Transformer Primary protection 250% NEC 450.3(B)

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ezdrocks

ezdrocks

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Houston, Texas, USA
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I&E Designer, Industrial Electrical and Instrumentation design (NOT an engineer but close)
I am having a debate with our company's Electrical Engineer. Truth be told, I am NOT an engineer, but I have been I/E doing design work for 13 years and have designed numerous transformer installations, as well as MCCs, Pump motors, Lighting, etc., so I know what I'm doing. I feel that he thinks that since he's an Engineer, he knows all and doesn't agree with me at all.

We have a 45kVA transformer with 480V 3P primary and a 240/120V secondary.

The primary is coming off a Square D Series 6 MCC Vertical section, feeding the transformer, and the secondary is going to a 120/240VAC electrical panel with a main breaker on the incoming side.

Sizing the primary is easy...45,000/480/1.732=54.12A...54.12A*1.25=67A. NEC says use the 70A as the next size up standard size breaker.
Sizing the secondary is the same...45000/240=187.5A...187.5*1.25=234A. NEC says use 250A as the next standard size up.

Here's where the debate comes in.
NEC table 450.3(B) shows that if the transformer has protection on the primary AND secondary, the primary OCPD is 250%. He is making me size the primary OCPD at 135A (he said use a 125A), as if it is required. I know it's "up to" 250%, but can't find in the NEC where it says that. He always goes to the highest possible rating in the NEC and doesn't take industry standard into account.

Sorry to vent, but if someone could point me to the NEC where it shows that the 250% OCPD rating is optional, it would help me win this argument.
 
ezdrocks said:
I am having a debate with our company's Electrical Engineer. Truth be told, I am NOT an engineer, but I have been I/E doing design work for 13 years and have designed numerous transformer installations, as well as MCCs, Pump motors, Lighting, etc., so I know what I'm doing. I feel that he thinks that since he's an Engineer, he knows all and doesn't agree with me at all.

We have a 45kVA transformer with 480V 3P primary and a 240/120V secondary.

The primary is coming off a Square D Series 6 MCC Vertical section, feeding the transformer, and the secondary is going to a 120/240VAC electrical panel with a main breaker on the incoming side.

Sizing the primary is easy...45,000/480/1.732=54.12A...54.12A*1.25=67A. NEC says use the 70A as the next size up standard size breaker.
Sizing the secondary is the same...45000/240=187.5A...187.5*1.25=234A. NEC says use 250A as the next standard size up.

Here's where the debate comes in.
NEC table 450.3(B) shows that if the transformer has protection on the primary AND secondary, the primary OCPD is 250%. He is making me size the primary OCPD at 135A (he said use a 125A), as if it is required. I know it's "up to" 250%, but can't find in the NEC where it says that. He always goes to the highest possible rating in the NEC and doesn't take industry standard into account.

Sorry to vent, but if someone could point me to the NEC where it shows that the 250% OCPD rating is optional, it would help me win this argument.
Click to expand...

The 250% on the primary is the maximum allowable OCPD size per T450.3(B), not the "required" size. You may want a larger than 125% primary OCPD to mitigate tripping on inrush when energizing the transformer.

Your secondary calculation is wrong.
 
david luchini said:
The 250% on the primary is the maximum allowable OCPD size, not the required size.

Your secondary calculation is wrong.
Click to expand...

I know that the 250% is the max allowable, but I can't find where it says that. The table just says 250%. And to win this argument with him I have to show him where it says that.

On another note...how is my secondary calculation wrong. kVA of transformer/secondary voltage (it's single phase so no 1.732), then multiply by 1.25. 45,000/240=187.5, times 1.25 = 234.375A
 
ezdrocks said:
On another note...how is my secondary calculation wrong. kVA of transformer/secondary voltage (it's single phase so no 1.732), then multiply by 1.25. 45,000/240=187.5, times 1.25 = 234.375A
Click to expand...

XXXX. Never mind!! The transformer is still 3-phase even if the secondary isn't. The correct secondary is 135A or a 150A main breaker in the panel. Sorry.
 
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ezdrocks said:
I know that the 250% is the max allowable, but I can't find where it says that. The table just says 250%. And to win this argument with him I have to show him where it says that.

On another note...how is my secondary calculation wrong. kVA of transformer/secondary voltage (it's single phase so no 1.732), then multiply by 1.25. 45,000/240=187.5, times 1.25 = 234.375A
Click to expand...

The Table 450.3(B) says "Maximum" Rating or Setting. It doesn't say "Required" Rating or Setting.

45kVA is not a standard single phase transformer size. Your primary calculation is for 3 phase. The secondary rated current would be 108A.
 
ezdrocks said:
The transformer is still 3-phase even if the secondary isn't.
Click to expand...
Not true. :happysad:

A three phase transformer will have three primary windings, each coupled to its own secondary winding. It is certainly possible to connect all three phases of a three phase transformer to an upstream three phase breaker, and only connect secondary conductors to one of the secondary windings. A bit of a waste, but possible, and not a code issue. However, two of the three primary phases will have no current flowing. It will be exactly as if they do not exist. You would essentially have a single phase transformer with more copper and iron than you need for your application. The factor of 1.732 would not come into the calculation on either the primary or secondary side.

All that said, can you clarify your proposed installation? Your description does not make sense to me.
 
ezdrocks said:
I am having a debate with our company's Electrical Engineer. Truth be told, I am NOT an engineer, but I have been I/E doing design work for 13 years and have designed numerous transformer installations, as well as MCCs, Pump motors, Lighting, etc., so I know what I'm doing. I feel that he thinks that since he's an Engineer, he knows all and doesn't agree with me at all.

We have a 45kVA transformer with 480V 3P primary and a 240/120V secondary.

The primary is coming off a Square D Series 6 MCC Vertical section, feeding the transformer, and the secondary is going to a 120/240VAC electrical panel with a main breaker on the incoming side.

Sizing the primary is easy...45,000/480/1.732=54.12A...54.12A*1.25=67A. NEC says use the 70A as the next size up standard size breaker.
Sizing the secondary is the same...45000/240=187.5A...187.5*1.25=234A. NEC says use 250A as the next standard size up.

Here's where the debate comes in.
NEC table 450.3(B) shows that if the transformer has protection on the primary AND secondary, the primary OCPD is 250%. He is making me size the primary OCPD at 135A (he said use a 125A), as if it is required. I know it's "up to" 250%, but can't find in the NEC where it says that. He always goes to the highest possible rating in the NEC and doesn't take industry standard into account.

Sorry to vent, but if someone could point me to the NEC where it shows that the 250% OCPD rating is optional, it would help me win this argument.
Click to expand...

There is no industry standard for this kind of thing. It is a design choice. I'd be inclined to use as much of the 250% allowed as possible to avoid nuisance tripping. So I concur with his directive to use a 125A CB. Incidentally, the NEC does not require that you have to use the next size up CB. It allows you to do so. it is also a design decision. As a practical matter, the transformer is protected and you reduce the chances of nuisance tripping which can be a very expensive thing to have happen. The only thing it costs you is that the xfmr primary conductors have to be sized a little bit bigger.

Read carefully what table 450.3(B) actually says about the next higher standard rating. You seem to misunderstand this provision.
 
We install 45 kva transformers all of the time. Standard sizes for us: 15, 30, 45, 75, 112.5 kva, etc.
 
The 250% rule is pretty universal for inrush reasons. It is less of an issue on certain sizes. It is also a big problem when you have say one or two large motors that are close to the transformer capacity and will trip a primary fuse during starting. I’m reminded of a mine with two 200 HP motors on a 500 kVA transformer. It worked barely.
 
Explanation of magnetizing current:

https://electrical-engineering-portal.com/practical-considerations-of-transformer-inrush-current

It is however NOT infinite as the article says. Current is limited by the transformer primary resistance (inductance is saturated). The magnetizing current depends on the core material and the windings. In the mining business they use copper windings on mobile substations and equipment due to issues with ruggedness of aluminum transformers. Theoretically aluminum should be the same or better but not in actual use.

The smaller the transformer the higher the magnetizing current but it also settles down much faster (a couple cycles). Small ones can easily hit 40-100x FLA. Big ones will be maybe 10-20 but instead of one cycle it takes much longer especially in the MVA sizes.

https://www.sandc.com/en/support/coordinaide/

This handy web based calculator uses the “generic” numbers to calculate transformer inrush. Setting your fuse/breaker curves left of the calculated curve minimizes device size and avoids the issue. To the right of it using rules like 125-135% of primary FLA with a fuse you get a trip about 5-10% of the time on larger transformers. Again low impedance designs like copper coils are much worse. I’ve seen 2000-4000 kVA transformers with E-rated fuses rated at 135% (the number KBR likes) trip nearly every time. Upgrading to above the magnetizing curve 100% eliminated it.
 
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