Multiple secondary feeds from Xfmr

[wwhitney]

I know what you mean, but I am just taking issue with the use of the defined term "overload" as it is not used per its definition in these comments.

Ah, thank you. I'll use a different term. Maybe OCPD oversize factor.

Cheers, Wayne

 

[OldBroadcastTech]

I know what you mean, but I am just taking issue with the use of the defined term "overload" as it is not used per its definition in these comments.
There is no overload protection being provided for these conductors other than by the device at the load end of the conductors.

Which was the point of my original question: Why are the conductors to each 100-amp sub-panel required to be sixed for 173 amps if they are protected by a 100 amp breaker at the sub-panel ?

Sorry to be a pain in the arse, but that requirment makes no sense to me

Thank you all for your patience

Gary

 

[wwhitney]

Which was the point of my original question: Why are the conductors to each 100-amp sub-panel required to be sixed for 173 amps if they are protected by a 100 amp breaker at the sub-panel ?

Because if those conductors fault, the downstream OCPD is immaterial. So you need to look at the conductor size in relation to the 225A primary side OCPD. Those conductors effectively have a 900A OCPD protecting them in the case of an L-N fault, given the 4:1 transformer turns ratio. Make the conductors too small, and during a fault they will be damaged before the primary side OCPD can trip.

Cheers, Wayne

 

[jim dungar]

Which was the point of my original question: Why are the conductors to each 100-amp sub-panel required to be sixed for 173 amps if they are protected by a 100 amp breaker at the sub-panel ?

Sorry to be a pain in the arse, but that requirment makes no sense to me

Thank you all for your patience

Gary

See NEC 240.21(C), the conductor size is somewhat dependent on their length. It is hard to have multiple 100A feeders within the 10ft limitation so most people automatically go to the 1/3 rating.

As was mentioned, this all about complying with 240.4 which is about not having unprotected conductors. You really can't protect a conductor at the end of its run, as you need to break the circuit upstream from the fault. Because it is usually physically impossible to not have at least a little conductor upstream from a protective device, 240.21 gives the rules to protect these cases.

 

[OldBroadcastTech]

Because if those conductors fault, the downstream OCPD is immaterial. So you need to look at the conductor size in relation to the 225A primary side OCPD. Those conductors effectively have a 900A OCPD protecting them in the case of an L-N fault, given the 4:1 transformer turns ratio. Make the conductors too small, and during a fault they will be damaged before the primary side OCPD can trip.

Cheers, Wayne

OK

I see how that works now

Thanks to all.

 

[Jpflex]

Where the primary protection is 125% or less, there is no limit on the sum of the ratings of the secondary protective devices as no secondary protection is required.

I believe you are referring to a primary only OCPD but this is only allowed for limited applications such as a two wire single phase with no secondary neutral or a 3 wire delta delta and no neutral

 

[david luchini]

I believe you are referring to a primary only OCPD but this is only allowed for limited applications such as a two wire single phase with no secondary neutral or a 3 wire delta delta and no neutral

Primary Only Protection is allowed for ALL transformer applications per 450.3(B). where the primary ocpd is not more than 125% of the primary rated current (or the next standard size up.)

Even Delta-Wye 4 wire secondary applications.

 

[jim dungar]

Primary Only Protection is allowed for ALL transformer applications per 450.3(B). where the primary ocpd is not more than 125% of the primary rated current (or the next standard size up.)

Even Delta-Wye 4 wire secondary applications.

Very true. As far as 450.3 goes.

 

[Jpflex]

Primary Only Protection is allowed for ALL transformer applications per 450.3(B). where the primary ocpd is not more than 125% of the primary rated current (or the next standard size up.)

Even Delta-Wye 4 wire secondary applications.

So when did this code change?:

240.21 C?

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

 

[Jpflex]

Primary Only Protection is allowed for ALL transformer applications per 450.3(B). where the primary ocpd is not more than 125% of the primary rated current (or the next standard size up.)

Even Delta-Wye 4 wire secondary applications.

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

 

[electrofelon]

So when did this code change?:

240.21 C?

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

You are mixing up transformer protection (450.3) and secondary conductor protection (240.21(c)). They are two completely different things. You need to think about which one you are talking about. Sometimes/Often the same OCPD will meet both requirements.