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Air Compressor feeder conductor sizing #219051
01/13/18 05:49 PM
01/13/18 05:49 PM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
Did some calculations for feeder conductors for a new 3PH, 575V, 25HP Rotary Screw Compressor. Then I did them several more times and I still feel like I am missing something. Friday morning I contacted the manufacturer through email but I wasn't able to see if they responded as I had an appointment that afternoon. I will check Monday morning.

Here's my concern - inside the unit's control panel, where I will be terminating the feeder conductors, I see #6 AWG TEW conductors coming off the load side of the terminal block. That is significantly bigger than what I calculated for.

Then I did some research on the 'net and found this article from a different air compressor manufacturer (which may be comparing apples to oranges):

https://www.generalairproducts.com/...-air-products-air-compressors-explained/

I will definitely be interested in talking to the manufacturer of this compressor to see what they recommend. If there was any electrical installation recommendations that came with this unit I couldn't find it.

Anybody have experience with compressors of this size and finding their conductor sizing calculations differ from what the manufacturer recommends? Thank-you.


A malfunction at the junction
--------------------------------------
Dwayne
Tools for Electricians:
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219056
01/14/18 02:11 PM
01/14/18 02:11 PM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,225
Estero,Fl,usa
Just using the table (430.250) as a starting point, I see a FLA of 27a times 1.25% gets you to 33.75 or 8ga wire. If you see #6 there it is because they think that compressor may be starting under a load and be looking at LRA or a significant portion of it, longer than you would normally assume, based on the article you posted.
They normally deal with this in air compressors by using an "unloader" that drops pressure between the tank check valve and the compressor. HVAC systems use a time delay to allow the head to drop so they are not trying to compress liquid refrigerant.


Greg Fretwell
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219057
01/14/18 02:40 PM
01/14/18 02:40 PM
T
twh  Offline
Member
Joined: Mar 2004
Posts: 940
Regina, Sask.
Maybe they use the same compressor and wiring for other voltages and they don't change wire size just for Canada.

Re: Air Compressor feeder conductor sizing [Re: gfretwell] #219058
01/14/18 06:04 PM
01/14/18 06:04 PM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
Originally Posted by gfretwell
Just using the table (430.250) as a starting point, I see a FLA of 27a times 1.25% gets you to 33.75 or 8ga wire. If you see #6 there it is because they think that compressor may be starting under a load and be looking at LRA or a significant portion of it, longer than you would normally assume, based on the article you posted.
They normally deal with this in air compressors by using an "unloader" that drops pressure between the tank check valve and the compressor. HVAC systems use a time delay to allow the head to drop so they are not trying to compress liquid refrigerant.


Appreciate your feedback Greg.

Here's better information on the numbers I used to calculate conductor size, voltage drop, and O/C protection based on the 2015 CEC:

Motor nameplate:
- 3 PH
- 60 Hz
- 575V
- 24.5A
- INS. CL. "F"
- 1775 RPM
- TEFC

Min. Conductor Size

Rule 28-106(1)
24.5A x 1.25 = 30.6A -> Table 2 -> 75 Celcius -> 35A -> #10


Voltage Drop

Table D3 -> Max run length for #10, 24.5A @ 120V = 6.2m

Distance correction factor -> Table 2 -> 75 Celcius -> #10 -> 35A -> 24.5A/35A x 100% = 70%

Table D3 (note 3 chart) -> % of allowable ampacity = 70% -> rated cond. temp. = 75 Celcius -> Dist corr. factor = 1.03

Max run length = 6.2m x 3(%) x 1.03 x 600V/120V = 95.8m
95.8m x 3.3'/m = 316'


Max O/C device rating (breaker)

Rule 28-2009(a)(e)
Table 29 -> Max Setting Time Limit type CB -> 250% -> 24.5 x 250% = 61.3A

Instantaneous trip setting -> Rule 28-206(a)(b) -> 24.5A x 1300% = 319A


Non-Fusible Disconnect

Rule 28-602(1)(a)
Motor Horsepower = 25HP -> Disconnect must be rated for at least 25HP -> Therefore suitable Trade Size Disconnect for 3 PH, 600V, 25 HP motor = 60A (Max 50HP)

https://www.schneider-electric.ca/documents/electrical-distribution/en/shared/catalogue/DE2.pdf


A malfunction at the junction
--------------------------------------
Dwayne
Re: Air Compressor feeder conductor sizing [Re: twh] #219059
01/14/18 06:07 PM
01/14/18 06:07 PM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
Originally Posted by twh
Maybe they use the same compressor and wiring for other voltages and they don't change wire size just for Canada.


On Friday I spoke with another electrician who works at a different facility and we came to the same conclusion. That is essentially the question I posed to the air compressor company which I sent by email on Friday.


A malfunction at the junction
--------------------------------------
Dwayne
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219060
01/14/18 06:42 PM
01/14/18 06:42 PM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
Here's a rough visual of the way I see it:

[Linked Image]


A malfunction at the junction
--------------------------------------
Dwayne
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219062
01/14/18 09:01 PM
01/14/18 09:01 PM
T
twh  Offline
Member
Joined: Mar 2004
Posts: 940
Regina, Sask.
Oh, that's what you're looking at. When they changed the tables and included 14-104 they didn't seem to consider other rules, like motors and heaters, although the rule specifically says "other rules". Maybe they decided that it was okay to load wires to a higher amperage if the load is a motor or heater; or, they might not have been able to imagine someone writing a rule that didn't increase the cost of an installation.

Think of it this way: At one time, #10 r90 was acceptable for a 40 amp breaker on a range. I worked on a really old installation and there was only slight heating of the neutral bar. Additionally, I was in a row housing complex that was wired recently (recently to me) and I reported to the inspection department that the entire complex had ranges wired with #10 on 40A breakers and they didn't do anything about it.

I'm pretty sure the inspectors know you can run #10. Get the permit quick before the new book comes out.

Re: Air Compressor feeder conductor sizing [Re: Potseal] #219064
01/14/18 11:25 PM
01/14/18 11:25 PM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,225
Estero,Fl,usa
If you look at the article Dwayne linked, that compressor manufacturer is saying they really expect the product to start under a load that may demand more of the conductor than the code minimum. I also agree that MCC may be an off the shelf product that serves other loads in other places so simply the fact that they spent an extra buck on 6ga wire may not be important to this installation. I think that at the end of day, taking guidance from the manufacturer may be the way to go. Down here in the states we would say 110.4(B) trumps the minimum code.


Greg Fretwell
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219067
01/15/18 01:19 AM
01/15/18 01:19 AM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
As always, I appreciate everyone's feedback. Once I get input from the manufacturer I will get back here and report what the result will be.


A malfunction at the junction
--------------------------------------
Dwayne
Re: Air Compressor feeder conductor sizing [Re: Potseal] #219073
01/15/18 10:08 PM
01/15/18 10:08 PM
Potseal  Offline OP
Member
Joined: Feb 2013
Posts: 264
Saskatchewan
Haven't heard back from the manufacturer yet but I was able to get more detailed information from the supplier. The specifications show that for the model I am supplying power to #10 AWG is the minimum recommended conductor size. So, no surprises.


A malfunction at the junction
--------------------------------------
Dwayne
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