Power in old building

[theatrelighting]

Hi Everyone,
I have a quick (hopefully) question about rated amperage on some outlets in an old rec center-style building. This building seems to have some pretty old outlets... each circuit breaker is rated for 20 amps, but I am skeptical about the amount of power each outlet can hold on its own since they have seen better days. Is there any safe way to check if an outlet is rated for under 15amps? If it helps, there is only one socket per plate (usually there are two), which I assume to be associated with the age of the building.
I have no access to electrical plans or anything that might be helpful.
Any information you can provide would be very helpful in ensuring a safe event!

Thank you!

 

[Edrick]

The usual idea is if it's a straight blade only it's 15A if it has both straight blade and angled blade (look at the left slot on the plug is it a T or just a straight I, if it's like a sideways T then it's a 20A Receptacle.

That being said even if it is a 20A receptacle with the angled slot that doesn't mean the wiring is rated for 20A at that point you'd want to verify the breaker.

However going even further it's possible the place was wired with 14AWG and someone at some point swapped both the breaker and receptacle for 20A counterparts but left the 15A wiring.

So there's many ifs ors and butts.

 

[theatrelighting]

Thank you for your response.
If the last situation were true, what would happen if 20 amps were plugged into the outlet? Would the circuit breaker trip or would would it fry the 15amp wiring?

Is it safe to assume the outlets are AT LEAST 15amps? Or is it possible that it could be less?

 

[gbdesign]

Thank you for your response.
If the last situation were true, what would happen if 20 amps were plugged into the outlet? Would the circuit breaker trip or would would it fry the 15amp wiring?

Is it safe to assume the outlets are AT LEAST 15amps? Or is it possible that it could be less?

Hopefully the circuit breaker would trip, though probably not immediately. Worst case it wouldn't trip and the wiring would heat up ... and you know the rest. You could try removing one of the outlets (with power off) and checking to see if there is any type of rating indication on the outlet, and what type/ size of wire is feeding it.

 

[museav]

The assumption would be that since it has a 20A breaker, then it was properly installed as a 20A circuit. But that is an assumption.

I believe that per NEC, 15A circuits may only use 15A receptacles, however 15A and/or 20A receptacles may be used with 20A circuits. The reasoning for this is that the receptacle current rating is related more to the potential current draw of a connected device than to the current capacity of the physical receptacle.

In commercial construction you generally won't find anything less than a 15A circuit and probably mostly 20A or greater rated circuits.

So assuming everything is properly installed, based on the 20A breaker noted you likely have either a 15A or 20A receptacle on a 20A circuit. However, two potential factors to consider. One is that there may be multiple receptacles on the same circuit, so just because a receptacle could support 15-20A does not necessarily mean that you could have that amount of current draw at every receptacle. Second, as Edrick noted, there is the possibility that at some time the receptacle and/or breaker were improperly replaced and what appears to be a 20A circuit could have wiring based 15A.

 

[porkchop]

I would say open up one of the outlets (with the power off of course) and look at the wiring going to it. Most wiring has an indication of what gauge it is. If it was wired to be a 15 amp outlet the wiring could be 14 AWG, but if the outlet was wired to take 20A the wiring would have to be at least 12 AWG. There's always a chance that a different gauge wire is in the wall somewhere, but if it's smaller then the person doing the install was very lazy and unsafe, so I would say it's safe to assume the wiring in the receptacle will represent what the outlet was wired for.

 

[FMEng]

Yes, you could check the wire size, but let's not get carried away here. There is more risk and liability to opening up and looking than making reasonable assumptions. By opening and looking, you might loosen a splice and cause a problem or discover the hard way that there are additional hots and shared neutrals in the same raceway. Older wiring materials are not forgiving about being handled. Let old, sleeping dogs lie unless you are a licensed electrician.

Do we really think that if some #14 wire is loaded up to 16 amps, (80% of 20A which is max load) that the wire will instantly erupt in flames? It won't. The NEC is designed with some margin for safety. Give the engineers a little credit.

Ampacity of wire is determined under the worst case scenario of continuous load and tight conduit fills. Time and duty cycle is a factor in heating of electrical components. I doubt the lights will be on full for long periods of time, and conduits are seldom filled anywhere near what the tables allow.

I'd sweat the details if the installation were permanent, and the load at full for hours on end. Otherwise, it is a 20A breaker there so assume the wiring is good for a full load of 16 A.

As for the receptacle itself, a 5-15R is mechanically capable of just as much current as a 5-20R. The difference in blade configuration is mostly a handy way of knowing what the circuit is rated for, assuming there are no other loads on the circuit. Again, a 5-15R isn't going to melt at 20 A.

Of course, always follow the rules as thoroughly as you can, but also learn when and where to make reasonable judgements.