NEMA Receptacle Faces for 240 Volt and RV Circuits

Why this matters

Half the 240 V calls a residential shop takes start with a customer describing a plug: "the big one with the L," "the RV kind," "the one my welder came with." The other half end with a receptacle that does not match the cord on site. And one of these faces carries a trap that destroys equipment: the TT-30 RV receptacle looks like a 240 V device and is a 120 V device, and wiring it across two hots sends 240 V into a camper built for 120. This card puts the seven faces a residential and light-commercial electrician meets most on one page, with what each carries and what usually feeds it.

How to read a NEMA number

The configuration number tells you the system before you look at the slots.

• The first number is the family. 6 is two hots and a ground at 250 V, no neutral. 10 is two hots and a neutral at 125/250 V with no ground; it is a legacy family. 14 is two hots, a neutral and a ground at 125/250 V. TT is the travel-trailer configuration, 125 V.
• The second number is the rating in amperes. 20, 30, 50.
• R or P. Receptacle or plug. A 14-50R takes a 14-50P.

The number is molded into the face of every listed receptacle and onto every listed plug. Read it before you describe the shape. Shapes within one family are deliberately non-interchangeable between ratings, and the number settles which one you are looking at faster than the slot geometry does.

The faces

SCHEMATIC FACES - shows which slots exist and their shape,
not true angle, spacing or mounting orientation

6-20R  250V       6-30R  250V       6-50R  250V
[=]      [T]      [=]      [=]      [=]      [=]
    (o)               (o)               (o)
hot, hot, ground  hot, hot, ground  hot, hot, ground

14-30R 125/250V   14-50R 125/250V   10-30R 125/250V
    (o)               (o)           [/]      [\]
[|]      [|]      [|]      [|]
    [L]               [=]               [L]
hots, L neutral,  hots, straight    angled hots,
ground            neutral, ground   L neutral, NO gnd

TT-30R 125V
[/]      [\]
    (o)
angled hot and neutral, ground: 120 V ONLY

In the drawing, (o) is the round or U-shaped ground opening, [L] an L-shaped slot, [T] a T-shaped slot, [/] and [] slots set at an angle, and [=] or [|] a straight slot. The 6-20R's T-slot is what lets it accept a 6-15P as well as a 6-20P. The 6-30R and 6-50R differ in blade orientation and size so that neither accepts the other's plug; tell them apart by the number on the face.

What each one is for

Breaker and conductor sizes below are the usual pairing for copper, from 'Branch Circuit Wire and Breaker Pairing Card'; the equipment's nameplate and instructions govern where they differ.

Notes on the rows that generate calls:

• 6-50 and welders. Article 630, as adopted, sizes welder supply conductors from the welder's rated primary current and duty cycle, so a welder circuit can legitimately differ from the general pairing. Follow the welder's nameplate and instructions.
• 10-30. New installation of a 10-30 is not permitted. The exception in 250.140, as adopted, lets an existing three-wire dryer or range branch circuit stay in service under its conditions; a new circuit is four-wire. The upgrade is a HowTo: see related 'How to Upgrade a Three-Wire Dryer Circuit to Four-Wire', and the failure it prevents is the case in 'The Dryer That Shocked Through a Three-Wire Cord'.
• 14-50 on a 40 A circuit. Table 210.21(B)(3) pairs a 40 A circuit with a 40 A or 50 A receptacle, and a single receptacle on an individual circuit only needs a rating not less than the circuit (210.21(B)(1)), so a 14-50 on 8 AWG NM and a 40 A breaker is a legal range or charger circuit.
• 14-50 or 6-50 for an EV. EV charging is a continuous load, so a charger cord on a 50 A circuit draws no more than 40 A (see related 'Continuous Load and the 125 Percent Rule'), and 625.54, as adopted, requires GFCI protection for personnel on receptacles installed for EV charging. A 14-50 in a garage also falls under the garage GFCI item since the 2020 edition extended 210.8(A) to 250 V receptacles. See related 'AFCI and GFCI by Location in a Dwelling'.

Identifying the face before the truck rolls

Customers rarely know the number, but three questions on the phone usually get it, and they decide which receptacle and breaker go on the truck:

• How many prongs on the plug, and is one of them round? Three flat-ish blades and no round pin is the legacy 10-30. Four openings with one round is a 14-series. Three with one round is a 6-series or a TT-30.
• What is it for? A camper with a "30 amp" cord is almost always TT-30, 120 V. A camper or motorhome with a "50 amp" cord is 14-50. A dryer bought in the last few decades ships ready for a 14-30 cord, though the cord itself is often sold separately to match the house.
• Can they read the number on the plug? Ask them to photograph the face of the plug and the nameplate of the equipment. The photo settles it and the nameplate gives you the breaker.

The TT-30 trap

The TT-30 has two angled slots and a ground, and to a person used to 240 V devices it reads as a 240 V receptacle with a ground. It is not. One angled slot is the hot and the other is the neutral, and a recreational vehicle's 30 A system is a single 120 V circuit. Put 240 V across those two slots and every 120 V load in the camper, converter, microwave, air conditioner, sees double its rated voltage the moment the cord goes in.

The 14-50 is different: an RV with a 50 A service genuinely uses both legs, as two 120 V legs sharing a neutral, so 240 V between the two straight hot slots is correct there.

Worked example: the RV receptacle a friend installed

A customer's brother-in-law added a "30 amp RV outlet" on the side of the garage. The customer plugged in the camper and the converter and the microwave died. The camper owner's RV technician wants the house side checked before anything else gets plugged in.

Before any reading at a live receptacle, this is energized work: a CAT III or better meter with its leads in good shape, test probes into the slots from the front of the device only, the PPE your shop's NFPA 70E program assigns for that task, and nobody's hand near the open cover. The camper stays unplugged throughout.

1. Read the face. The molded number reads TT-30R. A 120 V device.
2. Measure, slot to slot. Between the two angled slots: 240 V. From each angled slot to the ground opening: 120 V each. A correctly wired TT-30 reads 120 V between the angled slots, 120 V from the hot slot to ground, and close to 0 V from the neutral slot to ground. This one has two hots.
3. Turn it off and open it up. Turn off the breaker feeding it, lock it out, and verify absence of voltage at the receptacle with the same meter proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024) and 29 CFR 1910.333(b)(2). Test all three slots to ground and to each other, because an installer who wired a 120 V device across two hots may also have fed it from two single-pole breakers with no handle tie, and one of them may still be on.
4. What was found. 10/2 copper NM with ground on a 2-pole 30 A breaker: black on one pole, white on the other pole, so the white is a second hot (and was not even re-identified as one). At the receptacle the black and white landed on the two angled-slot terminals. The cable size, 10 AWG, is right for 30 A.
5. The repair. Replace the 2-pole 30 A breaker with a single-pole 30 A breaker. Black to the breaker. White to the neutral bar. Bare to the ground bar (in a service panel these may share a bar; in a subpanel they may not). At the receptacle, black to the brass (hot) terminal, white to the silver (neutral) terminal, bare to the green. Torque each termination to the value marked on the breaker and the device with a calibrated tool (110.14(D)). The receptacle is outdoors on a garage wall, so GFCI protection and a weather-resistant device in an in-use cover apply. See related 'Tamper-Resistant and Weather-Resistant Receptacle Rules'.
6. Re-energize and prove it. Breaker back on, standing to the side of the panel rather than in front of it. At the receptacle: 120 V hot slot to neutral slot, 120 V hot slot to ground, under 2 V neutral to ground. Press the GFCI test and confirm the receptacle goes dead, then reset it.

The receptacle is now right and the camper is still damaged. Write on the ticket what was found, what was measured before and after, and that the RV's own equipment needs inspection by the RV technician before it is plugged in again; the house-side repair does not prove the camper's converter is safe to energize.

When the cord and the receptacle do not match

Adapters between these faces exist and some are listed, such as a 14-50 to TT-30 adapter that picks one leg and the neutral. An adapter from a 14-30 to a TT-30 does the same. What an adapter cannot do is change the overcurrent protection: a TT-30 camper on a 50 A circuit through an adapter has a 30 A cord protected at 50 A. The fix for a customer who will park the same RV there every season is the receptacle that matches the inlet, on the breaker that matches the receptacle.

References

• NFPA 70, National Electrical Code, 2023 edition as adopted by your jurisdiction: 110.14(D), 210.8(A), 210.21(B), 250.140, 625.54, Article 630
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333(b)(2)
• See related: Common Switches & Receptacles Reference
• See related: Branch Circuit Wire and Breaker Pairing Card; How to Upgrade a Three-Wire Dryer Circuit to Four-Wire