Next size up

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steve burke

steve burke

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Morning all!
Running three #4 THHN w #8 ground from 200A main to 100A sub in detached garage.
Since #4 THHN is 95A @90, can we put 100A breaker in main panel as OCPD for the Garage sub?
That said, 100A garage sub panel will also have a car charger. So that particular EV branch circuit must be protected (as i understand) at 125% of the of the #4 THHN feeding the garage sub. So that means max EV breaker size 76A — would have to have 70A since no one makes a 75 or 76, right?
Thanks!
 
#4 Cu 75C ampacity = 85A, you are limited to 75C by the terminations. So the feeder is good for a calculated load of up to 85A, and may be protected by a 90A breaker (next size up).

If the only load is a continuous load (e.g. the EVSE), that load is limited to 85A / 125% = 68A.

Cheers, Wayne
 
As WW said you cannot use a 100 amp OCPD for this feeder with #4 AWG conductors. What's the total calculated load?
 
tthh said:
I agree.

It is kinda interesting to think about...lighting and a lot of other loads have decreased, but now here come the electric cars...one car today, two tomorrow.
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Thanks gentlemen,
Three #4 THHN conductors pulled to detached garage sub panel. Bigger wire didnt go. That said the garage subpanel supports a 3.3 amp pond pump (running 24/7/365) and other modest incidental loads (eg garage opener etc) that are estimated at 10amps(probably much less).
Total amps available to garage=85A (per WW above). Breaking that down:
85A = 10A (not continuous convenience and lighting) + 4.125A (3.3 * 1.25) + EVSE * 1.25 ==> EVSE limited to 56.7amps, right?

So the EVSE must be sized to a 55A breaker in the Garage sub, right?
 
steve burke said:
Thanks gentlemen,
Three #4 THHN conductors pulled to detached garage sub panel. Bigger wire didnt go. That said the garage subpanel supports a 3.3 amp pond pump (running 24/7/365) and other modest incidental loads (eg garage opener etc) that are estimated at 10amps(probably much less).
Total amps available to garage=85A (per WW above). Breaking that down:
85A = 10A (not continuous convenience and lighting) + 4.125A (3.3 * 1.25) + EVSE * 1.25 ==> EVSE limited to 56.7amps, right?

So the EVSE must be sized to a 55A breaker in the Garage sub, right?
Click to expand...

55 amps is not a standard size overcurrent protective device so 60 amps is the next size up
 
Methinks there is a misunderstanding of the 125% rule.

For a continuous load, the wire must have an ampacity at least 125% of the load, the breaker must have a rating at least 125% of the load, and the breaker must be sized to protect the wire. The breaker cannot be matched to the load current.

Jon
 
winnie said:
For a continuous load, the wire must have an ampacity at least 125% of the load, the breaker must have a rating at least 125% of the load, and the breaker must be sized to protect the wire. The breaker cannot be matched to the load current.

Jon
Click to expand...

Seeing Dennis' review of the status of the thread, I am going to add:

If the EVSE is a 55A continuous load, then it would be placed on conductors with an ampacity 69A or greater. The OCPD would need to be rated 69A or greater, and also would have to properly protect the conductors.

Note that the _breaker_ does not significantly limit the current consumed by the EVSE in normal operation. Breakers are intended to trip on faults, and can take a very long time to trip on slight overloads. If you have an EVSE designed to draw 60A and install it on a 50A breaker it might _never_ trip. Usually EVSE devices have internal setting to adjust the maximum current they will draw. Given the other loads in the garage good design would dictate the EVSE set to 50 or 55A, and then breakers sized as described using the 125% rule.

-Jon
 
wwhitney said:
#4 Cu 75C ampacity = 85A, you are limited to 75C by the terminations. So the feeder is good for a calculated load of up to 85A, and may be protected by a 90A breaker (next size up).

If the only load is a continuous load (e.g. the EVSE), that load is limited to 85A / 125% = 68A.

Cheers, Wayne
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Wayne, it has always been my understanding that feeders don't go up to the next higher standard size. Branch circuits can go up if they meet the three requirements in 240.4 (B) Figuring the #4 at 75C (85A), I would think that the maximum OCPD would be 80 Amp. I may be wrong here and if so, someone please clarify. I was taught that feeders (roots) go down and branches go up. That's how I remembered it.
 
Rick 0920 said:
it has always been my understanding that feeders don't go up to the next higher standard size. Branch circuits can go up if they meet the three requirements in 240.4 (B) Figuring the #4 at 75C (85A), I would think that the maximum OCPD would be 80 Amp. I may be wrong here and if so, someone please clarify. I was taught that feeders (roots) go down and branches go up. That's how I remembered it.
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240.4(B) isn't limited to branch circuits.

#4 feeder conductors could be protected at 90A per 240.4(B).
 
The rules are in 240.4(B), no prohibition on feeders, that's the more usual case in fact. Sometimes prohibited for branch circuits, as you can see in part (1):

(2017) 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:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

Cheers, Wayne
 
table 310.12 will allow you to set the breaker on a #4 cu feeder at 100 amp, if the continuous load calcs out less than 85 amps, you are still within the requirements.
 
david luchini said:
320.12 does not apply to the feeder to the detached garage.
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310.12(B) would allow the #4 to be on a 100amp OCPD, although I agree the derating for the continuous load is going to effect the circuit sizing.
 
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