Motor contribution and AIC

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chris kennedy

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I have a situation where I am in question of the EE's calcs.

I have a 150 KW 208y/120 POCO pad mount transformer within 30' of the service. Z=3.66. Conductors are parallel 500's in PVC.

He has the available fault current at 26k. How does the motor contribution figure into this?

Thank you.
 
chris kennedy said:
I have a situation where I am in question of the EE's calcs.

I have a 150 KW 208y/120 POCO pad mount transformer within 30' of the service. Z=3.66. Conductors are parallel 500's in PVC.

He has the available fault current at 26k. How does the motor contribution figure into this?

Thank you.
Click to expand...

I often, but not always, add 100% of the FLA of operating motors. So, for many non-industrial installations (places where the maximum size motor is 40HP @480V), motor contribution is fairly negligible. I believe your engineer is doing a standard practice of anticipating the POCO transformer may some day be doubled in size.
 
jim dungar said:
I often, but not always, add 100% of the FLA of operating motors. So, for many non-industrial installations (places where the maximum size motor is 40HP @480V), motor contribution is fairly negligible. I believe your engineer is doing a standard practice of anticipating the POCO transformer may some day be doubled in size.
Click to expand...

Thanks Jim, but my question is how do motor loads increase the available fault current?
 
chris kennedy said:
This makes sense, but how does one figure the magnitude and duration of the motors contribution?
Click to expand...

Excerpt from Eaton/Cutler Hammer Consulting Application Guide, Section 01 ? Power Distribution Systems

To determine the motor contribution
during the first half-cycle fault current,
when individual motor horsepower
load is known, the subtransient reactances
found in the IEEE Red should
be used in the calculations. When the
system motor load is unknown, the
following assumptions generally
are made:

Induction Motors ? Use 4.0 times
motor full load current (impedance
value of 25%).

Note: For motors fed through adjustable
frequency drives or solid-state soft starters,
there is no contribution to fault current, unless
1) they have an internal run contactor or
2) they have a bypass contactor.

Synchronous Motors ? Use 5.0 times
motor full load current (impedance
value of 20%).

When the motor load is not known,
the following assumptions generally
are made:

208Y/120 V Systems
■ Assume 50% lighting and 50%
motor load.
or
■ Assume motor feedback contribution
of twice full load current of
transformer.
or

240/480/600 V 3-Phase, 3-Wire or
4-Wire Systems
■ Assume 100% motor load.
or
■ Assume motors 25% synchronous
and 75% induction.
or
■ Assume motor feedback contribution
of four times full load current
of transformer.

480Y/277 V Systems in Commercial Buildings
■ Assume 50% induction motor load.
or
■ Assume motor feedback contribution
of two times full load current
of transformer or source.
Click to expand...
 
Smart $ said:
Excerpt from Eaton/Cutler Hammer Consulting Application Guide, Section 01 ? Power Distribution Systems
Click to expand...

Nice Smart, thanks, I saved that.

Here are the EE's calcs. Do theses raise eyebrows other than mine?

No motor contribution 20,041 A.I.C.
W/motor contribution 23,376 A.I.C.

Trans. Full L. Amps 834 Amps

Trans size 300kva

Trans Impedance 4.166%

Fault current at main

No motor contribution 17,364 A.I.C.

w/motor contribution 20,669 A.I.C.

wire size 350 AWG(wrong, 500's)

length of runns 40 Feet

number of runs 2

Call me kookie, but this value seems very low to me.


 
chris kennedy said:
Call me kookie, but this value seems very low to me.
Click to expand...
Ok kookie :D-
My first guess is the engineer did not have the installed equipment data when the calc was done.

No, 26ka does not appear low for a 150kVA or a 300kVA, 350kcmil or 500kcmil. It seems a bit high. Where did the 26kA number in post 1 come from? It doesn't match any of the numbers you show in post 7.

Just curious, since the xfm is POCO, do the 500kcmil fall under the NEC as a service lateral?

cf
 
Chris,
300kVA, 4.166% Imp @ 208V 3PH, I get 20,012A with no Utility impedance (infinite primary) and no motor contribution. Keep in mind, that an infinite primary calculation is theoretical and will always be lower because the utility substation is not next to the building (or is it?).
What kind of building is this?
If it is an industrial building with lots of motors, conveyors, etc, or a commercial building?
Most folks discount motor contribution for motors less than 50hp, as they do not contribute enough to make it worth while tracking.
23kA with motor load doesn't seem impossible if the motor load is small.
 
Cold Fusion said:
Where did the 26kA number in post 1 come from? It doesn't match any of the numbers you show in post 7.
Click to expand...

That was of the top of my head before I went out and got the plans. Sorry for the confusion.

benaround said:
Chris, I would be concerned with an EE using AIC for Available Fault Current calcs.
Click to expand...

Typo? That is weird.

ron said:
What kind of building is this?
If it is an industrial building with lots of motors, conveyors, etc, or a commercial building?
Click to expand...

Mixed use, AC motor loads.

Thank you all for the help.
 
Not to go off topic, but it seemed related to the AIC...
In terms of an increase of AIC, would a large enough stage lighting dimmer rack contribute to the available fault current?
 
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