Transformer primary and secondary OCPD, feeder sizing

[shortcircuit1]

All,

I am trying to confirm my calcs for transformers. See below

225kVA, 480 delta-208/120 wye
Primary Amps=271A
Based on 450.3(b) Primary OCPD=271*1.5=406A, I can use a 450A breaker but i would prefer to stick with a 400A breaker (Reason for 1.5 was to avoid the nuisance tripping of the breaker due to inrush)
Primary wire size=2 sets of 3#3/0 CU

Secondary Amps=623A
Secondary OCPD=623*1.25=779A. Next size rule doesnt apply to taps. So 700A OCPD
Secondary wire size=2 sets of 4#500kcmil CU

Do these calcs look good?

Few questions I have are:

Do i need to size the primary wire based on primary OCPD or primary FLA?
Can i up size my primary breaker to next standard size i.e 60A if there is no standard size available?
Do i need to size the secondary wire based on secondary OCPD or secondary FLA?
Can i up size my secondary breaker to next standard size? I remember that up sizing doesn't apply to taps.

 

[david luchini]

All,

I am trying to confirm my calcs for transformers. See below

225kVA, 480 delta-208/120 wye
Primary Amps=271A
Based on 450.3(b) Primary OCPD=271*1.5=406A, I can use a 450A breaker but i would prefer to stick with a 400A breaker (Reason for 1.5 was to avoid the nuisance tripping of the breaker due to inrush)
Primary wire size=2 sets of 3#3/0 CU

Secondary Amps=623A
Secondary OCPD=623*1.25=779A. Next size rule doesnt apply to taps. So 700A OCPD
Secondary wire size=2 sets of 4#500kcmil CU

Do these calcs look good?

Few questions I have are:

Do i need to size the primary wire based on primary OCPD or primary FLA?
Can i up size my primary breaker to next standard size i.e 60A if there is no standard size available?
Do i need to size the secondary wire based on secondary OCPD or secondary FLA?
Can i up size my secondary breaker to next standard size? I remember that up sizing doesn't apply to taps.

Your secondary breaker can be 800A. Secondary conductors would need to be 2 sets of 600kcmil for 800A c/b. (or 3 sets of 300kcmil, etc.)

Primary conductors need to be sized based on primary OCPD.

Primary breaker greater than 125% of FLA cannot round up. The largest primary c/b would be 600A.

Secondary wire size need to be based on secondary OCP...there is no rounding up for the ampacity.

The secondary breaker can size to the next standard size up...800A.

 

[Ainsley Whyte]

Your secondary breaker can be 800A. Secondary conductors would need to be 2 sets of 600kcmil for 800A c/b. (or 3 sets of 300kcmil, etc.)

Primary conductors need to be sized based on primary OCPD.

Primary breaker greater than 125% of FLA cannot round up. The largest primary c/b would be 600A.

Secondary wire size need to be based on secondary OCP...there is no rounding up for the ampacity.

The secondary breaker can size to the next standard size up...800A.

agree

 

[shortcircuit1]

Your secondary breaker can be 800A. Secondary conductors would need to be 2 sets of 600kcmil for 800A c/b. (or 3 sets of 300kcmil, etc.)

Primary conductors need to be sized based on primary OCPD.

Primary breaker greater than 125% of FLA cannot round up. The largest primary c/b would be 600A.

Secondary wire size need to be based on secondary OCP...there is no rounding up for the ampacity.

The secondary breaker can size to the next standard size up...800A.

So primary conductors based on 1.5*271=406A which means i can not go next size up? I can only use 400A and my wire size would be 2 sets of 3/0 AWG CU?

In short I can not go to next standard size for primary OCPD. For example if my primary FLA after multiplying with 1.5A comes to a non standard size then I need to go to next standard size down. And for secondary OCPD i can go next size up and then size my conductors based on OCPD?

 

[david luchini]

So primary conductors based on 1.5*271=406A which means i can not go next size up? I can only use 400A and my wire size would be 2 sets of 3/0 AWG CU?

In short I can not go to next standard size for primary OCPD. For example if my primary FLA after multiplying with 1.5A comes to a non standard size then I need to go to next standard size down. And for secondary OCPD i can go next size up and then size my conductors based on OCPD?

The primary can be as large as 250%, not 150%. So 600A would be the maximum. As for the rest, you have that correct.

 

[Carultch]

All,

I am trying to confirm my calcs for transformers. See below

225kVA, 480 delta-208/120 wye
Primary Amps=271A
Based on 450.3(b) Primary OCPD=271*1.5=406A, I can use a 450A breaker but i would prefer to stick with a 400A breaker (Reason for 1.5 was to avoid the nuisance tripping of the breaker due to inrush)
Primary wire size=2 sets of 3#3/0 CU

Secondary Amps=623A
Secondary OCPD=623*1.25=779A. Next size rule doesnt apply to taps. So 700A OCPD
Secondary wire size=2 sets of 4#500kcmil CU

Do these calcs look good?

Few questions I have are:

Do i need to size the primary wire based on primary OCPD or primary FLA?
Can i up size my primary breaker to next standard size i.e 60A if there is no standard size available?
Do i need to size the secondary wire based on secondary OCPD or secondary FLA?
Can i up size my secondary breaker to next standard size? I remember that up sizing doesn't apply to taps.

Technically, your secondary conductors are not really taps. They are governed by a series of rules in 240.21(C), which are each analogous to each of the tap rules in 240.21(B), so it is an understandable reason you might call them that. The secondary conductor extends from the terminals of the transformer to the first OCPD of each secondary connected circuit. Once it lands on the first OCPD after the transformer, the circuit on the load side of that OCPD is now a feeder.

The primary can be based on the full load amperes, and where applicable, the next-size-up rule is allowed to be applied. It is treated as if it were any other load off of the source that feeds it. The secondary by contrast, specifically requires at least as much ampacity as the secondary OCPD. Using an 800A OCPD on the secondary, gotta use 800A of wire. Even if the load on the load side of that OCPD would only require 760A of wire, and be allowed to "round up" to 800A per 240.4(B). It's a little non-intuitive that you'd have to have 2 parallel 600 kcmil's on the secondary conductors, but you get to have only 2 parallel 500 kcmil's from your secondary OCPD onward, should the load require no more than 760A.

Some transformer topologies allow you to protect the secondary conductors by the primary OCPD (after scaling it by the voltage ratio to apply it to the secondary), due to the fact that fault currents are guaranteed to line up directly across the winding pairs. I like to call this "protect by proxy". This only applies specifically to 2-wire:2-wire single phase, and 3-wire:3-wire delta:delta systems specifically. Any time you have a wye system on either side or a system with a center tapped winding, forget it. A secondary fault could go "unnoticed" by a primary OCPD if there is a wye system on either side, which is why 350A primary OCPD on the 480V side doesn't automatically become 808A worth of secondary wire protection on the 208V side. It only does this on certain topologies of the transformer. Where it doesn't apply, which is most of the time in my experience, secondary protection is needed the instant you terminate the transformer secondary conductors in the first device after the transformer.

 

[shortcircuit1]

The primary can be as large as 250%, not 150%. So 600A would be the maximum. As for the rest, you have that correct.

Isn't 2.5x271=677.5A which means maximum breaker size allowed would be 700A?

Also, can you please point to me code reference where it says "Primary breaker greater than 125% of FLA cannot round up"

 

[david luchini]

Isn't 2.5x271=677.5A which means maximum breaker size allowed would be 700A?

You can't go up to the next standard size.

Also, can you please point to me code reference where it says "Primary breaker greater than 125% of FLA cannot round up"

See Table 450.3(B)

 

[Carultch]

Also, can you please point to me code reference where it says "Primary breaker greater than 125% of FLA cannot round up"

That applies when there isn't secondary protection. I.e. when your primary OCPD indirectly protects conductors on the secondary as the first part of 240.21(C) allows. FLA in this context, means the amps associated with the kVA of the transformer, and not necessarily the full load amperes of the load. When there is secondary protection, 250% of the amps associated with the transformer kVA becomes your limit on primary protection.

To walk through an example, consider a 10 kVA, 480V : 240V single phase transformer, that has 2 wire grids on both sides. The amps at 480V associated with 10 kVA is 20.83A. The maximum OCPD you therefore could use on the primary side, presuming none on the secondary side, would be 26.04A in theory, which in practice is 25A. The 25A OCPD on the primary, effectively protects the secondary at 50A, given a topology that allows indirect overcurrent protection of the secondary. The maximum continuous load allowable on the secondary would therefore be 40A, or 9.6kVA. That probably allows for most load amounts that would tell you to plan a 10kVA transformer.

What if I have a secondary load in the margin between 9.6 and 10 kVA continuous? Then you'd have a situation where you might need protection on both sides, even though it isn't the transformer topology requiring it. Because without secondary OCPD, you a 30A OCPD on the primary exceeds the limit of Table 450.3(B). Put in a secondary 60A OCPD, and now you can have 30A OCPD on the primary, to take care of loads this close to the margin of the transformer rating. You could have as high as 250% of 20.83A for the OCPD rating on the primary, which would be a 50A OCPD in practice.

 

[david luchini]

That applies when there isn't secondary protection. I.e. when your primary OCPD indirectly protects conductors on the secondary as the first part of 240.21(C) allows. FLA in this context, means the amps associated with the kVA of the transformer, and not necessarily the full load amperes of the load. When there is secondary protection, 250% of the amps associated with the transformer kVA becomes your limit on primary protection.

The "primary only" protection can apply to any transformer, not just one where the primary OCPD is protecting the secondary conductor

To walk through an example, consider a 10 kVA, 480V : 240V single phase transformer, that has 2 wire grids on both sides. The amps at 480V associated with 10 kVA is 20.83A. The maximum OCPD you therefore could use on the primary side, presuming none on the secondary side, would be 26.04A in theory, which in practice is 25A.

With primary only protection, the primary OCPD could be 30A.

 

[Carultch]

With primary only protection, the primary OCPD could be 30A.

How do you reconcile that with the 125% limit in Table 450.3(B)?

 

[david luchini]

How do you reconcile that with the 125% limit in Table 450.3(B)?

Table 450.3(B) says that when 125% does not correspond to a standard OCPD size, the next standard size up can be used. In your example, that would be 30A.

 

[Carultch]

Table 450.3(B) says that when 125% does not correspond to a standard OCPD size, the next standard size up can be used. In your example, that would be 30A.

So to confirm: the next standard size allowance applies to the 125% rule in the table for primary protection, but it does not apply to the 250% rule? Do I understand this correctly?

 

[david luchini]

So to confirm: the next standard size allowance applies to the 125% rule in the table for primary protection, but it does not apply to the 250% rule? Do I understand this correctly?

That is correct.

 

[kwired]

So to confirm: the next standard size allowance applies to the 125% rule in the table for primary protection, but it does not apply to the 250% rule? Do I understand this correctly?

see Note 1 and look at which scenarios reference it.

 

[shortcircuit1]

Thanks, David. I came across an article on the ECM website written by Mr.Mike Holt. In that article, he mentions that the neutral conductor on the secondary of a three-phase four-wire system counts as a current-carrying conductor. I can understand this scenario if you have non-linear loads fed from the transformer. One other interesting thing I read was "The neutral conductor of a 3-wire circuit from a 4-wire 3-phase 120/208V or 277V/480V wye-connected system is considered a current-carrying conductor".Generally, you would run 4 wires from the transformer to the panelboard. Is that scenario talking about where you feed a 120/208 single phase 3 wire panel from the secondary of the transformer?

 

[kwired]

Thanks, David. I came across an article on the ECM website written by Mr.Mike Holt. In that article, he mentions that the neutral conductor on the secondary of a three-phase four-wire system counts as a current-carrying conductor. I can understand this scenario if you have non-linear loads fed from the transformer. One other interesting thing I read was "The neutral conductor of a 3-wire circuit from a 4-wire 3-phase 120/208V or 277V/480V wye-connected system is considered a current-carrying conductor".Generally, you would run 4 wires from the transformer to the panelboard. Is that scenario talking about where you feed a 120/208 single phase 3 wire panel from the secondary of the transformer?

I don't know if what you read was wrong or if you are interpreting it wrong.

In a three phase four wire wye system with all linear loads the neutral only carries imbalance of the phase conductors and is considered not a current carrying conductor for the purpose of conductor ampacity adjustments. This means when only carrying imbalanced current it does not contribute additional heat to the raceway/cable containing the conductors.

However if you supply a circuit that only utilizes two phases and the neutral from a wye system - because of the 120 degree phase angle the current on the neutral is not the same imbalance. For 120/240 single phase (with 180 degree phase angle) with two ungrounded conductors carrying 10 amps each the neutral is carrying zero amps - heating effects are that of two conductors. Put same loads on a 120/208 three wire feed and the neutral will carry right about 10 amps as well- increasing the heating effects in the raceway/cable used.



Non linear loads increase heating in the neutral particularly in a three phase wye system because of additive effects of harmonic currents in the neutral.

 

[shortcircuit1]

I don't know if what you read was wrong or if you are interpreting it wrong.

In a three phase four wire wye system with all linear loads the neutral only carries imbalance of the phase conductors and is considered not a current carrying conductor for the purpose of conductor ampacity adjustments. This means when only carrying imbalanced current it does not contribute additional heat to the raceway/cable containing the conductors.

However if you supply a circuit that only utilizes two phases and the neutral from a wye system - because of the 120 degree phase angle the current on the neutral is not the same imbalance. For 120/240 single phase (with 180 degree phase angle) with two ungrounded conductors carrying 10 amps each the neutral is carrying zero amps - heating effects are that of two conductors. Put same loads on a 120/208 three wire feed and the neutral will carry right about 10 amps as well- increasing the heating effects in the raceway/cable used.



Non linear loads increase heating in the neutral particularly in a three phase wye system because of additive effects of harmonic currents in the neutral.

Shouldn't we be looking at whether the load is non-linear or not rather than considering if it's a single-phase 120/240v or 120/208v three wire feed from the transformer? Even if you have two ungrounded conductors carrying their loads, you may have a scenario where a majority of the load that is fed from this supply may be non-linear. In that scenario, even the neutral is considered current carrying.

 

[Carultch]

Non linear loads increase heating in the neutral particularly in a three phase wye system because of additive effects of harmonic currents in the neutral.

In general, 3rd order harmonics are common and understandably the reason for needing this rule. But some specific kinds of non-linear loads systematically never create harmonics that are multiples of 3, for which this logic applies. Such as VFD drives of motors, whose harmonics are one above and one below each multiple of 6. These kinds of harmonics will not accumulate on the neutral of either a 3-phase or split-phase system, because the harmonics are never multiples of 2 or 3, relative to the fundamental grid frequency. You'd need a hypothetical 5-phase electrical grid to be concerned about accumulating VFD harmonics on the neutral. Are VFD drives an exception to this rule?

 

[kwired]

Shouldn't we be looking at whether the load is non-linear or not rather than considering if it's a single-phase 120/240v or 120/208v three wire feed from the transformer? Even if you have two ungrounded conductors carrying their loads, you may have a scenario where a majority of the load that is fed from this supply may be non-linear. In that scenario, even the neutral is considered current carrying.

The additive effects in the neutral are not as bad on single phase with center tap neutral as they are on three phase wye systems.

First thing you only have effects from two ungrounded conductors in the single phase system to add up on the neutral. With three phase wye, you have three ungrounded conductors contributing to those effects and it will be worse.

That said, for the most part you need a data center for the kind of loading where this becomes a major concern for the neutral conductor.