Why does the voltage drop when my office circuit (115V) is under load?

Question

3 circuits (red, blue, black, neutral white; approx. 100ft in metal conduit [ed: alongside 2 other similar 4-conductor MWBC]) feeding several outlets in a couple rooms. Voltage tests 110V(ish) on all three, neutral-to-earth voltage is around 9V. Plug tester says everything's fine. (Y'know, that consumer plug with 2 yellow LEDs & one red LED.)

[ed: light industrial +office location, all work professionally done ~10 years ago, main is 250A 480V/3𝚽 𝚫 but this is off a 208/120 3𝚽 wye transformer, 12AWG solid copper wires, 20A breakers, boring office loads (few computers, router, 802.11 switch, a small printer, alarm, a space heater, micro-fridge)]

However, I plug something in, and get no juice. It's a verrrry localized brownout. Other circuits on the panel are fine. Does this sound like something obvious?

The first symptom we had was a short between blue and neutral, giving 210V on red and black outlets. (Yikes.) After dis- & re-assembling all the outlets and conduit junctions, the short disappeared, but (as above) the circuits are still not usable.

This seems like the most appropriate forum, but if this question goes somewhere else please let me know.

Thanks!

Answer 1

You have a lost neutral. Stop using the circuit until fixed.

This is a classic "lost neutral" we get all the time either on main panels or multi-wire branch circuits (MWBC, which is what this is). The whole thing is one large circuit since they share neutral. MWBCs are complicated and have special rules that need to be honored if you don't want things like this happening. Given the casual nature of speech and this question blowing over from ee.se, I suspect you may be out of your depth, no offense.

Particularly if "melting wires together" is a thing that is happening to you, it's really time to put the tools down and call a pro, or else "get religion" about doing good electrical work to Code and then read a book on electrical cover to cover and not skip through it as people with "smartphone attention spans" are prone to. Google cannot substitute for a book because it only answers questions,and your field knowledge is not sufficient to know which questions to ask.

This is a 3-phase MWBC, implying New York City residence. My advice on MWBCs here applies. Particularly the part about pigtailing neutrals. You are 3-phase so where it says 2, read 3.

The "symptom" for this problem is that if you plug a bulky load into one leg of the MWBC, say a resistive heater, and measure voltage at other legs of the circuit, it will be other than the expected 120V. Another giveaway is a high difference in voltage between neutral and ground, on a 100' run, 1-2 volts is to be expected but no more than that.

It sounds like you did not fix (or find) the problem with the neutral. If you insist on continuing to work on it, and you follow my advice, you'll be revisiting a lot of it, but also look in the service panel at the neutral termination there. The overcurrent may have fried it.

Answer 2

If you are looking for the obvious answer, uncommon voltage drops are usually caused by poor connections. So go through the entire three circuits and insure all connections are electrically and mechanically sound.

Unfortunately your description may indicate that it could be something other than a loose connection. You have a multiwire branch circuit meaning your neutral is being shared by 3 circuits. Are you troubleshooting a three phase circuit? If you aren't then you could be overloading the neutral. You say the circuits are 100 feet long. What are the loads they are powering? This could be just a standard voltage drop brought on by distance and load. Also what is the conductor wire size and what is the size of the breakers powering it? Also as it was pointed out you reading between your ground and neutral should be 0V not 9V. The original installer may be trying to run some of the circuits on the neutral and has switched the ground to use it as the neutral for some of the circuits.

So check the wiring for loose or poorly made connections and verify the wiring is properly installed. If you are still having the same problems, then it's not the obvious but something more complex and you will probably need a professional to help you sort things out.

Good luck.

Answer 3

After your short you now have a problem on 1 or possibly 2 circuits with a load no voltage. The short probably found a back stabbed connection and damaged it that's why you have 110 and the outlet tester shows good but under a load there is no power the damaged connection opens, when the load is removed you can measure voltage this happens. To find the problem plug in a load to this circuit breaker turned On. Now start checking outlets prior to this one if you find a good outlet in this circuit it will be at that box or the first dead one. Turn off the power and pull the outlet you are looking for a broken wire, loose wire nut or a back stabb on the hot or neutral. If you don't find a damaged connection after pulling the outlets but when you turn the power back on it works you need to replace the outlets if back stabbed because the connection will fail again and damage the wire further.

Answer 4

neutral-to-earth voltage is around 9V. It should be 0 volts. Check voltage between Red and Blue, Red and Black, Blue and Black .... and so on.