Questions about 480v – page 2 – electrician talk – professional electrical contractors forum electricity worksheets for grade 1


thermal cooling so we call it “ampacity”. The wire size depends on the average maximum current if the load and where it is run. Nether-McGrath irked out a very simple engineered design fir this about 50 years ago and most of the time we just use tables for it. Finally the third consideration is voltage drop over long distances. Your VFD will compensate fir that and this is normally more if a consideration over a few hundred feet.

I would agree with the comments about the electrician you have. Weight and flexing are important. Putting in 10 gauge wiring because it’s 480 V sounds like somebody that rarely if ever works on 480 three phase (the correct term) is out if their element. Still this is specialized equipment. Robots use very low inertia high torque motors and tight control loops for performance reasons. A

PMDC motor costs much more than a standard induction motor but produces several times more torque relative to its size, reducing inertia substantially.. The system has the cable and motor inductance and resistance programmed into it and uses the data for tight feed forward control. The extra low voltage signals from the motor are encoder feedback where it detects rotor position in fractions of a degree so that it can deliver maximum torque right down to zero speed even under rapid load fluctuations such as with a grinding or cutting head as it engages material without going out of control. If you don’t update the drive parameters that tight control is disturbed. Chances are it will still work but it might be pretty sloppy. I suggest calling a few larger motor service centers or check the drive manufacturer for a service group in your area. You are on the right track but you’re getting lots of bad information because you are working with people that know nothing about robots. Robots typically use feed forward (proportional and integral) control for rapid response to a command in parallel with PIV feedback control for trim to achieve tight control in the face of load variations at high speeds with very rigid frames so that mechanical bandwidth is well outside of the electrical performance envelope.

PE is the IEC term for a ground. IEC has a separate ground term for other grounds where we just call everything a ground in North America even when they should be separate so it gets confusing. Almost every drive uses PE in the manuals and most have a note that says this means ground or they label it with the letters PE and the symbol for ground so nobody gets confused. It is definitely NOT confusing for someone trained on drives no matter who built it because most drives are labeled that way. It’s just that in North America we use the word ground so we use the symbol, not the abbreviated term “Protective Earth ground” or PE and we say it’s grounded and bonded or usually just grounded not “earthed”.

The OP sounds like a lab manager or professor though with little or no experience with robots or drives and is just using the words from the manuals. I work for one of the largest motor and drive service centers in the Carolinas and I have many university customers. They often run into industrial equipment but the facilities guys know commercial/residential so they get lost, and lab personnel are often familiar with research lab procedures and rules which are very different from industrial rules because they typically don’t work over 50 Volts.

480 V VFD output does not exceed 1.45 times the AC RMS voltage intentionally at the drive output as DC which is equal to the peak (not RMS) AC voltage…so same as an AC RMS 480 V rating. When we consider reflected wave (ringing) voltages cannand do go higher but since this is momentary and 600 V cable is actually tested to something like 2.5 times it’s label, ordinary wire goes much higher and VFD manufacturers take advantage of and have tested this as long as you respect length limits. No special VFD cable needed. Same with motors up to a point. I do drives all the time and I’ve dealt specifically with this issue. Many vendors try to up sell customers on VFD cable. The only time it is necessary is when you are exceeding the VFD distance limit and not addressing it with filters but rather slapping in an upgraded (VFD rated) motor and using VFD cable which pushes you from around 1200 V peak rating to usually around 1800 V peak rating by Code but a lot of motors and cables exceed this even more. Usually installing an output dv/dt filter is cheaper. So the 600 V comment is from somebody that has never done drives or got taken to the cleaners by an unscrupulous vendor….with one exception see next paragraph.

As to the RF comment this is mostly true. Drives do generate significant RF in the first place but the shielding really isn’t for that. It’s to avoid induction problems where if you have two drives and two motor cables fed from them in a common raceway at some point an output pulse on one drive can induce a corresponding pulse in the other drive cable. If the other drive outputs at the same time it doubles the voltage which again gets us back to the reflected wave problem but much sooner. In robots all the cables run together in very tight wire trays so this is necessary. Breaks in shielding as long as they don’t coincide and the ground is carried through aren’t a problem…that’s what exists at the ends anyway. So depending on if you have other cables such as the knuckle motor cable it should be shielded. If the controls were NOT 24 V then induced issues would be a concern but like nearby VFDs this is an AC thing not DC.

As to Listing and wire size WTF? Might want to go back to reading up. NEC and UL have a minimum of 18 gauge in general power wiring for fault reasons. CEC might be different but US Codes allow much smaller even outside of power limited wiring. 16-18 gauge is commonly required on newer industrial controls because the terminals are getting too small as they try to jam more crap into smaller boxes, especially 24 VDC where typically the load might be 10 mA. You physically can’t get a 14 gauge wire onto the integrated terminals of a current model Allen Bradley PLC or a Honeywell DCS and the labels don’t allow it so it would be a Code violation. Keep in mind this is industrial, not residential. Romex has no place here and neither do residential rules. The assembly vs. genera rules do apply but the manufacturer didn’t use 20 gauge.