Machine Tool Transformer

[Gssc1414]

I am seeking some assistance in sizing wire sizes for a transformer that will supply power to a machine tool. I do not have a copy of the NEC code, so it's not that I am lazy and just won't look it up... The code book is on it's way, but I can't wait the week at this point, so I am hoping some nice folks here won't mind holding my had thru these calculations.

Project Scope:

• We are outfitting a vertical machining center (standard machine tool) that runs on 208vac with a 480vac -> 208vac transformer (45KVA)
• Before the transformer, there needs to be a fusible disconnect, which will be located in a separate electrical box, mounted to the side of the machine tool.
• The Machine tool comes standard with a 125amp MCCB
• So the plant bus bar will have a power drop (4 wire) directly to the fusible disconnect. Then, I need to wire fusible disconnection -> transformer -> Machine tool MCCB.
• The wires running from the disconnect box to the transformer will be inside rigid conduit.
• The wire running from the transformer to the machine tool will be inside rigid conduit.

I did the following:

With the help of this site http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm I did the following calculations:

Primary Current: 54.1 amps
Secondary Current: 124.9 amps
Primary Current Protection: 54.1*1.25 = 67.7 -> Next size up is 70 amp (Class J fuse is required).

Primary Conductor Size (Table 310.16)(will be using THHN, 90?C, copper): 6awg

• Because these are running thru conduit, do I need to apply a correction factor?

Primary Ground Size (Table 250.122): 8awg

• Based off 70amp current protection

**I have control of the wire type after the fusible disconnect, but before will be whatever the plant uses. Typically they use a cable, so in order to ensure adequate ampacities - should i spec the wire size and type taking into account the 80% correction factor (see below about wires in raceway/cable)?

Secondary Conductor Size(will be using THHN, 90?C, copper): ??awg

• This is where I get confused. My confusion comes from two things:
  • 1) Because the machine has an MCCB, I would think it would be considered protection for the secondary side of the transformer. So with that, do you size the secondary conductors based off the MCCB or based off the FLA of the secondary side of the transformer? I know it's the same in my case, but this won't always be the case.
  • Because there will be 4 wires are running through conduit (each phase and ground)- I think i need to apply a correction factor, however the correction factors are listed from "raceway" or "cable". Do i use the 80% correction factor anyways?
• So my initial shot would be: 125amp * 1.25 = 156.25amp *100/80 (correction) = 195.2amp -> Have to use 2/0 THHN wire
  • Could the MCCB being secondary protection change the requirements on these conductors?

My next big area where I am confused is grounding/bonding.

• Per the above, right now, I have a power drop with an 8awg ground. This ground connects into the disconnect box(terminal block inside of the box). From the disconnect box, Another 8awg ground will run to the transformer where a grounding bar will be mounted to the inside of the transformer enclosure.
• So I am confused for what sizes I need for the following connections
  • Transformer grounding strip -> X0 terminal on transformer (think I need 6awg based on area of hot ungrounded conductor on primary)
  • Transformer grounding strip -> transformer case
  • Transformer grounding strip -> Machine tool

Lastly, the machine tool came from the factory wired with 3awg (25mm?) wire from the MCCB to the first device (which is then daisy chained to other devices). I cannot see how this is ok as the ampacity of 3awg is 110amp at best (THHN @ 90?C). I would think I need to replace this with larger wires so that they are protected by the MCCB.


Sorry for the lengthy post. I just wanted to try and be as clear as possible and also show that I am trying to logically work thru this and not just get someone else to do all the work from me. Thanks for any and all help, I appreciate it!

 

[Gssc1414]

Is there something wrong with my op? I see there are many views, but no responses. Perhaps it was too wordy? I apologies again for the length - was just trying to get all the information out there.

 

[petersonra]

Is there something wrong with my op? I see there are many views, but no responses. Perhaps it was too wordy? I apologies again for the length - was just trying to get all the information out there.

Maybe people were too busy to respond.

I don't have any problems with your calculations however I think you are restricted to final ampacity not to exceed the 75 deg C column after any adjustment factors have been applied. Unless it is unusually warm there, I don't see any adjustment factors needed. I don't see that you can protect a #6 with a 70A fuse.

You have to protect both the primary and the secondary of the transformer and the primary and secondary conductors. Usually you can arrange things so the same OCPD can protect the primary and the primary conductors, and another OCPD can protect the secondary and the secondary conductors.

I don't know what 80% factor you are trying to add in. Generally, you can start with the 90 deg C column ampacities but after all adjustments are made you can't exceed the 75 deg C column.

In any case the ground wire is not a current carrying wire so no adjustment is needed there.

I do not think you are allowed to take advantage of the next higher standard size fuse rating on the primary side. However you are permitted to go to 250% on the primary side OCPD. If you were to do that the wire would get bigger.

 

[Gssc1414]

Maybe people were too busy to respond.

I don't have any problems with your calculations however I think you are restricted to final ampacity not to exceed the 75 deg C column after any adjustment factors have been applied. Unless it is unusually warm there, I don't see any adjustment factors needed. I don't see that you can protect a #6 with a 70A fuse.

You have to protect both the primary and the secondary of the transformer and the primary and secondary conductors. Usually you can arrange things so the same OCPD can protect the primary and the primary conductors, and another OCPD can protect the secondary and the secondary conductors.

I don't know what 80% factor you are trying to add in. Generally, you can start with the 90 deg C column ampacities but after all adjustments are made you can't exceed the 75 deg C column.

In any case the ground wire is not a current carrying wire so no adjustment is needed there.

I do not think you are allowed to take advantage of the next higher standard size fuse rating on the primary side. However you are permitted to go to 250% on the primary side OCPD. If you were to do that the wire would get bigger.

Thanks for your response. I understand if people are busy - I was just asking if there was some reason why no one had responded, because the post was so long.

About the 75?C column. I re-read the instructions on the website that I linked in the original post it says "Step 3. Size Primary Conductor: Feeder conductors supplying continuous loads shall be sized no less than 125 percent of the continuous loads based on the conductor ampacities as listed in Table 310.16, before any ampacity adjustment in accordance with the terminal temperature rating [110.14(C) and 215.2(A)(1)]."

So it looks like I read it wrong. I would need to use 4awg wire. Do you agree with this? Then the 70amp fuse can protect it.


Currently I have the fusible disconnect protecting the primary and primary conductors and the MCCB protecting the secondary and the secondary conductors. Is this not ok?


The 80% factor is because there are 4 current carrying conductors. Per the link I posted in the OP, it says that if there are 4 or more current carrying conductors (and specifically says the ground is considered current carrying), then you have to apply a correction factor of 0.8. Now this is listed as for raceway & cables. Rigid conduit is not explicitly listed... so I don't know if this rule applies to me or not. I could also be completely off and this has nothing to do with my situation.
Here is what I read: "But?. Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity shall be reduced in accordance with Table 310.15(B)(2)(a). For our examples, there are four current-carrying conductors on the secondary [neutral considered current carrying 310.15(B)(4)(c)], therefore the conductor ampacity after adjustment [based on 90?C ampacity [110.14(C)], must be no less than 175A for the 45 kVA transformer and 400A for the 112.5 kVA transformer."



I did the next higher fuse size because of a blurb I read on the same site I got all the other quotes:
"The primary winding of transformers shall be protected against overcurrent in accordance with the percentages listed in Table 450.3 and all applicable notes. Where 125 percent of the primary current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker as listed in 240.6(A), the next higher rating can be used [Note 1]."
The 250% rule confuses me. Why would you want to go to 250% on the primary OCPD? Doesn't this just mean bigger wires, larger fuses? What would the benefit to it be? Does this rule only apply if you don't have an OCPD on the secondary?


Thanks for your help and patience... just trying to understand this stuff.

 

[petersonra]

About the 75?C column. I re-read the instructions on the website that I linked in the original post it says "Step 3. Size Primary Conductor: Feeder conductors supplying continuous loads shall be sized no less than 125 percent of the continuous loads based on the conductor ampacities as listed in Table 310.16, before any ampacity adjustment in accordance with the terminal temperature rating [110.14(C) and 215.2(A)(1)]."

The load is based on the actual devices on the secondary side the xfmr is powering and not the rating of the xfmr. Most of us tend to use the rating of the xfmr as the load just to allow future use of the entire xfmr, but it is not required.

So it looks like I read it wrong. I would need to use 4awg wire. Do you agree with this? Then the 70amp fuse can protect it.

ok

Currently I have the fusible disconnect protecting the primary and primary conductors and the MCCB protecting the secondary and the secondary conductors. Is this not ok?

I never said it wasn't.

The 80% factor is because there are 4 current carrying conductors. Per the link I posted in the OP, it says that if there are 4 or more current carrying conductors (and specifically says the ground is considered current carrying), then you have to apply a correction factor of 0.8.

Ground is never considered a CCC. Grounded conductors (e.g-the neutral) may or may not be CCC.

Now this is listed as for raceway & cables. Rigid conduit is not explicitly listed... so I don't know if this rule applies to me or not. I could also be completely off and this has nothing to do with my situation.

Rigid conduit is a raceway.

Raceway. An enclosed channel of metal or nonmetallic materials
designed expressly for holding wires, cables, or busbars,
with additional functions as permitted in this Code.
Raceways include, but are not limited to, rigid metal conduit,
rigid nonmetallic conduit, intermediate metal conduit, liquidtight
flexible conduit, flexible metallic tubing, flexible
metal conduit, electrical nonmetallic tubing, electrical metallic
tubing, underfloor raceways, cellular concrete floor raceways,
cellular metal floor raceways, surface raceways, wireways,
and busways.

Here is what I read: But?. Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity shall be reduced in accordance with Table 310.15(B)(2)(a). For our examples, there are four current-carrying conductors on the secondary [neutral considered current carrying 310.15(B)(4)(c)], therefore the conductor ampacity after adjustment [based on 90?C ampacity [110.14(C)], must be no less than 175A for the 45 kVA transformer and 400A for the 112.5 kVA transformer."

You can start with the 90 deg C column for derating purposes, but the final number can't be any higher than the 75 Deg C column.

I did the next higher fuse size because of a blurb I read on the same site I got all the other quotes:
"The primary winding of transformers shall be protected against overcurrent in accordance with the percentages listed in Table 450.3 and all applicable notes. Where 125 percent of the primary current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker as listed in 240.6(A), the next higher rating can be used [Note 1]."

The correct table to use is 450.3(b). Your situation has both primary and secondary protection so you can go up to 250% on the primary. Read the table carefully. Note 1 only applies to secondary protection where you have both primary and secondary protection.

The 250% rule confuses me. Why would you want to go to 250% on the primary OCPD? Doesn't this just mean bigger wires, larger fuses? What would the benefit to it be? Does this rule only apply if you don't have an OCPD on the secondary?

The most important reason is to reduce nuisance trips on power up. It is only allowed if you have protection on both. And yes it does result in larger wires, but that is a design trade-off that is often worth making to reduce the chances of nuisance trips.

As for why no one else responded it may be that the post has some pretty basic questions and people are afraid that the poster is going to do something based on their comments and not fully understand the comments and get someone hurt. I almost didn't for that very reason.

This is the kind of thing that a more experienced person could help you with.