Service Entrance Amperage?

Nothing confuses me more than trying to determine amperage when no main breaker exists. Can somone please help with the following?

  1. What are the service entrance wire sizes (CU and AL) with their appropriate amperage capacities?

  2. How do you determine conductor wire size when you can’t read it on the conductor?

I did a 24 year old house yesterday–what would you say the amperage is based on the photo of the conductor feeding into the panel (photo of panel and service entrance conductor sheathing attached)? (I could not read the rating of the panel itself–hidden behind wires.)

Thanks for the help.

061506 059.jpg 061506 081.jpg

Many electrical supply companies have a plastic gauge for this purpose…

1/0 al can carry 100 to 125 amps so I would assume 100 amp service.
I know assume means make an *** of you and me.

Are you sure Jae? I’ve been looking for one for a long, long time and have yet to find one. Please let me know where to get one. I’ve been told that you can’t measure the outside diameter of the insulation to determine the gauge, i.e. you must measure the wire itself. That would be difficult even with a plastic gauge I think.

This is for the service entrance or MAIN FEEDER to the home, not sub panels
from 310.15(B)(6)
I give up, it won’t let you type a table

I had a burndy servit one made out of brass and it had two guage lines one for wire and onw with the insulation . .Now with the different thickness of the insulations have no idea how close it would be . ( misplaced it 30 years ago still keep looking )
Roy Cooke sr . RHI. CAHPI-ON

I ordered a guage yesterday from Professional Equipment (item E835): $13.95. But I obviously didn’t have one during this inspection. Some of you guys seem to be able to “eyeball it” but I certainly can’t.

Would some Guru please post the wire sizes in CU and AL and the appropriate amperage? I’ll buy you a beer for doing so. :smiley:

Also, does anyone make a special comment when the service is only 100 amps? I have been doing so (telling them to talk to a sparkie about the limitations of a 100 amp service). Wondering if that is what most do?

here’s one:

And another:

I owe you Larry. Thanks so much.

What is “THW”, “THWN”, & “TW” insulation? Anyone?

Some reading for you to enjoy.:slight_smile:
There is meaning in the letter sequences used to identify conductor
types, and these letters usually refer to properties of the conductor’s
insulating layer(s). Some of these letters symbolize individual
properties of the wire while others are simply abbreviations. For
example, the letter “T” by itself means “thermoplastic” as an
insulation material, as in “TW” or “THHN.” However, the three-letter
combination “MTW” is an abbreviation for Machine Tool Wire, a type of wire whose insulation is made to be flexible for use in machines experiencing significant motion or vibration.

C = Cotton
FEP = Fluorinated Ethylene Propylene
MI = Mineral (magnesium oxide)
PFA = Perfluoroalkoxy
R = Rubber (sometimes Neoprene)
S = Silicone “rubber”
SA = Silicone-asbestos
T = Thermoplastic
TA = Thermoplastic-asbestos
TFE = Polytetrafluoroethylene (“Teflon”)
X = Cross-linked synthetic polymer
Z = Modified ethylene tetrafluoroethylene

H = 75 degrees Celsius
HH = 90 degrees Celsius


N = Nylon


U = Underground
W = Wet
-2 = 90 degrees Celsius and wet

Therefore, a “THWN” conductor has Thermoplastic insulation, is Heat resistant to 75o Celsius, is rated for Wet conditions, and comes with a Nylon outer jacketing.
Letter codes like these are only used for general-purpose wires such as those used in households and businesses. For high-power applications and/or severe service conditions, the complexity of conductor technology defies classification according to a few letter codes. Overhead power line conductors are typically bare metal, suspended from towers by glass, porcelain, or ceramic mounts known as insulators. Even so, the actual construction of the wire to withstand physical forces both static (dead weight) and dynamic (wind) loading can be complex, with multiple layers and different types of metals wound together to form a single conductor. Large, underground power conductors are sometimes insulated by paper, then enclosed in a steel pipe filled with pressurized nitrogen or oil to prevent water intrusion. Such conductors require support equipment to maintain fluid pressure throughout the pipe.

You wowed me, Michael. Thanks a lot!

Well, that’s a really neat gauge but not what I’m looking for. More often than not, the service entrance wires are separated already and come to the SE panel individually thru a short piece of conduit. This gauge only works for 3 conductors still in the outer jacket. I think I’ll order one anyway. :slight_smile:

Save your money. I’ve had one for over 3 years and it has never come out of the tool bag because of what you stated above.

I bought the plastic guages and tried to use them, but they weren’t very helpful.
Then I went to Home Depot, and bought several one foot pieces of cable, split them apart, cut them down to 3" lengths and now have several very accurate sizes of wire. It has made sizing conductors very easy. Two tips: First, buy long enough lengths of cable so that the size is printed on the side or bring a pen and some masking tape so that you can mark the size. Secondly, go to Home Depot (or other store) when they are not busy, as you will drive them crazy cutting all that wire. I made a whole set, and most of the wire they gave to me for free, I only had to pay for some of the more expensive SEC. Home Depot was very helpful.

I inspected a small, older house that had a 100 amp service a few weeks ago. I asked the client if they planned on adding air conditioning once they bought the home. They said they were planning on purchasing an air conditioner sometime in the next couple of years. That’s when I told them they should get a licensed electrical contractor to check out the service and make sure that it could handle the extra load, especially as the hot water heater was electric. I would not make an issue of it if the furnace and hot water heater were gas, with no A/C and a small house. This was advice that came to me from an electrical contractor who has been wiring houses for over 25 years.

Congrats, that is the way to do it (and what I teach) either you are familiar with what you are inspecting or you aren’t. If you aren’t a set of plastic gauges is at best a poor substitute.



Yes it is, and I should have noted that that was one of the tips that I learned from your electrical course at the NACHI convention in Orlando last February. Many thanks for the great courses at the convention and your on-line electrical course.