power up refrigeration unit

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mikemac

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Hoping someone can help. I have a refrigeration unit here at the loading dock at work. I have 3 phase 480 volt power at the dock. The unit requires a 215 to 240 volt, 3 phase 10 horsepower 40 running amps feed. Am I right when I say I need a 480v to 220v transformer? If somehow I come out of a 120 volt 3 phase panel thats much further away, will I need a buck boost transformer to get my voltage up from 208 to 215/240? I'm trying to figure out the best option. Thanks.
 

charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Stepping down from nearby is better than stepping up from a remote supply. I'd go for a 3 phase, 30 KVA, 480-240 transformer, and install a local fused disconnect or enclosed breaker on the secondary, along with an appropriate outlet for the fridge.
 

philly

Senior Member
Hoping someone can help. I have a refrigeration unit here at the loading dock at work. I have 3 phase 480 volt power at the dock. The unit requires a 215 to 240 volt, 3 phase 10 horsepower 40 running amps feed. Am I right when I say I need a 480v to 220v transformer? If somehow I come out of a 120 volt 3 phase panel thats much further away, will I need a buck boost transformer to get my voltage up from 208 to 215/240? I'm trying to figure out the best option. Thanks.

Ignoring voltage drop of the remote feeders, the 208V supply will be sufficient for running this motor. 208V is about 96% of the lower end of the 215V rating of the motor, which is within the 10% tolerence.
 

mikemac

Member
Ignoring voltage drop of the remote feeders, the 208V supply will be sufficient for running this motor. 208V is about 96% of the lower end of the 215V rating of the motor, which is within the 10% tolerence.

Thanks for your reply Charlie. Philly, it says on the wiring info sheet for the trailor that "Lessee, at its expense, will install a buck boost transformer in the power supply line, if the unit running voltage is ever less than 215 volts". This is what scares me with the 208v.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
I agree with Charlie and definitely would not run equipment that specifies a "low end voltage" at a voltage lower than specs.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Stepping down from nearby is better than stepping up from a remote supply. I'd go for a 3 phase, 30 KVA, 480-240 transformer, and install a local fused disconnect or enclosed breaker on the secondary, along with an appropriate outlet for the fridge.
Agree.

Ignoring voltage drop of the remote feeders, the 208V supply will be sufficient for running this motor. 208V is about 96% of the lower end of the 215V rating of the motor, which is within the 10% tolerence.
Disagree.



I always prefer the higher end of a voltage range, or the greater voltage between dual voltages, when it comes to motors. I also prefer stepping down over stepping up, because of the higher currents involved with the lower voltage.
 

iwire

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Location
Massachusetts
Ignoring voltage drop of the remote feeders, the 208V supply will be sufficient for running this motor. 208V is about 96% of the lower end of the 215V rating of the motor, which is within the 10% tolerence.

A motor tagged only 230 volt is designed for a 240 volt supply and should not be connected to a 208 supply, that leaves no room for voltage drop or sags.

Of course there is a good chance it would run fine but it also might not. The low voltage might shorten the units life or cause the OLs to trip .... than you look like you do not know what you are doing.

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mxslick

Senior Member
Location
SE Idaho
What Bob and Charlie said, plus keep in mind that refrigeration compressors are frequently hard-starting and low voltage will easily burn them out despite the built-in overloads.

Step down from the 480 is your best and most economical bet. When you calculate the cost of running that LV feeder from the distant panel with labor, the transformer is gonna be cheaper.
 

mikemac

Member
A motor tagged only 230 volt is designed for a 240 volt supply and should not be connected to a 208 supply, that leaves no room for voltage drop or sags.

Of course there is a good chance it would run fine but it also might not. The low voltage might shorten the units life or cause the OLs to trip .... than you look like you do not know what you are doing.

ANSIC841.jpg
[/IMG]

Agreed guys. Especially when the motor its feeding is a refrigerated trailor full of meat. The major drawback I'm having is the cost of the xfmr ($1800.00 not counting labor) and room to install it.
But thanks to all for your feedback and advice.
 

mikemac

Member
Okay guys. We're going with the 480/240 volt transforemer. My next question is about calculation.
They are saying the specs are 215 to 240 volt, 3 phase, 10 horsepower, 40 running amps and to use a 60amp breaker with #6 wire.

Now that we are using a 480v to 240 v transformer (installed at the location of the motor), what is the wire size/amperage from the 480volt panel to the line side of the xfmr? And if anyone can show me the calculation, I'd appreciate it. Thanks for all the help.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
With a 2:1 voltage ratio, the current ratio will be 1:2, so the primary current will be half of the secondary current. #10 and 30a might work, but you may go up to #8 and 40a or 50a.

The 60a secondary breaker may not stand up to starting current. The wire is fine, but code will let you go higher for the breaker. Will you have a starter with proper overload protection?
 

mikemac

Member
With a 2:1 voltage ratio, the current ratio will be 1:2, so the primary current will be half of the secondary current. #10 and 30a might work, but you may go up to #8 and 40a or 50a.

The 60a secondary breaker may not stand up to starting current. The wire is fine, but code will let you go higher for the breaker. Will you have a starter with proper overload protection?

Larry, first of all thanks for your responses. If you don't mind, I'm going to give you all the info they gave me. I will type their words in bold. Some of it is a repeat of whats already stated above but here goes....

[I]215 to 240 volt, 3 phase, 10 horsepower, 40 running amps. Lessee at its expense, will install a buck boost transformer in the power supply line, if the running voltage is ever less than 215 volts[/I]
The buck boost would have been to up the voltage from a 3 phase 208 volt panel but I'm going to go with the 480 volt to 230 volt step down transformer.

Minimum of 215 to maximum of 240 volts, 3 phase, using a seperate 60 amp circuit breaker protected power line. (DO NOT USE FUSES)
Local electrical code will dictate the size of the power line required. Normally a number 6 gauge, 4 wire power line is adequate when the distance from the building electrical distribution panel to the electrical connection box located on the front of the trailer/container is less than 100 feet
.

All this leads me to trying to figure out (now that I'm using a transformer and knocking down the voltage) what the primary wire size and breaker should be and how to protect it.

Again, your help is really appreciated. Thanks!
 

david luchini

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Location
Connecticut
Occupation
Engineer
With a 2:1 voltage ratio, the current ratio will be 1:2, so the primary current will be half of the secondary current. #10 and 30a might work, but you may go up to #8 and 40a or 50a.

The 60a secondary breaker may not stand up to starting current. The wire is fine, but code will let you go higher for the breaker. Will you have a starter with proper overload protection?

I don't think the #6 secondary wire would be fine if you increase the secondary breaker size. It might not be large enough even using the 60A breaker size. The specs are listed as 40A, and use #6 wire. 40Ax1.25=50A. #8 has an ampacity of 50A at 75 deg, so lets assume they are using the 60 deg column, and the #6 has an ampacity of 55A.

Per 240.21(C)(1), the secondary conductors can be protected by the primary OCPD on a delta-delta, 3 wire transformer if the primary OCPD value does not exceed the secondary ampacity x sec/pri xfmr ratio. 55A x 0.5 = 27.5. The 30A (or 40A or 50A) primary breaker would not work for the 55A ampacity of the #6 secondary conductors.

If instead, you wanted to use the secondary breaker to protect the secondary conductors per 240.21(C)(2), then the ampacity of the secondary conductors must not be less than the rating of the OCPD where the secondary conductors terminate. The 55Amps of the #6 is less than the 60A c/b.
 

mxslick

Senior Member
Location
SE Idaho
I don't think the #6 secondary wire would be fine if you increase the secondary breaker size. It might not be large enough even using the 60A breaker size. The specs are listed as 40A, and use #6 wire. 40Ax1.25=50A. #8 has an ampacity of 50A at 75 deg, so lets assume they are using the 60 deg column, and the #6 has an ampacity of 55A.

Per 240.21(C)(1), the secondary conductors can be protected by the primary OCPD on a delta-delta, 3 wire transformer if the primary OCPD value does not exceed the secondary ampacity x sec/pri xfmr ratio. 55A x 0.5 = 27.5. The 30A (or 40A or 50A) primary breaker would not work for the 55A ampacity of the #6 secondary conductors.

If instead, you wanted to use the secondary breaker to protect the secondary conductors per 240.21(C)(2), then the ampacity of the secondary conductors must not be less than the rating of the OCPD where the secondary conductors terminate. The 55Amps of the #6 is less than the 60A c/b.

Since we're talking a refrigeration unit, I see no problem with the #6 secondary wire OR the manufacturer's required 60amp breaker. The running amps of the unit are well within the ampacity of the #6, and the 60 amp breaker can provide the required local disconnecting means.

I see no reason to over think this.
 

mikemac

Member
Since we're talking a refrigeration unit, I see no problem with the #6 secondary wire OR the manufacturer's required 60amp breaker. The running amps of the unit are well within the ampacity of the #6, and the 60 amp breaker can provide the required local disconnecting means.

I see no reason to over think this.

What I'm trying to figure out is what size wire and protection for the primary.
 

Volta

Senior Member
Location
Columbus, Ohio
What I'm trying to figure out is what size wire and protection for the primary.

The primary protection will be allowed to be 250% of the transformer's primary rating. What size transformer are you using?

If 25kVA, then you can protect it as low as 30 amps and as high as 75 per T450.3(B). Use the appropriate wire size depending on terminations and ambient temperature.
 

mikemac

Member
The primary protection will be allowed to be 250% of the transformer's primary rating. What size transformer are you using?

If 25kVA, then you can protect it as low as 30 amps and as high as 75 per T450.3(B). Use the appropriate wire size depending on terminations and ambient temperature.

Thanks Volta. I'm using a 480/240v 30Kva transformer.
 

Volta

Senior Member
Location
Columbus, Ohio
Thanks Volta. I'm using a 480/240v 30Kva transformer.

Then a 36 amp primary allows full use of the transformer capacity, you can go up to 90 amps for the 250%.

I would note several things:

210.63, and install a receptacle.

210.5(C). This will be the third nominal voltage system in the premises, so you will need to derive a method of identification to differentiate the nine separate phase legs on the property, or at least the three new ones.

200.6 If your Separately Derived System will be grounded (I'm sure it will be), then the Grounded Conductor will either be the third neutral, though only needs distinctive marking if sharing a box with one of the first two, or if corner-grounded it will be a phase leg, still the same.
 
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