The Comfort Zone
No. 05
by Maury Tiernan
Geary Pacific Corporation
May, 1996
Page 1 of 3
The Comfort Zone column appears regularly in the Modular Building Institute’s Commercial Modular Construction Magazine
AMPACITY VS. AMPERAGE . . . ARE THEY THE SAME ?
Imagine that the electrician calls from the job to tell you that there is not enough power to run the newly set
modular building. You know that you do not have enough money in the job to bring in more. Does this sound
familiar ? What happened ? One possible cause (on larger projects) is a common error in the electrical load
calculations.
The unit ratings and electrical data for each HVAC unit can be found on its specification sheet and data tag. Two
very common terms found in both places are AMPACITY and AMPERAGE.
●● AMPACITY is used to calculate the size of the wire and breaker size at the panel box to the
HVAC unit.
●● AMPERAGE is used many different ways. In the mobile/modular industry it is used to calculate
the electrical loads (in watts) for the panel schedule on the plans.
Are you thinking “Well, so what ? ...AMPACITY or AMPERAGE...it's all the same”? Not so. If you grab
the wrong number off of the HVAC equipment specification sheet, the mistake can cause you to miscalculate the
electrical load. That error snowballs as the electrical load is used to determine other electrical needs. It can even
cause you to choose a larger-than-necessary site transformer, wasting thousands of dollars.
Consider this rule of thumb when filling out your electrical panel schedule:
The AMPACITY number is approximately 25% larger than the AMPERAGE number
(the HVAC unit full load amps).
Let’s look at an example and do some sample calculations. For our modular building panel schedule we need to
calculate the number of watts required for the single phase HVAC unit. We use the UNIT AMPERAGE in a
formula to calculate the watts as follows:
Volts X Amps = Watts.
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
Usually the amps numbers can be obtained from an HVAC unit spec sheet. Instead of “AMPERAGE,”
most HVAC spec sheets show AMPACITY, sometimes UNIT FULL LOAD AMPS, and/or individual
COMPONENT FULL LOAD AMPS. Understanding what each number represents can help you choose the
right ones(s) to calculate the watts correctly.
Volts X Amps = Watts.
●● Use AMPERAGE, if shown.
●● Remember that UNIT AMPERAGE is the same as UNIT FULL LOAD AMPS. You may
need to calculate the unit amperage from the individual component full load amps data. You
would need to know which components’ data to add up.
●● Use AMPACITY if the above is not available, but remember that AMPACITY has a 25%
protection factor above AMPERAGE.
EXAMPLE
Let's work through an example of this watts calculation using sample data from a specification sheet for a 230 volt
single phase 3 Ton Wall Mount Heat Pump with a 5 kw auxiliary heat strip.
SAMPLE DATA FROM A SPEC SHEET:
AMPACITY
UNIT FULL LOAD AMPS
COMPRESSOR FULL LOAD AMPS
BLOWER FULL LOAD AMPS
FAN FULL LOAD AMPS
5 kw HEAT STRIP AMPS
53 amps
Not Shown
16.3/17.0 amps
2.2 amps
1.5 amps
Not Shown
Remember, you want to add up the individual COMPONENT FULL LOAD AMPS to obtain the UNIT
FULL LOAD AMPS (amperage) needed to calculate the watts. Mistaking AMPACITY for AMPS
(amperage) makes the calculation shorter, but approximately 25% larger wattage number.
FULL LOAD AMPS METHOD
Would you know which components to add up ?
AMPACITY (a.) METHOD
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
The Comfort Zone
No. 05
by Maury Tiernan
Geary Pacific Corporation
17.0 amps x 230 volts = 3,910
2.2
x 230
= 506
1.5
x 230
= 345
5 kw strips is watts = 5,000
UNIT WATTS = 9,761
May, 1996
Page 3 of 3
53 a. x 230 v. = 12,190
__
UNIT WATTS = 12,190
Therefore, the actual wattage is 9,761 watts. Using AMPACITY instead of FULL LOAD AMPS in our
formula the wattage would be 12,190 watts. That’s a difference of 2,429 watts, about 25% too high.
Let’s now take our example a little further in our scenario to look at how this information can result in unnecessary
expense. Wattage is used to determine the LCL (Long Continuous Load). Assuming the 5 kw heat strip could be
active more than three hours per day, using the full load amps watts of 9,761, the watts are multiplied by 1.25 (at
the bottom of your schedule) to get the Long Continuous Load figure of 12,201 watts. It is no coincidence that
this number already looks a lot like the wattage obtained using AMPACITY to calculate watts. The
AMPACITIY number already has 25% protection over the FULL LOAD AMPS number.
So here is the bottom line with AMPACITY vs. AMPERAGE calculations. While over-stating the wattage
is definitely not a health and safety issue, it can cause the Dealer or Building Owner to increase the main panel /
transformer size up to 25% larger than necessary. If you use the AMPACITY number from the panel schedule
and then multiply by 1.25 (found in the LCL section), you have over stated the load by an additional 25%. If you
use the AMPACITY number, simply exclude that from your LCL affected watts.
On large projects, multiplying 2500 watts by a large number of floors can drastically effect the main site
transformer selection. If incorrect wattage becomes part of your contract, and the dollars for the larger panel /
transformer were not included in the bid, someone ends up on the short end of the stick.
Keep your voltage up, and your electrical bills down, until the next time we enter . . . The Comfort Zone.
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940