XFMR protection 450

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don_resqcapt19

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Yet once again I will go to 240.21 (C) (1) which states primary OCPD will not protect secondary on a 3 wire to 4 wire delta/delta transformer... So if primary does not protect secondary than row 2 of table applies to secondary protection. The way I understand it is the general rule of 240.24 (C) just says OCPD is not needed at the transformer but when using 240.21 (C) (2) - (6) it is required after the taps based on table 450.3 (B)...
This is the issue here...the rules in Article 240 have nothing to do with the protection of the secondary winding of the transformer. The only rules that apply to the protection of the secondary winding of the transformer are found in Article 450 and in this case protection of the secondary windings is not required.

240.21 (C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
The section you have cited only applies to the protection of the conductors that are connected to the transformer and not to the transformer winding itself.
 

don_resqcapt19

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So I really could use #2 cu secondary with no main breaker - 240.21(C)(2)(1)b - the rest complies. This case really doesn?t meet the other 6 options of 21(C).

If so this is my comparable for value and application: #2 cu no main breaker or #8 cu with 40a breaker.

I think we?ll go with the 40a main.
If you use a #2 with no main in the panel, how do you provide the protection for the panel as required in 408.36?
 

tryinghard

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Primary OCPD can not provide protection on a multivoltage secondary of a transformer..............Clear as day in 240.21 (C) (1) last paragraph. So if primary cant provide it than secondary protection is required. Hense row 2 table 450.3 (B).

If a particular application meets 240.21(C)(2) then this criteria can be used, even though (1), (3), (4), (5), & (6) are not met. Actually it appears most often - if ever - any 2 would be met consecutively, and again most often none of the 6 gets met causing the requirement of 450.3(B) to apply.

The OCP for the primary is adequate protection for the whole XFMR. Notice a short calc on the secondary: 100a short x 240v 3ph = 41.57kva / 480v 3ph = 50a revealing a overcurrent situation on the primary OCPD.
 

gmtt

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The transformer cannot a feed a load of more than 36A anyway (assuming continuous load). Why then a humongous panel (B) of 125A was installed in the first place? Simply a 40A C.B would suffice. What is confusing me here, a 125A panel has several spaces in it (I don't know exactly how may). Why wouldn't any one else may attempt to load this panel using a CB in one its spaces? What would prevent them in doing so? Can any one please elaborate on this? Is there any code concerning this? Thanks.
 

yired29

Senior Member
If a particular application meets 240.21(C)(2) then this criteria can be used, even though (1), (3), (4), (5), & (6) are not met. Actually it appears most often - if ever - any 2 would be met consecutively, and again most often none of the 6 gets met causing the requirement of 450.3(B) to apply.

The OCP for the primary is adequate protection for the whole XFMR. Notice a short calc on the secondary: 100a short x 240v 3ph = 41.57kva / 480v 3ph = 50a revealing a overcurrent situation on the primary OCPD.
You still don't meet the requirements of 240.4 (F). I believe its the line to neutral that presents a problem.
 

don_resqcapt19

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You still don't meet the requirements of 240.4 (F). I believe its the line to neutral that presents a problem.
Just like 240.21(C), 240.4(F) only applies to the protection of the conductors. In the application in this thread, you only need protection for the secondary conductors, not the secondary of the transformer, and as long as the secondary conductors have an ampacity equal to or greater than the OCPD used at the load end of those conductors, the installation is code compliant.
 

don_resqcapt19

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If a particular application meets 240.21(C)(2) then this criteria can be used, even though (1), (3), (4), (5), & (6) are not met. Actually it appears most often - if ever - any 2 would be met consecutively, and again most often none of the 6 gets met causing the requirement of 450.3(B) to apply.

The OCP for the primary is adequate protection for the whole XFMR. Notice a short calc on the secondary: 100a short x 240v 3ph = 41.57kva / 480v 3ph = 50a revealing a overcurrent situation on the primary OCPD.
I don't understand...nothing in 240 kicks you back to 450. They are two seperate issues. 450 is only concerned with the protection of the transformer itself and 240.21(C) is only concerned with the protection of the transformer secondary conductors. All transfomer secondary conductors must meet one of the rules in 240.21(C).
 

yired29

Senior Member
I don't understand...nothing in 240 kicks you back to 450. They are two seperate issues. 450 is only concerned with the protection of the transformer itself and 240.21(C) is only concerned with the protection of the transformer secondary conductors. All transfomer secondary conductors must meet one of the rules in 240.21(C).
Well the point I'm try to get across is we need OCPD on the secondary. Table 450.3 (B) lays the ground rules for sizing the OCPD. We can't exceed the values given in Table 450.3 (B) if we exceed these values we run the risk of overloading the transformer. So a 36 amp FLC transformer with anything larger than 45 amp OCPD on a 4 wire secondary has a potentioal to overload the transformer. Say we use 125 amp OCPD in the panel from the secondary what limits the load from exceeding the 36 amp FLC of the transformer?
 

raider1

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Well the point I'm try to get across is we need OCPD on the secondary. Table 450.3 (B) lays the ground rules for sizing the OCPD. We can't exceed the values given in Table 450.3 (B) if we exceed these values we run the risk of overloading the transformer. So a 36 amp FLC transformer with anything larger than 45 amp OCPD on a 4 wire secondary has a potentioal to overload the transformer. Say we use 125 amp OCPD in the panel from the secondary what limits the load from exceeding the 36 amp FLC of the transformer?

Provided that the primary overcurrent protective device is sized no more than 125% of the current rating of the transformer we are not required to provide additional overcurrent protection of the secondary winding of the transformer. Take a look at 450.3(B)

Again 450.3 is only dealing with overcurrent protection of the transformer itself, not the secondary conductors or the panelboard.

The secondary conductors must be protected in accordance with 240.21(C) and the panelboard must be protected in accordance with 408.36.

Chris
 

jim dungar

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Provided that the primary overcurrent protective device is sized no more than 125% of the current rating of the transformer we are not required to provide additional overcurrent protection of the secondary winding of the transformer. Take a look at 450.3(B)

Again 450.3 is only dealing with overcurrent protection of the transformer itself, not the secondary conductors or the panelboard.
Absolutely.

450.3(B) requires a transformer to be protected only with a primary device sized at not more than 125%.

450.3(B) allows the transformer primary device to be larger than 125% if a properly sized secondary device is provided.
 

tryinghard

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The transformer cannot a feed a load of more than 36A anyway (assuming continuous load). Why then a humongous panel (B) of 125A was installed in the first place? Simply a 40A C.B would suffice. What is confusing me here, a 125A panel has several spaces in it (I don't know exactly how may). Why wouldn't any one else may attempt to load this panel using a CB in one its spaces? What would prevent them in doing so? Can any one please elaborate on this? Is there any code concerning this? Thanks.

No code, this is a 3ph panelboard cu bus'd and minimum is 125a rated, I believe a 100a is so rare it may cost more. Remember the rating is the busing not the main. This project is a distribution upgrade for some old archaic knife switches, the upgrade is accounting for minimum sized (rated) equipment but there is no valid reason to upsize the XFMR past a calcd load (that includes over 10.5kva spare/future loads). But in the event 15 years from today they need this additional load the XFMR will simply be upsized along with the required main in the subpanel, everything else would remain. If the future loads were no concern a main lug only with a back-fed 40a would suffice just fine.
 

tryinghard

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California
I don't understand...nothing in 240 kicks you back to 450. They are two seperate issues. 450 is only concerned with the protection of the transformer itself and 240.21(C) is only concerned with the protection of the transformer secondary conductors. All transfomer secondary conductors must meet one of the rules in 240.21(C).

True, I gotcha.
 

yired29

Senior Member
Absolutely.

450.3(B) requires a transformer to be protected only with a primary device sized at not more than 125%.

450.3(B) allows the transformer primary device to be larger than 125% if a properly sized secondary device is provided.
The fundamental requirement of 240.4 specifies that conductors are to be protected against overcurrent in accordance with their ampacity, and 240.21 requires that the protection be provided at the point the conductor receives its supply. Section 240.4(F) permits the secondary circuit conductors from a transformer to be protected by overcurrent devices in the primary circuit conductors of the transformer only in the following two special cases:
1. A transformer with a 2-wire primary and a 2-wire secondary, provided the transformer primary is protected in accordance with 450.3
2. A 3-phase, delta-delta-connected transformer having a 3-wire, single-voltage secondary, provided its primary is protected in accordance with 450.3
Except for those two special cases, transformer secondary conductors must be protected by the use of overcurrent devices, because the primary overcurrent devices do not provide such protection. As an example, consider a single-phase transformer with a 2-wire secondary that is provided with primary overcurrent protection rated at 50 amperes. The transformer is rated 480/240 volts. Conductors supplied by the secondary have an ampacity of 100 amperes. Is the 50-ampere overcurrent protection allowed to protect the conductors that are connected to the secondary?
The secondary-to-primary voltage ratio in this example is 240 ? 480, a ratio of 0.5. Multiplying the secondary conductor ampacity of 100 amperes by 0.5 yields 50 amperes. Thus, the maximum rating of the overcurrent device allowed on the primary of the transformer that will also provide overcurrent protection for the secondary conductors is 50 amperes. These secondary conductors are not tap conductors, are not limited in length, and do not require overcurrent protection where they receive their supply, which is at the transformer secondary terminals.
However, if the secondary consisted of a 3-wire, 240/120-volt system, a 120-volt line-to-neutral load could draw up to 200 amperes before the overcurrent device in the primary actuated. That would be the result of the 1:4 secondary-to-primary voltage ratio of the 120-volt winding of the transformer secondary, which can cause dangerous overloading of the secondary conductors.
This is straight out of the 2008 handbook. (Explanation under 240.4 (F) )
Please read it all then respond.
IMO it says if you have a multi voltage secondary than OCPD is required on the secondary no matter how you size the primary OCPD
If secondary OCPD is required then we have to use table 450.3 (B) row 2
 

raider1

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The fundamental requirement of 240.4 specifies that conductors are to be protected against overcurrent in accordance with their ampacity, and 240.21 requires that the protection be provided at the point the conductor receives its supply.

Correct, but read on in 240.21(C)

"A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

This section specifically permits transformer secondary conductors to be run without overcurrent protection provided that we comply with one of the sections (C)(1) through (C)(6).

Section 240.4(F) permits the secondary circuit conductors from a transformer to be protected by overcurrent devices in the primary circuit conductors of the transformer only in the following two special cases:

Correct, this section says that transformer secondary conductors are not considered protected by the transformer primary OCPD except for the 2 situation mentioned in that section. So for transformer secondary conductor protection we go to 240.21(C).

IMO it says if you have a multi voltage secondary than OCPD is required on the secondary no matter how you size the primary OCPD
If secondary OCPD is required then we have to use table 450.3 (B) row 2

What that section is saying is that transformer secondary conductors must be protected by an overcurrent protective device. As has been pointed out many times in this thread, transformer secondary conductors overcurrent protection is specified in 240.21(C) and not by 450.3.

450.3 is only for transformer secondary winding protection not secondary conductor protection.

Chris
 

jim dungar

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If secondary OCPD is required then we have to use table 450.3 (B) row 2
No, that is not true.
You are confusing two independent sections of the NEC.

Per 450.3(B) secondary protection is always optional when the primary device is not more than 125% FLA. A combination of primary and secondary protection is required only if the primary device is >125% FLA

There is nothing in 450.3 that requires you to look at any section of 240.
There is, however, a requirement in 240.3 that directs you to look at 450.
 

tryinghard

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I?ve gone through this topic a few times in my tenure, I was hoping my case would spark some dialog. Even when I put the design together I thought I was clear, maybe this time it?ll stay with me. :-?
 

yired29

Senior Member
No, that is not true.
You are confusing two independent sections of the NEC.

Per 450.3(B) secondary protection is always optional when the primary device is not more than 125% FLA. A combination of primary and secondary protection is required only if the primary device is >125% FLA

There is nothing in 450.3 that requires you to look at any section of 240.
There is, however, a requirement in 240.3 that directs you to look at 450.
I agree with you if it is a single voltage secondary. If it is a multi voltage secondary would 240.4 (F) require secondary OCPD? 240.4 (F) is seperate from 240.21 and does require this.
 

yired29

Senior Member
I thought the OCPD on the secondary must be sized so that current in excess of the the transformers FLC x 125% will not be exceeded. I do understand the 240.21 rules. I think 240.4 (F) overrules 240.21 (C) (2-6). I just dont agree with the thought of putting a 125 amp OCPD on the secondary conductors of a transformer with a FLC of 36 amps.
 

jim dungar

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I agree with you if it is a single voltage secondary. If it is a multi voltage secondary would 240.4 (F) require secondary OCPD? 240.4 (F) is seperate from 240.21 and does require this.

240.4(F) requires the conductors to be protected by something other than the transformer primary device. One method of meeting 240.4(F) is the tap rules in 240.21(C). 240.4(F) says absolutely nothing about protecting the transformer.

Imagine a 3kVA 480-120/240V transformer feeding a single set of 500Kcmil conductors. 240.4(F) would allow these conductors to be protected by a 400A breaker - without considering the transformer secondary current at all. 450.3(B) says secondary protection is not required if the primary device is not greater than 125%.
 
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