Electrical Panel Amps:

How to determine the residential electrical service panel age, type, and size or capacity in Amps & Voltage:

This article explains how to estimate the electrical service size, ampacity rating, (or "electrical power" or "service amps") at a building by visual examination of the electrical service panel, main switch, and other details.

In this article series we describe how to recognize different vintages, ages, types of electrical panels and how to find the components that define the ampacity rating of the service equipment.

Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are also discussed.

We include photographs and sketches illustrate electrical panels, meter bases, and electric meters.

Page top photo: an antique electrical fuse panel at la Fabrica Aurora, San Miguel de Allende, Guanajuato, Mexico, photographed by the author.

Electrical Panel Ampacity - Guide to Inspecting Electrical Service Panels for their Ampacity Rating

A frequently asked question at home inspection education conferences is "How do I determine the electrical service amperage or size?"

This article describes inspecting the ampacity or size of the main disconnect and/or main electrical panel.

Safety concerns attend any electrical system inspection. See SAFETY for ELECTRICAL INSPECTORS at Residential Electric Panels.

[Click to enlarge any image]

Watch out: Do not attempt to open an electric panel without making yourself familiar with safe procedures.

Watch out: Do not open or even touch electrical equipment if you find dangerous conditions, or simply uncertain conditions which give cause for alarm.

If you elect not to examine a component, indicate to your client that you did not inspect the component, explain why, and recommend appropriate steps to make access and inspection safe and possible. Document this information in your report.

Home inspectors or electrical inspectors who touch electrical wires or move them in order to better read the data tag are performing a dangerous act.

Determine the Electric Service Panel Ampacity, Age & Type by Visual Inspection

We look for an electrical panel brand, model & data tag (usually paper) or embossed rating (on fuse pull outs) on the panel itself which often includes the amperage rating of the panel.

This information is usually present in newer panels on a panel side, or on the panel cover.

This data tag or one of multiple tags usually found inside the electrical panel door or cover but sometimes also found inside the electrical panel enclosure itself, perhaps on the interior surface of a the load center or panel side typically provides several categories of information including

• UL listing
• Electrical Panel or load center manufacturer
• Electrical Panel or load center AMPS rating
• Electrical Panel or load center model number and serial number.
  
  To decode a serial number to find the age of an electrical panel
  
  see ELECTRICAL PANEL AGE.

Actual dimensions of an electric panel are not a reliable determinant of ampacity. For example, many larger panels can be fitted with a variety of bus-bar and main switch assemblies of varying ampacity.

How to Recognize an Older 30- 60- & 100 Amp Fuse Panels

Older 30-amp fuse panels typically have round 15-amp or 20-amp fuses installed on both hot and neutral conductors,[Figure at above left] and sometimes a separate main fuse switch.

There are safety issues with ELECTRICAL PANEL FUSED NEUTRAL WIRE HAZARDS that risk fire or shock. Look for fuses on the neutral wires in old fuse panels or fuse blocks.

Simpler 30-amp panels or sub-panels

Simpler 30-amp panels or sub-panels like the smaller four-fuse panel shown at immediate left are usually auxiliary rather than main service panels in most buildings.

An exception which the author's have found is that this panel was used as a main service for small lake and other remote cottages.

Where such buildings have in more recent years had additional electrical circuits installed, for use of electric heaters, for example, such panels are often overcrowded and very often the individual circuits are over fused - a fire hazard.

Identify Older 60-amp Fuse Panels

Older 60-amp fuse panels [first panel shown above] and some 100-amp panels [Figure at left] often have a single or a pair of Bakelite^TM fuse holders, one marked "main," the other marked "range."

Also see ASBESTOS in PLASTICS

In typical installations, the "main" fuse pullout shuts off power in the panel except the range circuit.

This main should contain 60-amp or smaller fuses. Both main and "range" pullouts must be removed to cut all power.

The example at left used a 60A fuse supplying the lighting circuits (fuses in row along the panel bottom) and a 40A 240V appliance circuit is supplied by the "range" fuse pair.

Identify Single-Pull-Out 100A Fused Electrical Panels

100-Amp fuse panels: The next panel shown above is a 100-amp (or larger) fuse panel using a main fuse holder centered in the top of the panel.

The two fuse holders for two 240V circuits at the panel bottom are not mains and were often used to supply a range circuit and clothes dryer circuit, or range and sub panel circuits.

125-amp CB panels are often fitted with a 100-amp main disconnect CB or fuse-pair.

In this case the electrical inspector should ignore the larger equipment panel size (perhaps indicated by an inside-panel label) and instead s/he should rate the electrical service ampacity based on the main CB or fuse-pair.

Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.

Identify 100-Amp Circuit Breaker Panels

Based only on apparent panel size and provision for 24 circuits, this larger panel at left, showing a modern main CB at the top and two 12-breaker columns is at least a 100 Amp box, maybe 150 Amps.

Better check the service conductors, main CB, and panel label.

Identify Multi-Pull-out 100-Amp Fuse Panels, Split Bus

In the multi-pullout 100-amp fuse panel shown next at left, all four of the top pullouts are mains.

The top two switch 240-volt appliances and the bottom two power the left and right hand groups of screw-in type fuses in the bottom of the panel. (The upper left-hand 240 V appliance circuit's fuse is not connected so is not in use.)

All four should be labeled mains.

This is a tougher one since, as Hansen points out, [Hansen, op. cit.] the sum of the ratings of the different disconnects may exceed the capacity of the conductors.

For example you can have a 125A service with two main disconnects, one rated at 100 amps and one rated at 50 amps. In this case we'd look at the rating of the service conductor and the service equipment (panel) for help.

Details about split-bus electrical panels are

at SPLIT BUS ELECTRICAL PANEL HAZARDS

Electrical Panel Age Encoded in Panel Serial Number - General Electric

IF your electrical panel data tags include a GE serial number (as do GE appliances), then you can translate a GE serial number to month and year of manufacture using the following procedure:

GE serial number: first alphabetic character = month of manufacture, in this order:
A - January, D - February, F - March, G - April, H - May, L - June, M - July, R - August, S - September, T - October, V - November, Z - December

GE serial number: second alphabetic character = year of manufacture, in this order

F = 1991, 2003, or 2015

G = 1980, 1992, 2004, 2016

H = 1981, 1993, 2005, 2017

L = 1982, 1994, 2006

M = 1983, 1995, 2007

R = 1984, 1996, 2008

S = 1985, 1997, 2009

T = 1986, 1998, 2010

V = 1987, 1999, 2011

Z = 1988, 2000, 2012

A = 1989, 2001, 2013

D = 1990, 2002, 2014

...

---

---

ADVERTISEMENT

---

---

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

How do I verify the amperage rating of my electrical box if it h as no tags or labels?

The electrical inspector is asking me to verify the amperage rating of my service box and there are no tags or anything on it how do I do that - On 2020-10-06 by Michael

Suggestion by (mod) -

You could try identifying the circuit breakers by brand and look at electrical panels of that brand of the physical dimensions of yours.

Also look at the main circuit breaker and the service entry cable size.

We explain in detail

at AMPS, LIMITING FACTORS

that the smallest (in capacity) of the panel, main breaker, or electrical service entry cable determines that actual service ampacity at a building.

How to Decide the Main Disconnect Breaker / Fuse Rating

I have 4, 125 amp panels in a larger office trailers. Do I have to have a to have a primary panel rated at 500 amps or can I derate the feeders. On 2020-01-21 by ES

Reply by (mod) -

do I have to have a to have a primary panel rated at 500 amps or can I derate the feeders.

Great question, ES. I don't know an authoritative answer, but in general I think the answer depends on how the panels are fed.

For example it's common to see on service entry feed multiple electrical panels at a building or complex.

Here are some useful excerpts of the US NEC (If you are a U.S. reader) pay particular attention to Exception No. 2 below. Let me know what your local code inspector says.

225.31 Disconnecting Means.

Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.

225.32 Location.

The disconnecting means shall be installed either inside or outside of the building or structure served or where the conductors pass through the building or structure. The disconnecting means shall be at a readily accessible location nearest the point of entrance of the conductors. For the purposes of this section, the requirements in 230.6 shall be utilized.

225.33 Maximum Number of Disconnects.

(A) General. The disconnecting means for each supply permitted by 225.30 shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be no more than six disconnects per supply grouped in any one location.

And the exceptions to NEC 230.40 on Service Entrance Conductors may be helpful

[230.40], there are five exceptions:

Exception No. 1: A building with more than one occupancy can have service entrance conductors for each occupancy.

This exception commonly applies to strip malls and even to duplexes, triplexes, and other multiple-family buildings (e.g., apartment houses) to allow locating metering equipment at or near the end of the service drop or service lateral instead of at multiple locations. The rules on the location and grouping of service disconnecting means in 230.71 and 230.72 still apply.

Exception No. 2: Service entrance conductors can supply two to six service disconnecting means as permitted in 230.71(A).

Exception No. 3: A single-family dwelling unit with a separate structure, such as a garage, can have one set of service entrance conductors run to each structure from a single service drop or lateral.

Exception No. 4: A two-family dwelling or multifamily dwelling can have one set of service entrance conductors installed to supply the circuits covered in 210.25.

Exception No. 5: One set of service entrance conductors connected to the supply side of the normal service disconnecting means can supply standby power systems, fire pump equipment, fire and sprinkler alarms, and alternate power sources like fuel cells and solar photovoltaic systems [230.82(5) and (6)].

How can I use an existing 480A elecrical panel to provide 600 Amps for multiple chargers?

I have 68 chargers in a warehouse with 3 phase 480A the existing chargers pull 6 amp and the new chargers that will be installed pull 14.4amp.

The feeds come from a 200 amp panel and a 400 amp panel

is there any way to use the existing panels without adding a new 600 amp panel to feed the new chargers. replacing 68 with 68 On 2019-01-31 by aj

Answer by (mod) - you can't unless you use timers

AJ

If your existing electrical panels of 200A and 400A are powering the whole building then they cannot be entirely dedicated to your bank of 68 chargers (that draw a total of over 1000A) unless you split up the chargers into banks that are operated by timers to assure that only a limited subset can operate at one time.

Keep in mind that you also need to run lighting, other electrical receptacles, circuits, appliances, etc.

Is it OK if the Panel Rating Exceeds the Service Entry Size?

i have a 200 amp service and my main panel rating is 125 amp max. Is this ok? On 2016-04-22 1 by Mike Burchfield

Reply by (mod) -

Fusing at a LOWER amperage is generally a safe step, understanding that the fuse is protecting what is electrically "downstream" from that point.

So, for example, if you have 200A coming in at a meter and a 125A main panel with a 125A main breaker, as long as the cable connecting the meter outlet to the main panel/breaker inlet is rated for 125A or more, you're ok.

...

Continue reading  at MAIN DISCONNECT AMPACITY or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AMPS VOLTS DETERMINATION - home
  • AMPS, LIMITING FACTORS
  • AMPS & SEC SIZES
  • AMPS & VOLTS, DETERMINE VISUALLY
  • AMPS MEASUREMENT AUTOMOTIVE DC
  • AMPS MEASUREMENT METHODS
  • AMPACITY, MAIN DISCONNECT
  • DEFINITIONS AMPS VOLTS WATTS
  • ELECTRIC METERS & METER BASES
  • ELECTRIC MOTOR HORSEPOWER & CIRCUIT WIRE SIZE
  • ELECTRICAL PANEL AMPACITY
  • VOLTAGE at the SEC
  • UNDERGROUND SERVICE LATERALS
• AMPS MEASUREMENT AUTOMOTIVE DC
• AMPS MEASUREMENT METHODS
• ELECTRICAL INSPECTOR SAFETY PROCEDURES important basic safety procedures, clothing, and equipment for home inspectors and electrical inspectors
• ELECTRICAL PANEL AGE - how to decode panel data tags to obtain the age of a load center or electrical panel
• VOLTS / AMPS MEASUREMENT EQUIP

Suggested citation for this web page

ELECTRIC PANEL AMPACITY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Questions & answers or comments about how to estimate the electrical panel size or capacity - amperage rating of an electrical panel.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.

---

 

IF above you see "Comment Form is loading comments..." then COMMENT BOX - countable.ca / bawkbox.com IS NOT WORKING.

In any case you are welcome to send an email directly to us at InspectApedia.com at editor@inspectApedia.com

We'll reply to you directly. Please help us help you by noting, in your email, the URL of the InspectApedia page where you wanted to comment.

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---