When provisioning servers in colocation facilities, power capacity is typically measured in amps (A) at standard voltages (usually 120V or 208V in North America). Providers offer tiered power packages like 0.25A, 0.5A, 1A or 2A circuits. To properly size your allocation, you need to calculate your server's actual power draw.

The fundamental formula for calculating amperage is:

Amps (A) = Watts (W) ÷ Volts (V)

For example, if your server's power supply is rated for 500W and operates at 120V:

500W ÷ 120V = 4.17A

There are several ways to determine your server's actual power consumption:

1. Using Manufacturer Specifications

Check the server's technical documentation for:

• Power Supply Unit (PSU) wattage rating
• Typical and maximum power consumption
• Input voltage range

Example for a Dell PowerEdge R750:

PSU Rating: 800W (Platinum)
Typical Consumption: 300-450W
Input Voltage: 100-127V or 200-240V

2. Measuring with IPMI/BMC

Most enterprise servers provide power monitoring through their management controllers:

# Dell iDRAC example
racadm get system.Power.RealtimeReading

# HP iLO example
show /system1/powercontrol1

3. Using Power Monitoring Tools

Linux power monitoring example:

# Install powerstat
sudo apt install powerstat

# Run measurement for 60 seconds
sudo powerstat -d 60

Let's walk through sizing a colocation power allocation for a web server cluster:

Server configuration:
- 2x Intel Xeon Gold 6338 (32 cores)
- 512GB RAM
- 6x NVMe SSDs
- Dual 800W PSUs
- 208V power input

Measured average load: 450W

Calculation:
450W ÷ 208V = 2.16A

Recommended allocation: 3A circuit (provides 30% headroom)

• Always account for power supply efficiency (80 Plus ratings)
• Include headroom for peak loads (typically 20-30%)
• Consider redundant power supplies (double the circuit requirement)
• Factor in PDUs, switches and other infrastructure
• Monitor actual usage and adjust allocations over time

Here's a Python script to help with power calculations:

def calculate_amps(watts, volts=120, efficiency=0.9):
    """Calculate required amperage with optional efficiency factor"""
    return (watts / volts) / efficiency

def recommend_circuit(amps):
    """Suggest standard colocation circuit size"""
    standard_sizes = [0.25, 0.5, 1, 2, 3, 5, 10]
    for size in standard_sizes:
        if amps <= size * 0.7:  # 30% headroom
            return size
    return max(standard_sizes)

# Example usage
server_watts = 450
input_voltage = 208
required_amps = calculate_amps(server_watts, input_voltage)
recommended = recommend_circuit(required_amps)

print(f"Required: {required_amps:.2f}A")
print(f"Recommended circuit: {recommended}A")

---

When leasing rack space in a colocation facility, power allocation is typically specified in amps (A) at a specific voltage (usually 120V or 208V in North America). Common allocations include 0.25A, 0.5A, 1A, or 2A circuits. The key formula to remember is:

Power (Watts) = Voltage (Volts) × Current (Amps) × Power Factor

To determine your server's amperage requirements:

1. Check the server's power supply specifications (usually listed in watts)
2. Identify your facility's voltage (120V or 208V)
3. Assume a power factor of 0.9 for modern servers unless specified otherwise

Example calculation for a 500W server at 120V:

Amps = (Watts) / (Volts × Power Factor)
     = 500W / (120V × 0.9)
     ≈ 4.63A

Here's a Python function to calculate required amperage:

def calculate_amps(wattage, voltage=120, power_factor=0.9):
    """Calculate required amperage for server power allocation.
    
    Args:
        wattage (float): Server power consumption in watts
        voltage (float): Facility voltage (120 or 208)
        power_factor (float): Typically 0.8-0.95 for modern servers
        
    Returns:
        float: Required amperage
    """
    return wattage / (voltage * power_factor)

# Example usage:
server_wattage = 750  # Dell PowerEdge R740 typical consumption
required_amps = calculate_amps(server_wattage, voltage=208)
print(f"Required amperage: {required_amps:.2f}A")

For more accurate measurements:

• Use server's built-in iDRAC/iLO/IPMI power monitoring
• Implement power monitoring via SNMP (example OIDs for APC PDU):

# SNMP walk command for APC PDU power monitoring
snmpwalk -v 2c -c public pdu_ip .1.3.6.1.4.1.318.1.1.12.2.3.1.1.2

When planning for multiple servers:

Server Model	Typical Wattage	Amps @120V	Amps @208V
Dell R640	400W	3.70A	2.14A
HPE DL380	450W	4.17A	2.40A
Supermicro 1U	350W	3.24A	1.87A

Always include a 20-30% buffer for power spikes and future expansion.