XFMR secondary OCPD

George Stolz · Dec 5, 2005

It's weird. This morning, I saw and commented to this thread, and coincidentally, I get a call from the boss today on the same topic.

He asked, "I got a code question for you. I have a 480V primary, 208 secondary transformer. The primary is 92 amps. I have a breaker, and a disconnect before the transformer; and I have an MLO panel on the secondary. Do I need overcurrent protection to this panel?"

Off the top of my head (because 1.) I always leave my code book at home whenever other people need me to use it

, and 2.) It's Monday, and I tend to bring it out of the truck over the weekend), I remembered the other thread this morning, and said, "I think the secondary needs protection at 125% of the rating of the transformer." Also, vaguely remembering Note 2 to Table 450.3(B), I told him I think he needed up to six handles on the MLO, and then he'd need a main. "But I'm not certain."

So, I get home and look it up, and I am confused. When is overcurrent protection on the secondary required? According to Note 2 of Table 450.3(B), it appears that it's up to the AHJ to declare if OC protection is required...?

But under closer review, it appears that this falls under 408.36(D), which would require OCPD on the secondary. The exception doesn't seem to apply, because if it's 480 on the primary and it spits out 208, then it would have to be a delta-wye transformer, right?

So the correct answer (aside from, ask someone qualified

) would be:

Yes, you are required to have overcurrent protection ahead of the MLO panel (408.36(A or B) & (D)).
The conductors and the OCPD should be sized to 125% of the output of the secondary. (Table 450.3(B))
You're permitted to size the OCPD to the primary to 250% of the rating of the primary if you are so inclined. (Table 450.3(B))
A 75 kVA 480/277 -> 120/208 transformer would draw 156A of 480 and output 360 amps of 208, so he could install a 400 amp OCPD on the primary, and a 450 amp OCPD on the secondary ahead of the MLO.
The input conductors could be 2/0 CU on the primary, and either 500 MCM CU, or parallel 3/0 CU on the secondary.
The GEC for the transformer would be #4 CU.

Needless to say, I felt a wee bit unqualified to be answering this question. I could digress onto a tirade down that path, but I shall refrain and simply say, this is not my area of expertise. So how did I do?

iwire · Dec 5, 2005

Re: XFMR secondary OCPD

George you have an error here that effects the rest.

Originally posted by georgestolz:
A 75 kVA 480/277 -> 120/208 transformer would draw 156A of 480 and output 360 amps of 208,

75 kva 3 ph transformer

Primary current 90 amp

Secondary current 208 amps (Yes thats 208 a at 208 v

)

[ December 05, 2005, 07:43 PM: Message edited by: iwire ]

George Stolz · Dec 5, 2005

Re: XFMR secondary OCPD

So his 90 amp statement was accurate. (Not that I doubted it.)

Otherwise, correct?

iwire · Dec 5, 2005

Re: XFMR secondary OCPD

To answer you bosses question yes he needs secondary protection.

A typical 75 kVA set up in my area would be 100 to 125 amp primary breaker, 225 amp secondary breaker. The 225 amp breaker is usually the main breaker of a 225 amp 208Y/120 panel.

The 100 amp breaker is a large cost savings as the breaker fits into the standard bus position.

The 125 amp breaker is better for inrush currents however for all but a few panels will requirer a larger tub and different bus bar arraignments.

frenchelectrican · Dec 5, 2005

Re: XFMR secondary OCPD

A 75 kVA 480/277 -> 120/208 transformer would draw 156A of 480 and output 360 amps of 208, so he could install a 400 amp OCPD on the primary, and a 450 amp OCPD on the secondary ahead of the MLO.

Good catch Bob

i am glad someone is watching this one close here but really in fact i do agree with bob with OCPD on both primary and secondary side

this is my common way i do with primary and secondary side.

the primary side i useally normally use 100 A breaker it do fit most common panelboard without going oversize unit

secondary side it useally either 200 A or 225 A panelboard and it do meet the NEC code very easly here

but i did not get a chance to review on secondary side requrement for distance from transfomer to secondary OCPD distance limit so ,, i leave this question open for now

Merci , Marc

bob · Dec 5, 2005

Re: XFMR secondary OCPD

A 75 kVA 480/277 -> 120/208 transformer would draw 156A of 480 and output 360 amps of 208, so he could install a 400 amp OCPD on the primary, and a 450 amp OCPD on the secondary ahead of the MLO.
The input conductors could be 2/0 CU on the primary, and either 500 MCM CU, or parallel 3/0 CU on the secondary.

George
AS Iwire said, that secondary amps is 208. Had your caculations been correct, you are not allowed to go the the next size breaker on the secondary. The secondary conductors must match the breaker size at the panel. If the breaker is 450 amps the conductor capacity must equal 450 amps. If the breaker was a 400 amp you would need 600 kcm. 240.21.C.2b

[ December 06, 2005, 12:06 PM: Message edited by: bob ]

jim dungar · Dec 5, 2005

Re: XFMR secondary OCPD

No, a 600V or less primary transformer does not ever need secondary protection. But, if secondary protection is provided the primary OCPD can be sized greater than 125% of the primary current.

However,
The panelboard may, actually probably will, need a single main per 408.35, .36

The secondary conductors may need protection based on the requirements (and tap rules) in 240. The main for the panel may also be used for the conductor protection depending on the length of the conductors and the location of the OCPD.

To make it easier to follow, always start at the load and work your way to the source.

George Stolz · Dec 5, 2005

Re: XFMR secondary OCPD

Primary current 90 amp

So, how do they get that from a 75 kVA transformer?

Bob #20 wrote:
The secondary conductors must match the breaker size at the panel. 240.21.C.2b

I had assumed it was covered by 240.4(B).

So, conductors from a secondary are always "tap conductors"?

Jim wrote:
No, a 600V or less primary transformer does not ever need secondary protection.

Because of 240.21(C)?

To make it easier to follow, always start at the load and work your way to the source.

Can you elaborate?

jim dungar · Dec 5, 2005

Re: XFMR secondary OCPD

George,

Choose the correct OCPD and or main for the panelboard. By following 408.xx rules you will know if the panel requires a main or not, regardless of it's source.

Then decide if the required main device will be located integral or remote to the panel.

Now look at the conductors supplying the panel. Can they be proteced by the panel remote main directly or by the integral main as a tap rule in 240.

Finally look at the transformer and decide if you want to use the panel main (or the conductor's protection) as a transformer secondary protective device, which will allow you to upsize the primary protection.

iwire · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by georgestolz:

Primary current 90 amp

Click to expand...

So, how do they get that from a 75 kVA transformer?

I have assumed since the start you where asking about a 3 phase transformer as you listed the secondary voltage as 208 V.

75 x 1000 / 480 / 1.73 = 90.31 amps.

75 x 1000 / 208 / 1.73 = 208.42 amps.

[ December 06, 2005, 05:39 AM: Message edited by: iwire ]

iwire · Dec 6, 2005

Re: XFMR secondary OCPD

75 kVA 480-208Y/120 transformers are the most common transformer we install followed shortly by 300 kVA.

The typical 75 kVA will be located as close to the panel it serves as is possible. This is best as far as costs and voltage drop issues.

By locating it close we can forget the separate OCPD for the secondary conductors as Jim said that can be accomplished with the panelboard main.

In you bosses case all he has to do is install a separate OCPD that will protect the panel and secondary conductors.

A standard set up we install will have 125 amp primary protection with 1 AWG CU to the transformer.

Secondaries will often be 4/0 or 250s feeding into a 225 amp main breaker panel board.

But different engineers have different ideas. I have been directed to feed a 75 kVA with a 225 amp breaker using 3 AWG.

I questioned the engineer on the 3 AWG but got an all to typical 'talking down to' and that I had no business questioning his design.

OK fine... :roll:

augie47 · Dec 6, 2005

Re: XFMR secondary OCPD

I questioned the engineer on the 3 AWG but got an all to typical 'talking down to' and that I had no business questioning his design.

OK fine...

Good morning, mr inspector! lets go over here by this NEW panel and have that cup of coffee

George Stolz · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by iwire:
I have assumed since the start you where asking about a 3 phase transformer as you listed the secondary voltage as 208 V.

75 x 1000 / 480 / 1.73 = 90.31 amps.

75 x 1000 / 208 / 1.73 = 208.42 amps.

You assumed correctly. It just seems weird to me, we always multiply the voltage by 1.73, not divide. Because of the magic mystery of 3?, the voltage is worth more.

By locating it close we can forget the separate OCPD for the secondary conductors as Jim said that can be accomplished with the panelboard main.

So secondary conductors are considered protected by the main OCPD they serve? (240.21(C)(6)(2) ? )

In you bosses case all he has to do is install a separate OCPD that will protect the panel and secondary conductors.

You're saying, install a main OCPD in the panelboard served?

What does this mean:

240.21(C)(6)(1)The secondary conductors shall have an ampacity that, when multiplied by the ratio of the secondary-to-primary voltage, is at least one-third of the rating of the overcurrent device protecting the primary of the transformer.

Would this be 480 / 208 = 2.3

So for a 225 amp panelboard with appropriate conductors, the rating of an overcurrent device on the primary side could not exceed
225 / 2.3 = 97.82 amps, which which would allow the primary OCPD to be up to but less than
98 * 3 = 294 amps?

My head's spinning. I'm sure the conductors are less than 25 feet long, and I feel like if I can conquer this section, the rest will fall into place.

I'm not trying to be difficult, I just trying to grasp what the NEC calls things, and what rules to apply when.

wirenut1980 · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by jim dungar:

Then decide if the required main device will be located integral or remote to the panel.

Now look at the conductors supplying the panel. Can they be proteced by the panel remote main directly or by the integral main as a tap rule in 240.

Finally look at the transformer and decide if you want to use the panel main (or the conductor's protection) as a transformer secondary protective device, which will allow you to upsize the primary protection.

I am fuzzy on how I decide on when it is ok to use the main panel OCPD as a transformer secondary protection. Is this a design consideration or in the Code? Can anyone clear this up for me?

charlie b · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by jim dungar: No, a 600V or less primary transformer does not ever need secondary protection.

What, never?
No, never!
What, never?
Well, hardly ever!

See 240.4(F)

charlie b · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by georgestolz: It just seems weird to me, we always multiply the voltage by 1.73, not divide. Because of the magic mystery of 3?, the voltage is worth more.

Start with this:

Apparent Power (in units of VA) equals the square root of three times voltage times current.

or

VA = 1.732 x V x I

Now, what do you know, and what are you trying to find? In this case, you know the rating of the transformer, in units of VA. You also know voltage. You are looking for current.

So you take the formula above, and divide both sides of the equation by (1.732 x V). On the left side of the equation, you now have (VA) / (1.732 x V). On the right side of the equation, all that is left is "I," the value of current that you are trying to find.

So it is not the voltage that is worth more. If you knew the output rating in terms of current, and you were looking for voltage, you would take (VA) / (1.732 x I ), and you would get "V." In fact, I did that calculation in a thread sometime last week. If I find it, I'll show you where I used it.

charlie b · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by charlie b: In fact, I did that calculation in a thread sometime last week. If I find it, I'll show you where I used it.

It was actually several weeks ago. Look at my first post in this thread.

I used (VA) / (1.732 x I), given a 300KVA transformer and a 45 amp primary fuse (presuming that was around 300% of primary rating), to figure out that the OP was talking about a 12,470 volt primary.

bob · Dec 6, 2005

Re: XFMR secondary OCPD

Iwire

A standard set up we install will have 125 amp primary protection with 1 AWG CU to the transformer. Secondaries will often be 4/0 or 250s feeding into a 225 amp main breaker panel board. But different engineers have different ideas. I have been directed to feed a 75 kVA with a 225 amp breaker using 3 AWG.
I questioned the engineer on the 3 AWG but got an all to typical 'talking down to' and that I had no business questioning his design.

Is you concern using the #3 with the 225 amp
primary breaker? 240.21.3, and .4 will allow that
installation. The secondary to primary ratio is 0.43. Then 0.43 x 225 = 97 amps. #3 is rated at 100 amps at 75C. I am looking at this and thinking you could use the 90C rating.

[ December 06, 2005, 05:21 PM: Message edited by: bob ]

jim dungar · Dec 6, 2005

Re: XFMR secondary OCPD

Charlie, 240.4(F) requires the secondary conductors to have protection not the transformer. There is no place in 450 that requires secondary protection for transformers 600V or less.

George, the turns ratio allowance is only for protection of the secondary conductors. Say that your primary protection is 125A, then 240.21(C)(6) would allow the secondary conductors to be as small as (turns ratio) X (primary OCPD/3) = (480/208)x(125/3) = 96.15A.

charlie b · Dec 6, 2005

Re: XFMR secondary OCPD

Originally posted by jim dungar: Charlie, 240.4(F) requires the secondary conductors to have protection not the transformer. There is no place in 450 that requires secondary protection for transformers 600V or less.

I'm probably not the only one who is confused by the transformer protection tables.

For one thing, how do you protect the conductors that connect the transformer's output to the load, per 240.4(F), and not at the same time be protecting the conductors that comprise the secondary windings? In other words, if I have to add an overcurrent device at the secondary terminals (or at a nearby location, using the tap rules), because 240 says so, and by the way most of the time I do have to do this, does that not automatically put me into the right hand side of Table 450.3(B)?

For another thing, why would Note 2 of Table 450.3(B) begin with "Where secondary overcurrent protection is required," if it is never required?

XFMR secondary OCPD

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