Thread: wire size

Question> Todays service disconnect was a 70 amp breaker with a #4 aluminum wire. All the other breakers in the meter room were also 70 amp with #4 copper wire. There seems to be some conflicting information on whether or not this wire should actually be a # 2 wire. What do you guys think??

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I think the larger issue here William is that service panel. It appears to be a Federal Pacific with Stab-lok breakers. They have a known history of failure associated with them.

Sorry if you were already aware of that but as soon as I seen those orange handles on the breaker arms, the alarm goes off in my head.

Nick,

Yes, I do know that. The whole complex has Federal Pacific service disconnects in the meter room. They probably also had Federal Pacific panels in each apartment (not sure if they were fuses or breakers). The box in the unit I inspected has had the guts improperly replaced. I appreciate your response, but my only question here is the wire size.

Bill,

Originally Posted by william siegel

Question> Todays service disconnect was a 70 amp breaker with a #4 aluminum wire.

#4 aluminum wire has a rating of 55 amps in that panel.

All the other breakers in the meter room were also 70 amp with #4 copper wire.

#4 copper wire had a rating of 70 amps in that panel.

wire should actually be a # 2 wire

Aluminum? #2 aluminum has a rating of 75 amps in that panel.

Thus they would have two choices (if the terminals were rated large enough for #2 conductors):
- #4 copper wire had a rating of 70 amps in that panel.
- #2 aluminum has a rating of 75 amps in that panel.

Or other choices to use larger conductors within the rating of the terminal for size.

Thanks Jerry,

Thats what I thought. I did find a chart on line that showed me something different, however I just found out that I read it wrong. This should be fun, as this is a project development in Miami. What I cannot understand is why all the other cables to th other units are copper and this one is aluminum.

If these breakers are serving as the disconnecting means for the subpanel in the apts, than that fals under a different area of the NEC.
It was also stated that 1 Fed P guts were improperly changed, There is a mfg that makes replacement guts that are UL aproved. This allows the use of a diffent breaker other Fed Pac.

#4 (THW) AL is rated at 65-amps and it used to be acceptable to go to the next size breaker so a 70-amp breaker would have been allowed for #4 THW when this was built.

If you look in NEC table 310.15 (B)(6) you will see that #2 THW AL is good for 100-amps for residential services. The table doesn't go any lower than 100-amps. However if you look at the ampacity of all the sizes to their respective service size allowed as shown in that table they all seem to correspond to the next size up breaker from the allowable ampacity in table 310.16.

The #4 THW CU is good for 100-amps.

Originally Posted by Lou Romano

#4 (THW) AL is rated at 65-amps and it used to be acceptable to go to the next size breaker so a 70-amp breaker would have been allowed for #4 THW when this was built.

Still is

2008 NEC

240.4 Protection of Conductors.

Conductors, other than
flexible cords, flexible cables, and fixture wires, shall be
protected against overcurrent in accordance with their ampacities
specified in 310.15, unless otherwise permitted or

required in 240.4(A) through (G).

(B) 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 multioutlet
branch circuit supplying receptacles for cordand-
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.

In the case here, 65 AMPs is not a standard rating.

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes.

Originally Posted by william siegel

Question> Todays service disconnect was a 70 amp breaker with a #4 aluminum wire. All the other breakers in the meter room were also 70 amp with #4 copper wire. There seems to be some conflicting information on whether or not this wire should actually be a # 2 wire. What do you guys think??

Those are not "building services", possibly main power feeders for the individual occupancies, individually metered (poco or private), but NOT service entrances.

Originating indoors, apparently. "meter ROOM"

Main power feeders originating INDOORS - important consideration. Baby table allowable ampacity does not equate size. Not allowed to be blindly used, adjustments for allowable ampacity as per for example temperature restrictions - as this originates indoors. This has been clarified with changes and corrections in the last few code cycles.

Likely building service is via transformer, inside or outside.

Main disconnect does not equal service disconnect for a building or structure, even if the equipment and disconnect is required to be 'service rated'.