Wire Sizing, 3ph vs 1ph, same deal or not?

I've generally sized my wire for my 3ph machines just like I do for 1ph things. Look at total max load and pick a wire that's next up from that. But just a few minutes ago while looking at what I'll do to hook up a new lathe that is coming, I ended up a little puzzled. I know this matters very little on small equipment, and anything big will be 3ph anyway.(not considering voltage drop and such here)

So, If you have a 3ph motor of 7.5HP, 240V. You end up with about 23amps. Does each of the 3 load carrying wires ever see the full 23amps draw like a 1ph wire would?, or does it only see a faction of it on 3ph? I saw one formula while googleing a minute ago, that had a .866 factor into it for 3ph, so I'm thinking that you can actually figure a smaller load per wire?


Next part of the question would be in regard to a steady resistive load like a large electric water heater that was 3ph. Say it draws 100amps total, would you technically have to use wire that carries 100amps, or would you say each of those wires is only seeing a portion of the total amperage? and what would that portion normally be?

What made me think of this is that right now my two 3hp-3ph/240v machines are on 10Awg wire, and then I thought, how weird my new 7.5hp 3ph machine could also use this same 10awg wire, I thought I was gonna go with #8, but looks like I wouldn't really have to go that big, mostly if each wire doesn't even see the full 23amps.

Thanks for any info on this.

SND said:

What made me think of this is that right now my two 3hp-3ph/240v machines are on 10Awg wire, and then I thought, how weird my new 7.5hp 3ph machine could also use this same 10awg wire, I thought I was gonna go with #8, but looks like I wouldn't really have to go that big, mostly if each wire doesn't even see the full 23amps.

Thanks for any info on this.

Click to expand...

But it does. 3 ph. ratings are amps / phase, and since it's a motor, it will see 4 - 5 times that current for peak starting load, but only for an instant. My handy-dandy Square D "motor data calculator", based on the 1990 NEC (hmmmm, maybe it's time to get a new one?) shows your 7.5 HP 3 ph. motor draws 22A on 240V, and should be wired with 10 AWG. The same HP on a 240 Single phase system draws 40A, and requires 8 AWG. This makes perfect sense, as the load each conjductor sees is 1.732 (the square root of 3) higher on a single phase syetem for a given HP.

Dennis

Yes,

You will see up to 23amps per phase if fully loaded and when starting there will be an even higher inrush current for a few seconds. You will need wire capable of handling at least 25 amps or more on each phase.

JC Chamberlin III
Precision Electric Inc

I agree with the above post. Each leg will have to carry the full load (25amps) on 3ph. You will have to consider the length of run in your equation.
Chris

23 amps if it were run on single phase. It's not, it's running on three phase.

Divide by root 3 to get the FLA per conductor.

jim rozen said:

23 amps if it were run on single phase. It's not, it's running on three phase.

Divide by root 3 to get the FLA per conductor.

Click to expand...

The load for that motor is 23A on each phase. 10AWG min is required.

Here's the 2 formulas that came up while searching google. It won't allow me to copy/paste.
It shows the .866 factor on the 3ph one. For a 3ph motor to draw full load on one wire alone, wouldn't it mean the other 2phase aren't doing a thing? Its been a few yeas since I read on this, but I don't remember it working quite that way. Guess I might have to borrow that book again.


Wire Size Calculator

1/root 3 = .866

Misinterpreting or misapplying formulas does not help.

A 7.5HP 3-Phase 240V motor requires 22 amps on EACH wire or 10AWG minimum.

A 7.5HP Single Phase 240V motor requires 40 amps on EACH wire or 8AWG AWG minimum.

7.5 hp X 750 watts per hp = 5625 watts.

5625 watts / 240 volts = 23.4 amps, which are supplied by three wires.

Hence the divide by root three:

23.4 / 1.73 = 13.53 amps per wire

Or am I doing something wrong.

Peter help me out.

I'm not sure either how the 7.5hpx746watts(I normally use the 750 too, close enough)
turns into 40amps single phase, except for the part where you would need to oversize the breaker for it to take the start up current, and generally the wire should be matched by code, so maybe that's what was added to the 22-23amps? I'm running my little 2hp 1ph lathe on 20amps/240V.


Either way, this machine got ruined in shipping so I won't be hooking up anything anytime soon.

You can't really use the W/HP calculations on electric motor calculations. Small motors are especially inefficient. Efficiency increases as the size of the motor increases.

Actual Continuous duty motors have fairly well established current draw versus voltage and HP ratings. The values below also have a "fudge factor" built in for line-voltage variations. There is no "free lunch" when it comes to horsepower. See the following table for some generic values:

Single Phase Alternating Current Motors Full Load Current in Amperes:
-----------------------------
HP_____ 115V_____ 230V
-----------------------------
1_____ 16_____ 8
1.5_____ 20_____ 10
2_____ 24_____ 12
3_____ 34_____ 17
5_____ 56_____ 28
7.5_____ 80_____ 40
10_____ 100_____ 50

The above values are from the National Electric Code and reflect their use in calculating branch circuit requirements. Also notice 115V values are twice the 230V values.

The nameplate ratings on a motor will generally be lower than the above values.

The following values are taken from the catalog of a major manufacturer of high efficiency industrial motors for compressors. They will more closely resemble the values you should see on motors on high quality compressors or machinery.

Single Phase Alternating Current Motors Full Load Current in Amperes:
---------------------------
HP_____ 115V_____230V
---------------------------
1_____ 16_____8
1.5_____ 20_____10
2_____ 24_____12
3_____ 32_____16
5_____ 42_____21
7.5_____ --_____33
10_____ --_____44

You can see that efficiencies increase as motor size/HP increases.

Jim, you are doing two things wrong......

1) you left out efficiency.

2) you left out power factor.

The book value for 7.5 HP at 230V is 22A. (standard EASA "gimme" book value)

Your calculation is 746 x 7.5 = 5595 W.

Efficiency is probably around 87%, so 5595/.87 = 6431W

6431 / 230 = 27.9A.

power factor at full load might be anywhere from 0.6 to 0.75...... say it is 0.7

so 27.9/0.7 = 39.85A

Then 39.85A / 1.73 = 23.....

Pretty close for some WAG numbers.

Good call on the efficiency, missed that. Thank you.

Would be interesting to see the nameplate rating for that motor.

There are some convenient "gimme" tables on the web. I have this one bookmarked to look up motor wire sizes, conduit fill, conductor ampacities, etc:

Three Phase AC Motors


PS. SND, sorry to hear about machine.