disconnect on the primary and 100A breaker panel on the secondary is suffice per NEC code?

Yes. (I'm assuming your 60A disconnect on the primary is a 60A overcurrent protective device.)
 

notmyspace

Member
Location
Maryland
You have indicated that the transformer is 600-208/120V. To me this would indicate a Delta-Wye configuration. There will be no place to connect a neutral conductor on the primary (delta) side.



Yes. (I'm assuming your 60A disconnect on the primary is a 60A overcurrent protective device.)

Yes, you are right. For a Delta-WYE transformer - (3 wires + gnd for primary and 4 wires + gnd for secondary)?

and the 60A disconnect would be on my primary side.

thanks
 

Smart $

Esteemed Member
Location
Ohio
A wye-configured secondary always requires secondary protection.
A wye-configured secondary does NOT always require TRANSFORMER secondary protection (that is to say that Primary Only transformer protection from 450.3(B) can be used for a delta-wye transformer.) A wye-configured secondary will always require secondary conductor protection.
Technicality point so noted. :roll:
 

vw55

Member
Location
California
Could someone interpret this for me? I am slightly confused here.

I have a site that has an existing 45KVA transformer 600V 3 phase 208Y/120V rated at 29A primary and 83A on the secondary

There is an existing 60A disconnect with (4) #6 and (1) #8gnd connected to the transformer. On the secondary side, we have a 100A breaker panel connected to the transformer with a (4) #1 and (1) #6 gnd. The total load on the secondary side is 63.83A that is calculated with 125%.
Distance transformer to the breaker panel is around 10ft.

Based on the article 450.3(b) and 240, do I cover the overprotection to the transformer and conductor?

I saw article 450, there are two methods

Method 1: Primary protection only ? set no more than 125% primary current rating, and does not required secondary protection

Method 2: Secondary Protection set no more than 125% provided the primary current protection set less than 250%.

So, given those requirement, I should have less than 250% primary current protection and slightly less than 125% (around 120.5%) right?

thanks

I understand the two methods of transformer protection, but I have concerns when the transformer conductors are oversized to allow Method 2 where the secondary is to be protected by a panel's main circuit breaker. For instance, if a transformer has a primary rated current of 75A, and a secondary rated current of 150A, the contractor installs a 150A breaker in a distribution panel protecting the primary feeder, runs #1/0 primary conductors to the transformer, no secondary overcurrent protection at the transformer, and then runs #3/0 secondary conductors that terminate in a 200A panel with a 200A main circuit breaker, this installation will be compliant with both articles 450 and 240. Although the conductors are protected, is the transformer truly protected?

What happens to the transformer windings, if for whatever reason, the transformer draws 99A on the primary for an infinite amount of time? Neither breaker would trip because primary breaker rating is not exceeded, but the transformer primary is subjected to 99A while only rated at 75A. Would the secondary be self-limiting and only allow 150A to flow during this time or would 198A actually flow?

I understand they are probably over-engineered and will withstand such conditions, but for how long?
 
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