Transformer Secondary/ Motor Loads

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augie47

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Staff member
Location
Tennessee
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State Electrical Inspector (Retired)
I think I am correct on this but would like some input before I require a "change".
I have a motor load connected to a 208Y/120 transformer.The 50 HP motor has a fusible disconnect with 250 amp fuses with a 3/0 conductor to the motor. All Code compliant.
The disconnect is within 25 ft of the transformer and the E/C continued the 3/0 from the line side of the disconnect to the transformer.
I contend that 240.21(C)(6) requires the conductor from the transformer to the LINE side must have an ampacity as great as the fuses even though it is oversized for a motor load.
Is there any exception that would allow the 3/0 in this situation ?
 

stew

Senior Member
even with al 3/0 conductors seems to be correct or am i missing something? 3/0 al is good for 240 and the next fuse size up can be used so 250 seems correct. The motor required conductor ampacity is only 178 so 3/0 also is fine.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I think the last sentence of 240.21(C) tells that the "next size up" is not allowed for transformer secondaries so that shoots me down unless there is an exception for motors somewhere
 

RUWired

Senior Member
Location
Pa.
Augie, i'm with you. I don't see anything that would allow the conductors to be rated less than the fuse rating.

Rick
 

dana1028

Senior Member
even with al 3/0 conductors seems to be correct or am i missing something? 3/0 al is good for 240 and the next fuse size up can be used so 250 seems correct. The motor required conductor ampacity is only 178 so 3/0 also is fine.

?? Maybe I'm missing something. My code, Table 310.16 shows 3/0 copper has a value of 200A, AL has a value of 155A...neither of these is close to the 250A OCP.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
?? Maybe I'm missing something. My code, Table 310.16 shows 3/0 copper has a value of 200A, AL has a value of 155A...neither of these is close to the 250A OCP.

Agreed, however, Art 430 will allow the 3/0 on a 250 amp OCP for the motor. Art 240.21(C) does not seem to make the same allowance for the transformer secondary conductors supplying the same load.
 

kwired

Electron manager
Location
NE Nebraska
I think I am correct on this but would like some input before I require a "change".
I have a motor load connected to a 208Y/120 transformer.The 50 HP motor has a fusible disconnect with 250 amp fuses with a 3/0 conductor to the motor. All Code compliant.
The disconnect is within 25 ft of the transformer and the E/C continued the 3/0 from the line side of the disconnect to the transformer.
I contend that 240.21(C)(6) requires the conductor from the transformer to the LINE side must have an ampacity as great as the fuses even though it is oversized for a motor load.
Is there any exception that would allow the 3/0 in this situation ?

If the transformer were a delta transformer I would say the primary overcurrent device would be allowed to be the protection and the conductor could be 3/0, and this would be allowed by 240.21(C)(1). But since it is not a delta transformer I think you may need to increase the conductor size, seems pointless but not finding any way around it. I assume the neutral of the wye is grounded. If it were a grounded phase with floating neutral ... but it is hard to find anything that allows that also. There was a thread where this possibility was mentioned for backing up a corner ground with a wye connected generator and grounding a phase - but there was no real consensus as to whether or not that was acceptable IIRC, but is not exactly the same thing as doing something like that with a transformer either. I don't know - just something to think about.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Thanks...
It is a wye with grounded XO as there is another load requiring the neutral
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I think I am correct on this but would like some input before I require a "change".
I have a motor load connected to a 208Y/120 transformer.The 50 HP motor has a fusible disconnect with 250 amp fuses with a 3/0 conductor to the motor. All Code compliant.
The disconnect is within 25 ft of the transformer and the E/C continued the 3/0 from the line side of the disconnect to the transformer.
I contend that 240.21(C)(6) requires the conductor from the transformer to the LINE side must have an ampacity as great as the fuses even though it is oversized for a motor load.
Is there any exception that would allow the 3/0 in this situation ?

Auggie -
Just so I am clear: I'm not picking on you. The suituation is odd and a bit absurb. I'm making fun of the install - not you

This started out sounding like a dedicated xfm feeding a single motor load And of course its kind of silly for the NEC to require bigger conductors between the xfm and the motor disconnect than is required between the disconnect and the motor. There are no safety, reliability, operability issues that would require the xfm conductors to be larger - only a legal reason. And we all know that the laws of man trump the laws of physics. And the EC doesn't have any stamped drawings, didn't ask for a variance up front, and she wants to save the extra cost of 25 ft of larger conduit and 25 ft of more copper.

Tough luck. "The Law Says ....., and You Are Going To Follow It." (Delivered in a loud, deep, authoritive voice while fingering badge and gun)

Then we get this:
Thanks...
It is a wye with grounded XO as there is another load requiring the neutral
Now it gets freaky. Somewhere between the 250A disconnect and the motor controller there is an undefined tap to an undefined load that includes loads requiring a neutral.

So I draw it out.

Hummm ...
1. If the 250A disconnect is also the 125% xfm secondary protection, that limits the xfm size to 75kva.

2. LRC for a 208V, 50 Hp motor is maybe 800A - 900A. Figuring a nominal 5%Z, that ought to suck a 75kva xfm right to its knees. Hope it isn't a fire pump. It likely isn't going to make 15% VD. I wonder what happens to the undefined load when the VD falls off the end of the earth.

3. The presence of a disconnect means there is also a separate controller. Wonder how the tap is made. If the disconnect is not rated for two conductors, then use split bolts in a gutter?

I wonder what the tap rules are for tapping an art 430 motor circuit. Should be in art 430 - just something I don't use much.


So, where am I going:
Auggie -
I don't think we are getting the rest of the story here. I'm thinking there's more to it

Just a thought. (You should had seen the rabbit trail when I considered using a mag-only CB in a combination controller as transformer secondary protection and secondary conductor protection):sick:

ice
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Wow ! Super perception :D

I was actually looking at a job with both a motor load and lighting load fed by the same transformer.
I attempted to make my original post simple and ignore the lighting load, but then the XO question arose and I justified the XO ground.
My concern was directed at the transformer conductors to the motor load and, I agree, they should not have to be any larger than Art 430 requires, but I can't justify that in 240.21
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Wow ! Super perception :D

I was actually looking at a job with both a motor load and lighting load fed by the same transformer.
I attempted to make my original post simple and ignore the lighting load, but then the XO question arose and I justified the XO ground.
My concern was directed at the transformer conductors to the motor load and, I agree, they should not have to be any larger than Art 430 requires, but I can't justify that in 240.21
So, you're telling me I guessed pretty close. If so, I'd say the whole design is screwed (here I'm using the precise engineering definition of "screwed" - not the common definition)

I'd be really curious on how the idea that only one disconnect was needed for motor loads and non-motor loads is justified. I have no clues on how to justify that one.

From the information we have, and knowing we don't have the rest of the story. I'd say at least two disconnects are needed and possibly three.

Questions (and possibly ask in this order):
1. How much of this is already installed?

2a. Is there an engineer of record figuring out the motor starting criteria?
2b. If there is not an engineer of record, who is figuring out the motor starting criteria?
2c. Anybody look at (2011) 430.24 and 430.28

3. Motor starting criteria
3a. How hard is the motor to start? Does anybody even know if the motor will start with 250A fuses?
3b. How important is it the motor start easily (VD at the motor during starting)?
3c. How important is it the lights not blink when the motor starts? HID could easily switch off for minutes.

4. What does the loadcalc show for the total loading?
4a. Is the 3/0 big enough for both the motor loads and the non-motor loads?
4b. Is the transformer big enough for the motor loads and the non-motor loads?

5. How important is it to keep the contractor happy?

I know this isn't what you asked, but this is not a case of bumping 25 feet of wire from 3/0 to 250kcmil and all is well. I'd say it is a mess - not a very expensive mess, but a mess just the same.

ice
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
So, you're telling me I guessed pretty close. If so, I'd say the whole design is screwed (here I'm using the precise engineering definition of "screwed" - not the common definition)

I'd be really curious on how the idea that only one disconnect was needed for motor loads and non-motor loads is justified. I have no clues on how to justify that one.
The non-motor load was already connected to the transformer when the electricain discoverd one piece of equipment was 208 not 480.
From the information we have, and knowing we don't have the rest of the story. I'd say at least two disconnects are needed and possibly three.

Questions (and possibly ask in this order):
1. How much of this is already installed?

A major portion


2a. Is there an engineer of record figuring out the motor starting criteria?
2b. If there is not an engineer of record, who is figuring out the motor starting criteria?
2c. Anybody look at (2011) 430.24 and 430.28
No engineer involved. Building does not meet State criteria for required drawings.

E/C and I both looked at 430.24 and 430.28 and did not feel they were applicable.


3. Motor starting criteria
3a. How hard is the motor to start? Does anybody even know if the motor will start with 250A fuses?
3b. How important is it the motor start easily (VD at the motor during starting)?
3c. How important is it the lights not blink when the motor starts? HID could easily switch off for minutes.
Data given in example is not actual load but was provided to simplfy the answer. One machine, multiple motors,, largest is 10 HP,

4. What does the loadcalc show for the total loading?
4a. Is the 3/0 big enough for both the motor loads and the non-motor loads?
4b. Is the transformer big enough for the motor loads and the non-motor loads?

Transformer will be 85% loaded. 3/0 feeds only the motor load.. a second transformer tap feeds the lighting.

5. How important is it to keep the contractor happy?
Never is :D

I know this isn't what you asked, but this is not a case of bumping 25 feet of wire from 3/0 to 250kcmil and all is well. I'd say it is a mess - not a very expensive mess, but a mess just the same.

ice

feed to motor disconnect was already installed. changing is not major, but I didn't want to require unwarranted.

Original post was more for "learning" in the event I encounter this situation again.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Augie -
It sounded like I was cutting at you - not my intent. All of my questions were retorical in nature. You already know all of this.

The more I thought about this scenerio, it is not that uncommon. Generally it happens when there is a large motor load and small auxiliary load at the end of a long line. One puts in as small a transformer as will start the motors. Most then have a small panelboard on the secondary with a main and two feeders - one to the motor loads and the second to the aux loads panelboard (simplified install description). I don't recall any where a small transformer with only aux loads was just there waiting with sufficient capacity for a relatively large motor load.

If there were separate transformer taps and disconnects for the lighting and for the motor machinery, and I felt any need to keep the contractor happy, I would consider approaching the AHJ with 430.28.2 and ask for a variance. No reason not to ask unless the embarrassment factor was too high. In this particular case, there are no science, physics, safety, reilability, or operability issues - only an empty legal formality requiring the copper change. Most of the AHJs I know would accept that. But they do like to see that stamp on the request/drawings.

... No engineer involved. Building does not meet State criteria for required drawings. ...

So the EC did a design-build? And the owner didn't require stamped drawings? As an owner's agent, I have seen very few that did a good job for the owner without severe fighting with the contractor.

As a contractor, I really liked them. Lot more money.


... Never is important to keep the contractor happy :D ...
I don't often get that option. Any contractor that has the skilled people to do the job well, people don't do dope, shows up on time every day, and is still dumb enough to work in the middle of nowhere at -40, I can't afford to piss them off.

... Original post was more for "learning" in the event I encounter this situation again.

Are you a state agent for the AHJ? Or an inspector for the owner? Probably doesn't matter too much to me. I won't have any ideas on how to react from an inspector's view point. All of my ideas will be on how to not get there in the first place.

My norm is to treat remods as though they were new installations - even if I am tying into existing equipment. I really don't like cobbing on to existing equipment adding another layer of installation over the top of a myriad of others. When I cobb onto an existing system, I like it to be clean - I don't like marginal. I think maginal cuts into the reliability. Luckily for me, most of my customers, want safe, reliabile, operable. And of course, they want cost effective - but not at the detriment of all else. They find the money.

Where the majorityof the loads are 480V and one is 208V, put in a dedicated transformer just for that piece of machinery. Leave the existing lighting stuff alone. All you add with putting industrial load on the lighting is poorer lighting with extra flicker.

Nothing wrong with a dedicated xfm feeding one load. If there is enough money to make it worthwhile - ask the AHJ for the variance.

Just some thoughts

ice
 

DARUSA

Senior Member
Location
New York City
Wow ! Super perception :D

I was actually looking at a job with both a motor load and lighting load fed by the same transformer.
I attempted to make my original post simple and ignore the lighting load, but then the XO question arose and I justified the XO ground.
My concern was directed at the transformer conductors to the motor load and, I agree, they should not have to be any larger than Art 430 requires, but I can't justify that in 240.21

I will say that from the transformer to the 250 amp fuse DS need to be 250 kcmil wire to meet the secondary conductor rules ( next size up not permited.) from the disconnect switch to the motor 3/0 AWG will be fine!!!!
 
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