Replacing 50 amp breaker with 60 amp?

[MatildaModelY]

I currently have a 50 amp breaker that's running outside/underground to the v3 wall connector. I'd like to get the max charging speed out of it and so it seems I would need a 60 amp breaker here. The wire used is 125ft 6/3 UF-B with ground wire, which I believe is the right wire to accept 60 amp. House panel is 200 amp.

I've tried asking my electrician but so far it's crickets from him... I figured someone here might have an answer! TIA.

 

[thecavalry]

Fairly sure it’s rated to 55amps. So, no. You should not swap the breaker to 60 amps.

 

[jcanoe]

The thing to do is contact another electrician.

Underground Feeder (UF)-B is similar to NM-B (Romex) except with UF-B the wires are encased in solid plastic (UF-B) instead being more loosely bundled, wrapped with paper as in the construction of NM-B.

The ampacity specifications for UF-B are the same as for NM-B, i.e. you can only use 6 gauge UF-B or NM-B with a circuit rated over 55 amps, up to 70 amps if you use 4 gauge wire. 6 gauge UF-B or NM-B is limited to circuits of 55 amp maximum.

http://www.cmewire.com/catalog/sec01-bwx/bwx-01-nmb-ufb-amp.pdf

Changing to a circuit (and wire) rated for 60 amps would enable charging at 48 amps. You would be able to charge, add up to 42 miles per hour when charging versus 36 miles of range with the current 50 amp circuit that enables charging at 40 amps. Is the extra 6 miles per hour something you absolutely need?

 

[MatildaModelY]

Fairly sure it’s rated to 55amps. So, no. You should not swap the breaker to 60 amps.

I did just look that up and yes, correct, unfortunately it's only rated to 55 amps. Bummer...

But then some place are saying amp ratings are typically rounded up.

How many amps can a 6 3 wire carry?

6/3 is normally protected with a 60-amp breaker. Since there are no 55-amp breakers, code allows you to round up to the next larger standard size.

askinglot.com

 

[MatildaModelY]

I actually was looking at a similar question on a nissan leaf forum. The tesla wall connector pulls a max 48 amps, so I'm thinking this should actually be fine. They say:

"NM cable is limited to the base 60C ampacity (*). So #6 NM typically has an ampacity of 55A. Since 55A is not a standard breaker size, it is permissible to protect it with a 60A breaker, provided that the calculated load is 55A or less."

 

[thecavalry]

I actually was looking at a similar question on a nissan leaf forum. The tesla wall connector pulls a max 48 amps, so I'm thinking this should actually be fine. They say:

"NM cable is limited to the base 60C ampacity (*). So #6 NM typically has an ampacity of 55A. Since 55A is not a standard breaker size, it is permissible to protect it with a 60A breaker, provided that the calculated load is 55A or less."

It’s not standard…so we should ignore the 80% continuous duty rating and exceed it purposely for extended periods of time, often?

No. Do not do this.

 

[jcanoe]

Since 55A is not a standard breaker size, it is permissible to protect it with a 60A breaker, provided that the calculated load is 55A or less."

This comes up frequently. It is true that the NEC allows for selecting the next higher amperage circuit breaker when there is no exact circuit breaker made, i.e. there is no 55A breaker so a 60A breaker can be used. The 80% rule would still apply when sizing the circuit for a continuous use equipment (charging a plug-in vehicle or EV is considered continuous use.) 80% of 55A is 44 amps, so drawing 48 amps while charging would not be in compliance with the 80% rule. To be in compliance with code the maximum amperage when charging would be limited to 40 amps as the Tesla Wall Connector can be configured for a 50A circuit, wiring but not a 55A circuit, wiring.

 

[MatildaModelY]

This comes up frequently. It is true that the NEC allows for selecting the next higher amperage circuit breaker when there is no exact circuit breaker made, i.e. there is no 55A breaker so a 60A breaker can be used. The 80% rule would still apply when sizing the circuit for a continuous use equipment (charging a plug-in vehicle or EV is considered continuous use.) 80% of 55A is 44 amps, so drawing 48 amps while charging would not be in compliance with the 80% rule. To be in compliance with code the maximum amperage when charging would be limited to 40 amps as the Tesla Wall Connector can be configured for 50A but not 55A.

Thank you. The max amperage the wire would receive at any time would be 48amps - that still wouldn’t work even though the wire is rated for 55? Maybe I just have a poor understanding of this.

 

[jcanoe]

The wire (6 gauge UF-B or NM-B) is rated to 55A. The wire being limited to 55A is the limiting factor not the 60A breaker. Whenever charging an EV the rule is to always derate the circuit to 80% of the rated maximum amperage, so 44 amps would be the maximum when charging in this case. If the Tesla Wall Connector could be configured for a 55A circuit then charging using the Wall Connector would be limited to a maximum of 44 amps. Since the Wall Connector does not have a setting for a 55A circuit the next lowest setting must be used. In this case the Wall Connector would be correctly configured when set for a 50A circuit. This would enable charging at a maximum of 40A. (40A is also your current charging amperage with the current wiring.)

 

[MatildaModelY]

The wire (6 gauge UF-B or NM-B) is rated to 55A. The wire being limited to 55A is the limiting factor not the 60A breaker. Whenever charging an EV the rule is to always derate the circuit to 80% of the rated maximum amperage, so 44 amps would be the maximum when charging in this case. If the Tesla Wall Connector had a setting for 55A circuit then charging would be limited to 44 amps. The Tesla Wall Connector does not have a setting for a 55A circuit so the next lowest setting is for 50A circuit that will enable charging at 40A.

Is the derating rule to account for any unexpected spikes?

 

[jcanoe]

Is the derating rule to account for any unexpected spikes?

No; The 80% rule ensures that when the circuit is continuously operating, drawing 80% of the maximum amperage over many hours that the circuit wiring will not become so warm that the wire insulation starts to overheat and break down.

 

[MatildaModelY]

No; The 80% rule ensures that when the circuit is continuously operating, drawing 80% of the maximum amperage over many hours that the circuit wiring will not become so warm that the wire insulation starts to overheat and break down.

I guess I’m still a bit confused since the max setting for the model Y is 48amps which is below the wire rating of 55amps.

I.e it will never draw 55 or 60 amps in the first place, making the wire seemingly compliant.

 

[jcanoe]

We must apply a percentage (in this case 80%) to the circuit wire rating. For example, multiply 50 by .80 and you get 40. Multiply 55 by .80 and you get 44. Multiply 60 by .80 and you get 48. These results (40A, 44A, 48A) tell us that to be in compliance with code we can't exceed these amperage values when charging the Tesla Model Y (or any EV) when using wiring rate for a maximum of 50A/55A/60A.

You could charge at 44A and be in compliance with code with the current wiring that is rated for up to 55A but the Tesla Wall Connector does not have the capability of being configured for a 55A circuit. You have to go with the next lower circuit rating (50A) that limits charging to a maximum of 40A.

 

[jmaddr]

I guess I’m still a bit confused since the max setting for the model Y is 48amps which is below the wire rating of 55amps.

Breakers and wire are rated for instantaneous max current, short term use such as an AC motor starting. 80% rule specifies wires for continuous use based on heat build up.

 

[Sophias_dad]

I'm probably gonna get crap for this, but at least for the underground-direct-buried section the wire should be fine. Per NEC Table 310.15(B)(2)(a), we can uprate wire that is known to be in favorable temperatures. In this case, underground in Illinois. From 11-15C(51-59F), its a 1.22 factor(55x1.22=67 amps), and for 16-20C(60-68F), its 1.15(55*1.15=63.25 amps), Even the 1.08 factor encountered from 21-25C(69-77f) is 59.4 amps.


So at least for the underground section, I wouldn't hesitate to consider this 6AWG suitable for 60 amp service. If an electrician were doing the swap, they may or may not want to upgrade the wire from the breaker to just before it enters the ground. IIRC there may also be special rules for short distances of non-compliant wire ratings,


You can also choose to ignore the uprating for the breaker and just set the HPWC to 48 amps. The 50A breaker is still protecting the wire from overheat, in fact its rated less than the 55 maximum even not considering temperature factors. It >might< trip depending on how long your charge sessions really are and what other heavily loaded breakers are right next to it, and how hot it is where your panel is. I've done this and have had plenty of long-lasting charge sessions where I monitored the cable(6-2 NMB) temperature and it got barely warm(88F of the maximum 140F) in my 65F basement. Would I do this if I knew that cable were running through an attic in Texas, or worse, an attic with insulation all around the cable... definitely not! But my cable is in free basement air for all its length, and except for maybe 20 feet of it's ~60 foot run, its not even tacked to a flat-face-of-joist-or-beam.

 

[Jjrss]

Rule of thumb (I may have just created it in the past):

Take the amount of your insurance deductible, add the value of your time by the hours to replace whatever might burn up in the vicinity of the equipment, then factor the value of any irreplaceables.

If that total cost is less than the lowest quote from 3 electricians who will get the work permitted legally, and the permit will be accepted by your claims adjuster...

By all means, exceed the 80% max continuous draw on your wiring, and setup the next owner of your home for a lawsuit against you.

 

[MatildaModelY]

Rule of thumb (I may have just created it in the past):

Take the amount of your insurance deductible, add the value of your time by the hours to replace whatever might burn up in the vicinity of the equipment, then factor the value of any irreplaceables.

If that total cost is less than the lowest quote from 3 electricians who will get the work permitted legally, and the permit will be accepted by your claims adjuster...

By all means, exceed the 80% max continuous draw on your wiring, and setup the next owner of your home for a lawsuit against you.

Lol.. yeah I'll probably just keep it as is at the 50 amp breaker. So be it.

 

[ChrisMPK]

If an improperly sized breaker was grounds for a lawsuit, there wouldn't be a real estate transaction in the US that didn't end up in court. If you are concerned that the wire will overheat despite being an adequate size for at least a 55A draw, leave the 50A breaker as suggested above. You could also lower your charge rate a couple of amps if you did experience tripping.

If the cable is truly underground, you're fine at 60A. This comes down to whether you have the capability to use your brain, or only read charts and rules on the internet. Talk to an experienced electrician, as they are usually pretty good at navigating both charts and the real world.

 

[MatildaModelY]

If an improperly sized breaker was grounds for a lawsuit, there wouldn't be a real estate transaction in the US that didn't end up in court. If you are concerned that the wire will overheat despite being an adequate size for at least a 55A draw, leave the 50A breaker as suggested above. You could also lower your charge rate a couple of amps if you did experience tripping.

If the cable is truly underground, you're fine at 60A. This comes down to whether you have the capability to use your brain, or only read charts and rules on the internet. Talk to an experienced electrician, as they are usually pretty good at navigating both charts and the real world.

Thanks. The wire is truly underground except for about 3 feet where it enters the house’s exterior and to the panel.

 

[thecavalry]

If an improperly sized breaker was grounds for a lawsuit, there wouldn't be a real estate transaction in the US that didn't end up in court. If you are concerned that the wire will overheat despite being an adequate size for at least a 55A draw, leave the 50A breaker as suggested above. You could also lower your charge rate a couple of amps if you did experience tripping.

If the cable is truly underground, you're fine at 60A. This comes down to whether you have the capability to use your brain, or only read charts and rules on the internet. Talk to an experienced electrician, as they are usually pretty good at navigating both charts and the real world.

NEC isn't just "rules on the internet". Every single requirement is there because of real life consequence.