277/480 WYE 4-Wire Woes


Code Historian
Electrical Design
Right not to be pedantic, I consider you a code expert around here, but with all due respect 90.2 states this code covers 'equipment' generally, not just hard wired equipment.
Art 100 has a pretty catch all definition of equipment, that includes things like heaters, AC units, appliances and the like that may be cord and plug connected.
Then we get into 590 for plug in stuff like holiday/Christmas/seasonal lights & displays.

OSHA checks cords all the time, using art 590, I am sure you aware of stuff like an 'assured equipment grounding program'?
OSHA hands out fines for frayed cords like I do beers at a cook out.
I have never been asked about any of that stuff by construction electrical inspectors.

Then we have all the equipment cords in hazardous locations, medical equipment, shore power, commercial kitchens
and large events like
concerts, trade shows, farmers market type events, fairs, film shoots (back when there were such things)
the fire Marshall/OSHA love to look at all those cords..
Consulting for that used to be part of my job (back when there were events)
Cheers Happy Friday


Senior Member
Dear Jon,

Thank you so much for writing.

I know Ohm's Law pretty well. Most assuredly amperage goes down when voltage goes up; that's why generating stations send electricity out at such high potentials. I was just going by the readings I was taking. Each machine is pulling about 10 amps at 277 volts; that's 2700 watts.

And too I know one isn't supposed to max a 20 amp breaker, much less a 400 amp breaker. Schools in on the harmonics thing; I have no experience with that, but I'm a quick study. Hell, I've been working on gravity for 30 years and I've got half the secret, but this isn't the forum for that.

Email me and I will reveal it. bluescarecrow2000@gmail.com


Retapping is questionable.

So standard on transformers is 5 taps at 0, +/-2.5%, and +/-5%. So if incoming voltage is where it should be then the best you can do is tap up 5%’(since the tap changer is normally on the HV side) so you go from 277 to 263 V. Power supplies will be 220 or 230 V rating (utilization vs supply) so even if you get 5% VD you are down to 237 V so you are just barely in the UL/NEMA range of +10% but if losses are only 3% you will be overvoltage. You can use a buck/boost to slightly lower the voltage though much cheaper than replacing transformers. If you had all kinds of money I’d be suggesting installing an AVR (automatic voltage regulator) which is a fancy transformer that automatically bucks/boosts as needed based on load.

The harmonic thing is a problem only near/at maximum load. So what happens if you overload a transformer (too much current)? The output voltage goes down. So with current harmonics if the peak current is less than the transformer kVA rating it does not matter at all to the transformer. But when the peak current reaches a point where the transformer output voltage dips temporarily then we have a problem with harmonics. This is a common mistake....if I show you the current harmonics with all those switching power supplies it will look horrible but as long as there are no voltage harmonics the transformer will be fine. Not so good for capacitors though if those see harmonics, and the neutrals can see currents and voltages different from the phase conductors. Again monitoring can easily tell you how bad it is but power monitors and expertise using them isn’t cheap. So suggest watching your voltage dips and just use full size neutrals to avoid issues.