I have an outlet in my lab that is NEMA 10-30 120V/220V and I am not sure if it is 3 phase. How do i tell?

Question

I need 3 phase power to hook up my machine and I believe this outlet is a 3-phase but I am not sure. Thanks!

Answer 1

You may be confused by the three terminals, but a NEMA 10-30 is not a three phase receptacle, and is actually most likely obsolete, being a non grounding receptacle. What you have is two single phase "hot" conductors (~220v to 240v) and a neutral, there will be half the voltage from either "hot" to neutral (~110v to 120v, the markings on your device don't quite make sense)

As someone else mentioned, a three phase receptacle would have three current carrying conductors, plus in almost all modern installations, a protective ground, for a total of 4 terminals. But that would be for a single voltage, if it was dual voltage (4 wire "Y") there would also be a neutral for a total of 5 terminals.

If some one did use a 10-30 for three phase, (a really bad idea IMHO) you could probably tell by measuring the voltage at all three pairs of terminals, if the voltages are all the same (about 208V) you may have an (improper) 3 phase circuit, but if one pair measures about 240 and the other two about 120 (which it should) you have single phase.

Answer 2

First, you don't have "ground/hot/neutral" on a 10-30. It's hot/hot/neutral. The "L" shaped prong is the neutral (white), the other two are hot (red and black). However - as of the 1996 NEC, the 10-30 has been replaced with a 14-30. The old 10-30 didn't have a true ground, and instead grounded the appliance chassis to the neutral conductor. The problem with this was that the timer motor and lights in a dryer require 120v, so the neutral is supplying current as well as acting as an equipment ground. If you pulled a building permit for the work, the building inspector won't sign off on the 10-30 receptacle. If you're doing this WITHOUT A PERMIT and you ever have a house fire, there's a real good chance that the insurance inspector will blame your non-permitted, non-code wiring as a contributory factor, cancel your policy, and reject the claim. Do the job right, replace your existing appliance cord with a 14-30 cord, install the approved 14-30 receptacle, and get a permit. I don't mean to be a hardass, but for the cost of a new cord and a building permit, why give your insurance company an excuse to refuse covering a claim if there's ever a fire (knock on wood).

Answer 3

If a nema 10-30 outlet is wired up properly then it does NOT supply 3 phase power.

A three phase outlet will have four prongs, one of which is ground and three of which are hot.

You can determine if your building even has 3 phase power by going outside (assuming the power lines are overhead and not under ground) and visually following the power lines from the building to the utility pole. Now count the number of transformers on the pole. If there are three you have three phase power available. If there is just one you do not yet have access to three phase power.

Answer 4

It would be highly unusal to have a 120V connection on a 3 phase outlet. 220V single phase outlets often have two hot wires and a neutral that is 110V to each hot wire. More modern 220 volt outlets have both a neutral and a ground wire in addition to the two hot wires. Both a neutral and a ground will measure zero volts to ground. With 3 phase 220V you will have three wires that each measure 220V to either of the other two.

A 208V 3 phase system could have a neutral connection that would be 120V to each of the 3 phase connections, but that would require 4 wires.

Answer 5

With 3 phase you need 4 wires or 4 holes in the outlet for your machine to plug into, and the breaker that controls the outlet would have three sections to it. I really doubt you having a three phase outlet of that size, you need to have an electrician check it for you.

Answer 6

It is not three phase.
A NEMA 10-30 outlet is an ungrounded single phase three wire outlet.
Two phases and a neutral.

Answer 7

You most likely dealing with one phase in your lab.