Electrician installed wall connector using Romex 8/3 + 60A breaker

[SageBrush]

Given that I’ve already provided proof of exactly this above… I’m in.

You have provided evidence that you are not an electrician, and can misinterpret the NEC with the best of them

 

[ucmndd]

There is no problem running romex in conduit, nor does such an installation need a de-rating as a result of running it in conduit (and even if it did, you would derate from the 90C column, so it wouldn’t matter here).

A 60A breaker is the correct (and code compliant) size breaker for almost all #6 NM-B installations. The code *specifically* requires you to install a breaker 125% of the continuous load (or the next “standard” size up, of which 55A is not one of them) on your circuit to reduce any occurrence of nuisance tripping.

No, the code *specifically* requires continuous loads to not exceed 80% of the max rated ampacity of the conductors.

48 amps > 80% of the 55 amp rated capacity of #6 NM-B.

Calculated load is what matters for the round-up rule.

 

[Rocky_H]

A 60A breaker is the correct (and code compliant) size breaker for almost all #6 NM-B installations. The code *specifically* requires you to install a breaker 125% of the continuous load (or the next “standard” size up, of which 55A is not one of them) on your circuit to reduce any occurrence of nuisance tripping.

Both a 44A and a 48A continuous load get a 60A breaker. A 40A continuous load could get a 50A breaker, but a 60A breaker is also appropriate if everything in the circuit is sized appropriately (as it is with #6 copper)

No, you made the same mistake of misunderstanding the round up rule as way too many electricians do. It's explained really well in this video:

The mistake you are making is that when you try to round up that appropriate breaker size from 55A to 60A, you then falsely assume it applies a full 60A rating to EVERYTHING in the whole circuit, including the wire, so you then think it can support the 48A continuous load. But the 55A rated wire is still 55A rated wire, even though you rounded up the breaker. You don't get to round up the rating on the wire, so 55A wire can only allow up to 44A continuous--not 48A.

 

[davewill]

No, you made the same mistake of misunderstanding the round up rule as way too many electricians do. It's explained really well in this video:

The mistake you are making is that when you try to round up that appropriate breaker size from 55A to 60A, you then falsely assume it applies a full 60A rating to EVERYTHING in the whole circuit, including the wire, so you then think it can support the 48A continuous load. But the 55A rated wire is still 55A rated wire, even though you rounded up the breaker. You don't get to round up the rating on the wire, so 55A wire can only allow up to 44A continuous--not 48A.

Not what the dude said. He merely said you could use the 60a breaker. He didn't say you could charge at 48a on#6 NM-B.

 

[heavync]

Mine runs through 3-5 ft of insulation and then conduit. All inside. All dry. Less than 20
Ft. Going to swap to 50
Amp breaker. Thanks again

 

[appleguru]

No, you made the same mistake of misunderstanding the round up rule as way too many electricians do. It's explained really well in this video:

The mistake you are making is that when you try to round up that appropriate breaker size from 55A to 60A, you then falsely assume it applies a full 60A rating to EVERYTHING in the whole circuit, including the wire, so you then think it can support the 48A continuous load. But the 55A rated wire is still 55A rated wire, even though you rounded up the breaker. You don't get to round up the rating on the wire, so 55A wire can only allow up to 44A continuous--not 48A.

I did not make that mistake. Re-read my posts… I merely said that a 60A breaker is the correct size 99% of the time for #6 NM-B, which it is. The breaker size alone doesn’t really tell you how many continuous amps you can put on the circuit.

Do not confuse the “round up” rule (240.4(b)), which applies to correctly sizing circuit breakers, with the “continuous load” rules (210.19 and 210.20), which apply to determining the ampacity of your conductors and correct size of your breakers for continuous loads respectively.

NEC 240.4(b) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met

NEC 210.19 Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load in accordance with 310.14

NEC 210.20(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

I find it helpful to remember *why* these codes exist in the first place:
1) the “round up” rule (240.4(b)) for breakers exists to allow for installations where the “exact” correct breaker size for a given circuit may not be readily available
2) 210.19 exists because a continuous load on a circuit can lead to more heat buildup in the wiring than a non-continuous load, so NEC requires “oversizing” wiring by 25% to account for that and keep things cool/safe
3) 210.20(A) exists because a continuous load on a circuit breaker can lead to nuisance tripping, which creates a “cries wolf” situation where a breaker can trip under normal conditions and so the user gets used to just resetting it, potentially even when an actual (dangerous) fault like a short occurs.

 

[SageBrush]

Calculated load is what matters for the round-up rule.

I'm reasonably sure (but not 100% positive) that the round up rule for a branch circuit breaker references the wire ampacity, not the calculated load.
See NEC 240.4.(b)

You can say that the ampacity itself is dictated by the calculated load so by inference so is the OCPD but the wire sizing step is a *minimum*; if you want, you can also install a larger wire. The breaker sizing step is a *maximum*, meant to protect the wire.

 

[Cosmacelf]

I love a civil, facts based discussion. Keep it up guys, I’m breaking out the popcorn myself.

 

[240vPlug]

I bet the OP didn't anticipate this kind of response

 

[appleguru]

* Except in Canada where they don't use NM-B wire. In Canada they use NMD-90 (Rated for 90C) instead. Romex is a brand name, not a wire type and yes, Romex sells NMD-90 wire in Canada.

I just want to clarify that NM-B in the US and NMD-90 in canada are very similar in terms of both ratings and physical construction. They are both 90C rated Thermoplastic High Heat-resistant Nylon-coated (THHN) cable assemblies:

https://assets.southwire.com/ImConvServlet/imconv/cc9ee246a3e9a87c880b862c8b599829546d060e/origin?hybrisId=otmmHybrisPRD&assetDescr=Romex%20Brand%20SIMpull%20Indoor%20Wire%20Copper%20NM-B%20Cable

The only reason we treat nm-b as 60C cable here in the US is because NEC says to. And the only reason it does that is because they make the assumption that nm-b is usually installed in walls in insulation and needs to be derated accordingly, and decided to “make it easy” for everyone and just make that assumption always apply.

Some states, like MA, have decided to disregard that assumption and instead let electricians actually determine the appropriate rating for nm-b cable assemblies based on how they are actually installed.

 

[Tronguy]

FYI, this is not about a short, the underlying issue is to not overheat the wire with a continuous load over 55/1.25 = 44 Amps. Since the TWC lets you choose 40A or 48A load, you choose 40A.

Um. Speaking as:

1. One of the guys designing in fuses in and around circuit boards in Big Honking Pieces of Electronic Machinery
2. Also one of the guys whose job it's been has been to take the Smoking Remains of stuff returned from the field, figure out why it happened, then make sure it never happens again..

The general idea is belt and suspenders. In no particular order, and all about cases I've had to contend with:

1. The general idea is to use the minimal sized fuse for a circuit. If one has a circuit that draws 5A, say, then one figures out what, if any, surge current occurs during startup, shutdown, or normal operation; the steady state current when none of that is happening; and then pick the fuse that, after looking very carefully at the fuse-blow I-vs-time curves, picking the minimum sized fuse that fits. Why that minimum sized fuse? Because that's the fuse that protects the circuit board when something catches afire. Take that 5A circuit. If one puts a 20A fuse on it (certainly doable, fuses are generally small), then a 10A partial short will gleefully char the circuit board until it becomes a short, after which Circuit Board Fires R Us. And, yes, I've seen circuit boards where stuff got so badly charred that it bypassed the fuse. You do not want to know the human safety issues surrounding stuff like this: Circuit board smoke is toxic. So, pick the smallest fuse that fits - then, when (not if) a fault occurs, we blow the fuse, not the system.
2. Yep, wires count. Most of my work has been with circuit boards; so the thickness of the traces on the boards, their width, and oft times their length (Fun, when one gets voltage drop on power traces across a backplane...) But, if one is using, say, a 7A fuse, then the natural assumption is to make sure that the trace is also good for 7A. I remember one notable case where it was discovered that the negative power lead on a CMOS op-amp had been given a 5-mil trace. Which was OK so long as the op amp was powered correctly. But under certain strange power-up conditions, it was possible for the negative trace to be more positive than ground, leading to a nasty condition called CMOS latch-up. It would have been nice if the device blew up; but the cross-sectional area of the bond wires in the IC was actually greater than the cross-sectional area on that 10 mil trace, so the board would blow, preferentially. And the fuse.. was a 5A guy for multiple circuits, so the trace blew first. Whee.

Think of it this way. In the problem we're facing on this thread, we've got a breaker and wire. Suppose we put an (AC) current source on this thing and crank the current up. Before modification, it's a nominal 55A wire and a 60A breaker. Crank the current up and, because we're doing Murphy's Law here, let it settle at each current. 0A: No problem. 10A: No problem. 50A: No problem. 55A: nominal limit of the wire. Yes, I've seen the discussion that if the wire's not surrounded by house insulation then it can handle more current. Screw that: If the NEC or whatever passes for local code says 55A, then 55A it is. The wire is presumably Not Going To Get Warm Enough To Degrade The Insulation Over the Long Term. Go to 57A: Oops. Under the right corner cases, we do degrade the insulation.

So, why 57A? Isn't the car max'd out at 48A? You don't have as suspicious a brain as what I've got. Crazy Ants. Corrosion. Humidity. Fungus, a la WWII Jungle Operations. Cracked plastic because, of course it does. Components in the car, or the wire, or whatever. Mice chewing on insulation. Salt water spray from taking US1 down to Key West. Murphy's law says everything fails eventually. All that junk happens and adds to the normal current. Remember what I said about the minimum fuse size for the load in question?

Heh. Capacitors across the feed. Look up Y-rated capacitors, designed to be connected between line and ground. It gets interesting: They're expensive, larger than normal caps of a particular size and (this is the critical bit) designed that when they fail, they fail open. The better not to shock and kill the innocent. X rated capacitors are similar, but, if they fail, they're allowed to fail short. Fun.

Going on, say we hit 60A.. nominal. That breaker has a bell-curve on the actual current that will pop the thing. At 65A, it will pop. 60+-1A, maybe not.

So, the NEC guys aren't idiots: If they say a wire is good for, say, 15A, that's conservative with fudge factors.

So, going back to belt and suspenders: Put at 50A breaker on 55A wire and the question about fire because of overheating wire simply doesn't occur. Put a 60A breaker on 65A wire, likewise. The basic idea: Make the breaker pop before the wire does.

Finally: It's like this. When it comes to safety related electrical, I don't believe in cutting corners. If a bunch of PhD's and such with replicable research, calculations, and hard experimental evidence come up with numbers, I'm not going to argue with them. Their job is to keep people from dieing, so I'm a-gonna follow their lead.

 

[NickFie]

You're reconfigured the TWC so it reports to the car that it's on a 50A circuit. Then, the car, which is the thing actually drawing the current, never draws more than 40A.

The TWC / EV Supply Equipment (EVSE) does the arithmetic and tells the vehicle the maximum Amps it can supply. That allows properly configured multiple TWCs / EVSEs sharing a single circuit to allocate power so multiple vehicles can charge simultaneously.

We have two HPWCs (Tesla Gen 2 High Power Wall Connector) on a 100 Amp circuit (2 gauge copper wires). My 2017 Model S can charge at 72 Amps (its maximum) if no other vehicle is charging. If my wife connects her Model 3 to the second HPWC to charge, each charger limits its attached vehicle to 40 Amps - half the 80 Amps ceiling for the circuit. As the Model 3’s charge current demand tapers off, the Model S’s HPWC ramps up Model S ceiling.

 

[ucmndd]

I'm reasonably sure (but not 100% positive) that the round up rule for a branch circuit breaker references the wire ampacity, not the calculated load.
See NEC 240.4.(b)

You can say that the ampacity itself is dictated by the calculated load so by inference so is the OCPD but the wire sizing step is a *minimum*; if you want, you can also install a larger wire. The breaker sizing step is a *maximum*, meant to protect the wire.

Yes, thank you for saying what I was trying to say more eloquently. What I was implying is that the round up rule does not negate or supersede other sections of code that require conductors for continuous loads to be rated at 125% of the max continuous load.

Here’s one of many examples discussing this on the Mike Holt forums.

#6 NM-B on 60 amp breaker for 48A EVSE

I have been asked to connect an EVSE to an existing piece of 6/2 NM-B (ROMEX) in a new home that was installed by the home builder. the EVSE has a draw of 48 amps, so the breaker needs to be 60 amp (48 x 1.25 = 60). My understanding is that a continues load (the charger will be drawing maximum...

forums.mikeholt.com

 

[Rocky_H]

Not what the dude said. He merely said you could use the 60a breaker. He didn't say you could charge at 48a on#6 NM-B.

How else is one supposed to read that? Here it is:

A 60A breaker is the correct (and code compliant) size breaker for almost all #6 NM-B installations.

Talking about #6 NM-B and says for these loads:

Both a 44A and a 48A continuous load get a 60A breaker.

In a paragraph talking about uses for #6 NM-B, it says it's for both 44A and 48A loads. It should have probably said the #6 couldn't be used for the 48A load.

So that's cool if you do understand the need to upsize the wire for the 48A setting, but that was not stated there, so it gave a very different impression.

 

[appleguru]

How else is one supposed to read that? Here it is:

Talking about #6 NM-B and says for these loads:

In a paragraph talking about uses for #6 NM-B, it says it's for both 44A and 48A loads. It should have probably said the #6 couldn't be used for the 48A load.

So that's cool if you do understand the need to upsize the wire for the 48A setting, but that was not stated there, so it gave a very different impression.

Let me be explicit:

If you have #6 NM-B wire for your EVSE:
-If your wiring is not run through insulation and your state allows for the full 75C rating of the cable, you can run your EVSE at the full 48A on a 60A breaker
-if your wiring *is* run through insulation, or you live in a state where the normal NEC applies and nm-b is limited to the 60C column, you can run your EVSE at 40A or 44A on a 60A breaker. If you are only drawing 40A, a 50A breaker is also allowed (but a 60A breaker is totally fine and code compliant)