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We are in the process of trying to find out why 1/0 AL wire over 400' of length to supply a 2HP motor for a Bifold Door is being pushed so hard by door manufacturer.

We have a door on order with a 2HP motor, rated at 230-208 volts with approximately 10 FLA @ 230 volts. The spec'd OCPD is 30 amp.

To supply this motor, we have a long run of 400' from the meter with 240 volts supply (242 measured).

In an extreme situation of minimum wire size (10 AWG) with triple the inrush current of FLA (30 amps) we would get a voltage drop of approximately 10% down to 216 volts which is apparently within the nameplate rating of the motor.

Why is large voltage drop a problem if the operating voltage is within nameplate rating on motor?

that is...

If the name plate says "230-208 volts", why would 216 volts be a problem?

We are aware of many solutions all involving a varying amount of cost and effort (larger wire, buck-boost transformer, etc) but we do not understand the root of the issue.

We have been warned of damaging the motor in short order if we deviate from the recommendation of 1/0 AL wire in any way but the recommended wire size would only have a 1.5% voltage drop @ 30 amps. We are trying to understand why the recommendation. Is it more than a voltage issue?

  • The motor likey wires differently when attached to 208 vs 230 systems. – Tyson Nov 30 '16 at 16:48
  • The motor can be wired for 115 or 230-208. We cannot wire for 230 or 208. – Damon Nov 30 '16 at 17:11
  • With a long feeder the wire size should be upsized to limit the voltage drop to 3%. Allowing the motor to run at lower voltages will cause brown out conditions causing higher current draws and increased heating of the motor. Some motors will stay in start mode and not make run speeds causing motor failure. – Ed Beal Nov 30 '16 at 17:50
  • @edbeal "...allowing the motor to run at lower voltages..." than 202 volts? (Which is a 3% voltage drop from 208 volts) – Damon Nov 30 '16 at 18:08
  • The question references inrush current. This does not even take into account that operating current is much lower and the voltage would be well above 230 volts during normal operation given we would never run 10 gauge wire 400 feet no matter what for a 30 amp OCPD. That is the operation mode in question is only for a few seconds or less out of the total operating time. – Damon Nov 30 '16 at 18:09
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If the inrush or starting current if not sufficient it will ruin your motor. This is not just opinion I am a professional electrician. The rated 230V current for a 2 hp motor is 12 amps. Code requires the book value to be used not the nameplate. I would use a minimum of 300% starting current some motors need up to 500% for short duration's. without the increase in wire size the motor will brown out as stated earlier. I just ran the numbers 300% x 12 amps at 230V for 400' is actually 1/0 aluminum with a 2.6% voltage drop. If you use 208V the FLA is 13.2. Do what you want ignore the Manufacturer and My advice that provides similar information, Just don't cry when you let the magic smoke out of the motor.

  • "If the inrush or starting current if not sufficient it will ruin your motor." If the voltage drops from 240 to 210 and the current at 240 would be 40 amps, what amperage would it be at 210? Is there a way to calculate the current given a voltage drop? This is all given the voltage is still within the nameplate rating of the motor since my question is about why voltage within a nameplate rating of a motor is a bad thing even if it has dropped from a higher voltage. It seems you are alluding to it being a current issue vs a voltage issue. – Damon Nov 30 '16 at 20:22
  • There was a liscensed electrician over the spring that installed #8 copper for 220 feet of the 400 knowing what the planned the motor was who is also a professional who said it would be enough; there was another door and motor on site that was pretty much equivalent that he looked at. Since we trust both the electrician and the door manufacturer we are trying to gain understanding. – Damon Nov 30 '16 at 20:27
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    Not all electricians are (well) trained on motors. Adding a long run adds another variable in the equation. Depending on the type of starting method the motor may not be able to come up to speed and change from the start winding to the run winding (common in house hold motors). Name plate values may be a good starting point but code requires the table values to be used ( 2014 NEC 430.6.A.1), Single phase values are provided in table 430.284. If this motor was connected to a branch circuit of ~50' 125% of the FLA book value would be enough for the wire size. – Ed Beal Nov 30 '16 at 23:05

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