Wye Delta starter feeder sizing

[hbeery10]

We are running two power feeds for 2 air compressors (480 VAC). Each air compressor has a 250 hp motor and a fan motor (didn't check for sure what the size of it is, but probably about 15 hp). The nameplate shows a total FLA (compressor and fan) of 313, but another nameplate shows a total amps of 325.6. I was told by both the installer and manufacturer that they have a wye delta starter. I had never heard of this before, but apparently it essentially lets the motor soft start. The compressor has a control box with a motor starter and overload built into it.

My question is how do I size the feeder and breaker for the MDP I am feeding it from? Does the wye delta start allow it to be a smaller feeder and OPD or not? Thanks a lot!

Garth

 

[jimdavis]

Wye-delta is a common starting technique for a motor of that size and creates a reduced starting current. Conductor sizing however is based on running current and not starting current. Your feeder or branch circuit conductors need to be sized at 125% of the compressor FLA plus 100% of the fan motor FLA. Since you have a nameplate rating for the whole package I would size the conductors at 125% of that.


As for the OCPD, The NEC directs us to table 430.52 regardless of the starting technique. However I would seek recommendations from the manufacturer since the wye-delta starter provides for a substantially reduced starting current.

 

[domnic]

wye - delta starter

wye - delta starter

When you use a wye - delta motor starter it starts on wye and runs on delta. at what point does it switch is it based on RPM or time or current and how does the wye use less current to start the motor are the motors wound for wye or delta or both ?

 

[roger]

The conductors from the controller to the motor shall be based on 58 percent of the FLA, see 430.22(C) (quoting 2011 and earlier)

Roger

 

[jimdavis]

The OP's question pertained to the wiring from his distribution panel to the machine. He stated that the compressor had an attached control panel containing the wye-delta starter. The conductors from the controller to the motor would be factory-wired in that case.

 

[roger]

The OP's question pertained to the wiring from his distribution panel to the machine. He stated that the compressor had an attached control panel containing the wye-delta starter. The conductors from the controller to the motor would be factory-wired in that case.

I was not disputing your post, I was pointing out that there are provisions to reduce the conductor sizes for Wye/Delta start applications but they apply to the conductors from the controller to the motor.

Roger

 

[Smart $]

When you use a wye - delta motor starter it starts on wye and runs on delta. at what point does it switch is it based on RPM or time or current and how does the wye use less current to start the motor are the motors wound for wye or delta or both ?

There are several methods used, but the most typical is a centrifugal switch... so based on rpm.

There are three windings with no interconnection. All six leads are brought out of the frame. Contactors are used to change connection from wye to delta. Wye connection is always 1/sqrt(3) voltage per winding than delta. With the frequency and inductance being the same, you get 1/sqrt(3) current per winding than delta.

1/sqrt(3) is roughly 58%

 

[jimdavis]

I was not disputing your post, I was pointing out that there are provisions to reduce the conductor sizes for Wye/Delta start applications but they apply to the conductors from the controller to the motor.

Roger

I didn't think you were disputing my post. I just thought perhaps you misread the OP. I myself do that sometimes if I read too quickly.

 

[roger]

I myself do that sometimes if I read too quickly.

Jim, I am extremely well versed with that and have grown fond of the taste of my foot.

I could have made my post more clear as to what I was saying.

Roger

 

[hbeery10]

Wye-delta is a common starting technique for a motor of that size and creates a reduced starting current. Conductor sizing however is based on running current and not starting current. Your feeder or branch circuit conductors need to be sized at 125% of the compressor FLA plus 100% of the fan motor FLA. Since you have a nameplate rating for the whole package I would size the conductors at 125% of that.


As for the OCPD, The NEC directs us to table 430.52 regardless of the starting technique. However I would seek recommendations from the manufacturer since the wye-delta starter provides for a substantially reduced starting current.

Another thing I can't figure out is why the total FLA is 313 (this is on a label inside the controller box) and the rated amps (shown on a label on the outside of the controller box) is 325.6? Either way, at 125% it looks like I need to run 600 kcmils based on 310.15(B)(16) 75C column. The weird thing is it looks like the lugs in the controller box for L1, L2, and L3 are only sized for 350 kcmils??? What did they size those off of??

As far as the OCPD, I did call the manufacturer this morning and tried to see if it could be reduced and the engineer I talked to basically wouldn't tell me anything, just that I needed to install per local codes. So... if I use an inverse time breaker based on 430.52, I would need an 800 amp breaker, correct?

My issue here is the existing MDP has a main breaker of 1000 amps. Hopefully I can find two 800 amp breakers for the compressors (remember, there are 2 compressors) that will fit in the panel. Hopefully, the compressors won't kick on at the same time and blow the main in the panel.

Thanks a lot for your comments!

 

[Smart $]

Another thing I can't figure out is why the total FLA is 313 (this is on a label inside the controller box) and the rated amps (shown on a label on the outside of the controller box) is 325.6? Either way, at 125% it looks like I need to run 600 kcmils based on 310.15(B)(16) 75C column. The weird thing is it looks like the lugs in the controller box for L1, L2, and L3 are only sized for 350 kcmils??? What did they size those off of??

Only the manufacturer can relieve those doubts. With HP stated, you have to go by values in Table 430.250, so 125% of 302A plus 21A for the fan. Yet, as you have assessed, you will need 600kcmil copper conductors, or go parallel 3/0. Sounds like a lug change is in order.

As far as the OCPD, I did call the manufacturer this morning and tried to see if it could be reduced and the engineer I talked to basically wouldn't tell me anything, just that I needed to install per local codes. So... if I use an inverse time breaker based on 430.52, I would need an 800 amp breaker, correct?

Not necessarily. Don't forget with a wye start, delta run motor, the starting current is reduced, whereas 430.52 establishes a maximum rating based on regular starting (i.e. across the line current on start). Nonetheless, you can go as high as 250%, even 300% under Exception 2 and still be compliant. To go less than 250% is possible...

My issue here is the existing MDP has a main breaker of 1000 amps. Hopefully I can find two 800 amp breakers for the compressors (remember, there are 2 compressors) that will fit in the panel. Hopefully, the compressors won't kick on at the same time and blow the main in the panel.

Two 800A breakers may exceed manufacturer limitations. Check panel label and/or documentation.

Sounds like the main was sized prior to these compressors being a known addition. Perhaps the reason for wye start, delta run motors being chosen. Hopefully someone at least did a load calculation. Should have done a feasibility assessment, too. Just the compressors alone allow up to a 1200A main. No other loads were mentioned, but I have to assume there are other loads for an existing panel. A 1000A main OCPD could actually be a violation of 430.63.

 

[Jraef]

Another thing I can't figure out is why the total FLA is 313 (this is on a label inside the controller box) and the rated amps (shown on a label on the outside of the controller box) is 325.6? Either way, at 125% it looks like I need to run 600 kcmils based on 310.15(B)(16) 75C column. The weird thing is it looks like the lugs in the controller box for L1, L2, and L3 are only sized for 350 kcmils??? What did they size those off of??

As far as the OCPD, I did call the manufacturer this morning and tried to see if it could be reduced and the engineer I talked to basically wouldn't tell me anything, just that I needed to install per local codes. So... if I use an inverse time breaker based on 430.52, I would need an 800 amp breaker, correct?

My issue here is the existing MDP has a main breaker of 1000 amps. Hopefully I can find two 800 amp breakers for the compressors (remember, there are 2 compressors) that will fit in the panel. Hopefully, the compressors won't kick on at the same time and blow the main in the panel.

Thanks a lot for your comments!

There are no provisions for using a smaller OCPD or FEEDER conductors on a motor circuit with ANY kind of reduced voltage starter, including Wye-Delta. The rules are exactly the same. The only difference is if it is a VFD, and in that case the only difference is that feeder conductors must be sized at 125% of the VFD max amps, not the motor HP from the tables in 430.

 

[Jraef]

When you use a wye - delta motor starter it starts on wye and runs on delta. at what point does it switch is it based on RPM or time or current and how does the wye use less current to start the motor are the motors wound for wye or delta or both ?

The transition is based on time, I have NEVER seen it done with anything else. There are special little timers, called "Star-Delta Timers" that are frequently used for this, because there must be an "open" transition delay, where the motor circuit is open, no power, to prevent the connections from creating a short circuit. But it can also be done with off-the-shelf timers, it just usually takes two of them, or a timer and a relay with special contacts that are "early-make / late-break". This starter is usually ONLY used by OEMs who want the cheapest thing they can use and call it "reduced voltage", but the entire complicated process can cause SERIOUS issues and is the reason why I NEVER use Wye-Delta, but that's just a personal preference based on some very expensive lessons learned. YMMV.

The reason for the lower current is that when started, the motor windings are connected in a Wye (star) pattern referenced to each other, so the effective voltage across the windings is reduced by the sq. rt. of 3, 1.732, so 1/1.732 = .577, hence why we say the voltage is reduced to 58%. When you reduce voltage on an accelerating motor, the torque is reduced by the SQUARE of the voltage reduction, and current follows torque. So your torque and current are reduced to .58 squared = .3333, or 33% of what it normally would be. The transition from Wye to full Delta / Run must take place AFTER the motor gets to at last 80% speed, otherwise the entire exercise was pointless. So in most cases with an OEM, the designer knows the acceleration time and torque curves so they can properly set that timer. Otherwise it is a total crap shoot, hence most people do not retrofit Wye-Delta starters to existing machines. That's why very few electricians have ever worked on one (another reason why I never use them).

And finally, yes, the motor must be wound and terminals brought out in such a way as to allow this to be done.

 

[GoldDigger]

Would the relay not need early break/ late make contacts rather than what you said?
Or interlocking contacts as used in a non-bridging transfer switch?

 

[Smart $]

There are no provisions for using a smaller OCPD or FEEDER conductors on a motor circuit with ANY kind of reduced voltage starter, including Wye-Delta. The rules are exactly the same. ...

The rules are exactly the same... and your statement is correct for feeder rating, as the provisions yield a minimum. However, there does not have to be any provisions for reduced OCPD, as it is already built into existing provisions. OCPD rating provisions yield a maximum. You can size the OCPD as low as you want, even so low it will trip every time the motor is started.

 

[Jraef]

The rules are exactly the same... and your statement is correct for feeder rating, as the provisions yield a minimum. However, there does not have to be any provisions for reduced OCPD, as it is already built into existing provisions. OCPD rating provisions yield a maximum. You can size the OCPD as low as you want, even so low it will trip every time the motor is started.

You are of course, right. Point taken.

If it were me however, I would NOT want to run the risk of nuisance tripping. Wye-Delta is usually used on larger motors, and larger motors usually have much more restrictions on the maximum number of starts-per-hour they can handle. So if you have a motor rated for 2 starts per hour and you use up one in a failed attempt that resulted in the breaker tripping, you had better make sure it doesn't happen again or you are down for an hour. If you have under-sized the breaker to try to save money, and you cannot start your machine, what have you saved?

 

[GoldDigger]

And, of course, if you have a device like an A/C compressor/fan whose nameplate lists Minimum Circuit Ampacity, you are not allowed to use a smaller breaker, and even that size may result in lots of nuisance trips.

 

[Jraef]

Would the relay not need early break/ late make contacts rather than what you said?
Or interlocking contacts as used in a non-bridging transfer switch?

Here in the US, we tend to rely upon the mechanical interlock and aux contacts on those contactors as the way to ensure everything works with the correct timing. But then some people fail to actually use the mechanical interlocks because they are impatient in building the starters, or the mechanical components wear and jam, so they get removed, then the assurance is gone and things blow up.

In IEC countries, they must have BOTH an open control transition, AND the mechanical interlocks so that if one system fails or gets removed, the other is still in place. That's why they do their control circuit differently with either the special timers or with added relays. Having worked at a couple of IEC manufacturers, once I learned that philosophy, I stuck with it. I had forgotten that NEMA starters don't do that until I looked just now.


So in this diagram, 3 opens and 4 does not close for a fraction of a second.

 

[Smart $]

You are of course, right. Point taken.

If it were me however, I would NOT want to run the risk of nuisance tripping. Wye-Delta is usually used on larger motors, and larger motors usually have much more restrictions on the maximum number of starts-per-hour they can handle. So if you have a motor rated for 2 starts per hour and you use up one in a failed attempt that resulted in the breaker tripping, you had better make sure it doesn't happen again or you are down for an hour. If you have under-sized the breaker to try to save money, and you cannot start your machine, what have you saved?

I'm not saying 'reduce' the rating to the point of nuisance tripping.

Here's one rationale for reducing the rating. NEC permits an OCPD rating of up to 300% FLA for over 100A FLA if 250% is insufficient for starting... but this assumes ATL starting. You yourself stated wye current will be 33%... and this includes LRC, correct? If so, logic follows that 1/3 of 300% (100%) FLA_DELTA OCPD should be sufficient to start a wye-start, delta-run motor (assuming the OCPD's trip curve exhibits the same characteristics of the larger OCPD). So now you have a range of 100% to 250% FLA_DELTA to get the motor started without applying the exception, while having a reasonable assumption of no nuisance tripping.

Anyway, my reason for even discussing a lower rated OCPD isn't to save money, but rather to get the rating within possible manufacturer OCPD rating limitations in the MDP.

 

[StarCat]

Some Plant Experience Notes

Some Plant Experience Notes

Wye start Delta Run is widely used for larger Centrifugal Chiller starting. AS JRAEF has suggested, I would not want to be around if there were a failure.
We had 3 each 1000 ton units.
The other thing noted on the ones we ran is they have a large set of bleed off resistors inside that are used at the transition point.
ON the air compressor subject, you can bet they will start at the same time possibly unless something is done control wise to moderate that.
We once installed a staging panel in a plant that had 6 duplex 5 HP air compressors. The plant Electrician had set it up where they were ALL brought on the line at once with the result that everyone would assume. The staging panel was " brilliantly " set up with a bunch of adjustable timer relays etc.
After the fact, the plant Electrician then went in and set all the timers to the SAME timeout.
Also before the retrofit, the main feed off that 6 comprssor line was running through a 3/4" fitting before it hit the plant.
Best laid plans of the bosses and living in a world where its near impossible to design around a lack of common sense.