voltage drop start up ?

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Sierrasparky

Sierrasparky

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USA
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Electrician ,contractor
What is the best way to deal with conductor sizing and voltage drop for starting currents of motors, compressors , or refrigeration?
Do we size for starting current or normal running current.
 
I follow the same philosophy that CharlieB mentioned in a thread earlier this month, and that's to use the fire pump voltage drop criteria in NEC 695.7, 5% running VD and 15% on startup.
 
mayanees said:
I follow the same philosophy that CharlieB mentioned in a thread earlier this month, and that's to use the fire pump voltage drop criteria in NEC 695.7, 5% running VD and 15% on startup.
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So for start-up do you use the max current that the refrigerant compressor uses or the current just after before the unit stabilizes.

For example:
The initial start can be 25 amps for a second or so. Then it drops to 15 amps , then calms down to 7 amps.
 
Sierrasparky said:
So for start-up do you use the max current that the refrigerant compressor uses or the current just after before the unit stabilizes.

For example:
The initial start can be 25 amps for a second or so. Then it drops to 15 amps , then calms down to 7 amps.
Click to expand...

If you follow the MCA on the name plate your voltage drop is already incorporated into ther formula. AHRI standard for refrigeration equipment voltage drop is 2%.
 
Sierrasparky said:
So for start-up do you use the max current that the refrigerant compressor uses or the current just after before the unit stabilizes.

For example:
The initial start can be 25 amps for a second or so. Then it drops to 15 amps , then calms down to 7 amps.
Click to expand...

E'rr I guess this is only an example, because most motor loads especially high torque applications such as compressors including air compressors, can pull up to the LRA when starting which is about 6 to 10 times the FLA, a refrigeration compressor that pulls 7 amps normally can have as high as 42 to 70 amps at start up which is why we are allowed to have a larger breaker then the wires are sized for.

But as has been pointed out, for a branch circuit if you size the circuit for the minimum circuit size and breaker for the max circuit size the compressor will be fine, if you have a long run say over 60' then you need to think about up sizing the conductors, I use a "up one size" for every 60' run (one-way) in length of conductor rule which works out good, so for a 120' run I would up size my conductors two wire sizes over the minimum circuit size, also some manufactures also have one up size built in to their MCA, I had a condenser unit that had a 13 amp FLA but the MCA was 25 amps so one up size was already built in.

The only time this can be a problem is when a compressor is short cycled which is why I tell all my customers to make sure their HVAC installer installs a time delay to prevent short cycling because the head pressure will not have time to equalize and it will force the overload to trip which is not a good thing for the compressor to go through, most newer electronic furnace boards and thermostats have this protection built in but I would make sure.

Also if you have a feeder or service conductors that are long it can be important to think about motor starting current when sizing them to prevent dimming lights, have a 150 foot run of #4 on a 100 amp service that is loaded to 65 amps, and the AC unit will easily cause a voltage drop that will cause lights to dim when it starts up, which can make for some irritated customers who think because the house is new this shouldn't be.

The other thing to watch out for is when you have two or more AC units on a whole house generator, when power goes out it is possible that both units could be calling for cooling and starting up both can bring even a good size generator to its knees, I did a fix once for a friend had already had a generator installed (17kw) and he had a 3 ton and a 5 ton AC unit, who ever installed it did a poor job of calculating the demand load, it was installed in the late fall at a time they were not using the AC, but the next summer when he tried to use both AC units the generator about stalled, if the 3 ton was already fired up and the 5 ton tried to fire up it would stall out the generator, so I had to install a simple time delay and interlock system that would shut down the 3 ton and let the 5 ton start then bring the 3 ton back online, it worked perfectly and he never had a problem, it still running that way after 13 years, the circuit only ran while on generator power also, saved him from having to buy a new larger generator.
 
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Gregg Harris said:
If you follow the MCA on the name plate your voltage drop is already incorporated into ther formula. AHRI standard for refrigeration equipment voltage drop is 2%.
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MCA doesn't incorporate voltage drop, it is the minimum ampacity of conductor needed to supply the equipment, should you need to apply ampacity adjustments to the supply conductor your final adjusted ampacity still needs to be equal or greater than the MCA, but making ampacity adjustments does result in larger conductors and less voltage drop.

A larger conductor can lead to higher starting current also, there is pro's as well as con's to this. Reduced voltage is a starting method often used to limit the surge on a line when starting large motors and a some reduction of voltage for smaller motors doesn't hurt either - it is kind of a crude "soft starter" to some extent.
 
ptonsparky said:
Add some length to the circuit and the MCA will mean little. I believe the MCA is on the unit because HVAC techs could not do the math for motor loads.
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Canton said:
:lol:....lol....you should see the ones I work around...lol....:lol:
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I can believe it, if they put actual conductor size instead of ampacity on the nameplate I'm guessing you would still see some that can't seem to get that right.
 
kwired said:
MCA doesn't incorporate voltage drop, it is the minimum ampacity of conductor needed to supply the equipment, should you need to apply ampacity adjustments to the supply conductor your final adjusted ampacity still needs to be equal or greater than the MCA, but making ampacity adjustments does result in larger conductors and less voltage drop.

A larger conductor can lead to higher starting current also, there is pro's as well as con's to this. Reduced voltage is a starting method often used to limit the surge on a line when starting large motors and a some reduction of voltage for smaller motors doesn't hurt either - it is kind of a crude "soft starter" to some extent.
Click to expand...

That is correct the formula does not incorporate for voltage drop. It is telling you that from point A to point B is X. So depending on the distance the conductor may need to be increased to deliver X at the utilization equipment.
 
kwired said:
A larger conductor can lead to higher starting current also, there is pro's as well as con's to this. Reduced voltage is a starting method often used to limit the surge on a line when starting large motors and a some reduction of voltage for smaller motors doesn't hurt either - it is kind of a crude "soft starter" to some extent.
Click to expand...

Lower then rated voltage or voltage drop is not a good thing for most single phase AC motors (Asynchronous), voltage drop increases slip which cause the motor to take longer to reach full sync speed as well as raise the FLA, the more a motor slips the higher the current it will pull and the more heat damage it will cause to the windings, sure a larger conductor will allow a higher starting current, but it will be the design starting current and the motor will reach full speed much faster which will not produce as much winding heat, running a motor up to 5% over its design voltage can even reduce the time the motor takes to reach full speed thus reducing the amount of heat developed while starting, torque is also increased as well as the motor will have less slip, keep in mind that a motor starting up with the full rated voltage will never pull more then it's designed LRA or start up amps.

Take a look at this chart:


Notice that starting torque is +10% at 5% over voltage, while starting amps stayed proportional to the over voltage, this means that the motor will reach full running speed allot faster which doesn't allow time for the windings to heat, FLA and EFF remain normal, but with a 5% VD you have the direct opposite, you have reduced starting torque to -10% EFF has dropped while FLA has increased for the same given load, so the motor now takes longer to reach full run speed (longer time at LRA), has more slip and runs hotter as well as produces more heat in the windings.

The only time a lower voltage is good for a motor is when the motor is over sized for the load, a motor that is loaded 75% can gain a little EFF back by reducing the voltage 5%

High torque applications such as air compressors, and HVAC compressors can actually run better and last longer with a 5% over voltage, while a constant torque applications such as pumps and fans or blowers can have a slight negative effect and be better with rated voltage to no more then 2% over voltage.

Reduced voltage starting is done on 3 phase 12 lead motors and is a whole different animal then a single phase HVAC compressor, very rarely are these reduced voltage set ups started under a load, and never used on a single phase motor without controlling the frequency/voltage/current, do that to an air compressor and the motor would most likely stall.

Of course the above doesn't apply to universal (AC/DC) motors found in most hand tools, as these motors are voltage speed controlled but also loose allot of torque so can be stalled much easier at a reduced voltage which can be damaging.
 
I would really like to know how oversized wire can add to starting current? The larger the conductor the less volatge drop , thus normal starting torque as the motor then starts with rated voltage and normal starting LRA is accomplished. Overvoltage can be detrimental to a motor as well as undervotage. Keep the voltage within Nema tolerance which is 5% above or below nameplate and vitually any single or 3 phase motor will do its job as designed without any overheating.Above 5% of nameplate voltage and you will see the saturation curve start to affect the current. Get to 10% and look out! . Have seen many 3 phase motors 460 volt which have been operated on nominal 3 phase systems of 490-495 that were crispy critters!
 
stew said:
I would really like to know how oversized wire can add to starting current? The larger the conductor the less volatge drop , thus normal starting torque as the motor then starts with rated voltage and normal starting LRA is accomplished. Overvoltage can be detrimental to a motor as well as undervotage. Keep the voltage within Nema tolerance which is 5% above or below nameplate and vitually any single or 3 phase motor will do its job as designed without any overheating.Above 5% of nameplate voltage and you will see the saturation curve start to affect the current. Get to 10% and look out! . Have seen many 3 phase motors 460 volt which have been operated on nominal 3 phase systems of 490-495 that were crispy critters!
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I am going to venture that most of the 480v motors in this area operate closer to the 490+ range than anywhere close to 460. No problems.
 
stew said:
I would really like to know how oversized wire can add to starting current? The larger the conductor the less volatge drop , thus normal starting torque as the motor then starts with rated voltage and normal starting LRA is accomplished. Overvoltage can be detrimental to a motor as well as undervotage. Keep the voltage within Nema tolerance which is 5% above or below nameplate and vitually any single or 3 phase motor will do its job as designed without any overheating.Above 5% of nameplate voltage and you will see the saturation curve start to affect the current. Get to 10% and look out! . Have seen many 3 phase motors 460 volt which have been operated on nominal 3 phase systems of 490-495 that were crispy critters!
Click to expand...

You kind of answered your own question. The larger the conductor the less resistance, the more current can be delivered to the motor. This does result in more torque and faster acceleration. Exactly how it effects heating of windings is a little more complicated, as you need to consider torque demand and acceleration time in there as well. More torque demand will increase current, undersized conductors (or a soft starter) will limit that current but just how the current and time all balance out is what means the most to how hot the motor windings will get, as well as how frequently the motor is started. Throw in the features of a VFD and properly regulate the frequency when the voltage is lowered and you can run at lower voltage indefinitely - provided you still have adequate air flow for cooling.


Light load conditions usually result in utility delivered voltages around here of right near 215, 250, 500 for 208, 240, 480 volts nominal systems respectively. We really don't see many motors that have burned up because of overvoltage, if we do it is usually a condition where voltages were higher then those I mentioned.
 
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